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Year : 2018  |  Volume : 14  |  Issue : 11  |  Page : 895-901

Radiation Physics

Date of Web Publication29-Nov-2018

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How to cite this article:
. Radiation Physics. J Can Res Ther 2018;14, Suppl S4:895-901

How to cite this URL:
. Radiation Physics. J Can Res Ther [serial online] 2018 [cited 2022 Oct 2];14, Suppl S4:895-901. Available from: https://www.cancerjournal.net/text.asp?2018/14/11/895/246381

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A protocol for craniospinal axis localization and adaptive dosimetric verification using cone beam CT

K. Mohamathu Rafic, J. Sujith Christopher, S. Ebenezer Suman Babu, B. S. Timothy Peace, B. Rajesh, B. Selvamani, B. Paul Ravindran

Aim: Limitations in the longitudinal field-of-view (LFOV, ~16 cm) and Hounsfield units (HU) inaccuracies of typical cone beam CT (CBCT) restricts its potential use for localization and adaptive verification of large target volumes encountered in radiotherapy. In the present study, we aim to develop a protocol for comprehensive localization and adaptive dosimetric verification along the craniospinal axis (CSA) using CBCT with extended LFOV (CBCTeLFOV). Materials and Methods: Hybrid immobilization comprising of a thermoplastic mask and a whole body Vac-Lok™ was used to immobilize the anthropomorphic phantom as well as patient, from head to mid-thigh in supine position. Planning CT (pCT) images were acquired with 3 mm slice width and 500 mm reconstruction field-of-view (RFOV). Dual isocentre (one at C1 and the other at T1 vertebrae) VMAT treatment plans were computed with four partial arcs. According to our protocol, CBCT was acquired with half-fan geometry and fixed acquisition parameters (450 mm RFOV and 2 mm slice width). Multiple longitudinal translations of table in steps of fixed increment ‘Δ’, resulted in 1 cm the overlap were acquired. The custom scripts coded in MatLab capable of handling upto seven image sets simultaneously, generates fused CBCTeLFOV (eLFOV of 105 cm) that are assigned with same DICOM unique identifiers as the first series. The inbuilt misalignment management algorithm determines the optimum registration coefficient iteratively prior to generate CBCTeLFOV. A new quality assurance approach was demonstrated for validation of fused images by combining Catphan-604 and Catphan-504 phantoms. Contour mapping accuracy/association between pCT and CBCTeLFOV was evaluated using volumetric dice-similarity-coefficient (DSCvol) metric. End-to-end (E2E) treatment workflow demonstrating the actual clinical scenario of craniospinal irradiation (CSI) was investigated using both anthropomorphic phantom and two patients. Results and Discussion: Slice geometry, spatial linearity and HU accuracy of CBCTeLFOV were in excellent agreement with pCT. Although there is no major difference in the volumes of mapped structures, a spatial displacement in vital structures upto 2.5 cm (±0.5 cm) was recorded especially in the spinal PTV and kidneys. This could be due to combined effect of a minor rotational shift, treatment setup geometry and change in anatomy during online localization. Because there is no anatomical deformation and treatment related uncertainties in the anthropomorphic phantom, superior DSCvol results with lower standard deviation (0.97 ±0.02) was recorded. Consistent but lower value (0.74 ±22 and 0.70 ±0.28 respectively) than the desirable results were observed in the two patients. Although E2E dosimetric verification of patient paradigms demonstrated the likelihood of under-dosing the target volume, it is unknown whether these dosimetric deviations would transform into the reduced rate of tumor-control. Further investigations with large patient populations relating dosimetric outcomes with common inter-fraction uncertainties within the planned course of treatment would be required to demonstrate the clinical significance. Conclusion: CBCTeLFOV based localization and dosimetric verification is necessary for determining uncertainties beyond the actual LFOV, especially in the treatment beam boundaries and abutting regions. Our protocol enables regular image guided adaptive radiotherapy (not limited to CSI) with clinically desirable accuracy without affecting the overall machine throughput.

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Python scripting for radiobiological evaluation of robustness in treatment plans using optimized proton pencil beam scanning technique

M. P. Noufal, S. S. Dayananda, P. Kartikeshwar, A. Manikandan, K. Ganapathy, T. Rajesh, C. Srinivas1, R. Jalali1

Department of Medical Physics,1 Department of Radiation Oncology, Apollo Proton Cancer Centre1, Chennai, Tamil Nadu, India

Introduction: Proton therapy is susceptible to larger uncertainties, primarily due to beam range and setup uncertainties. Traditional treatment planning approach based on planning target volume (PTV) margins may not be adequate to cover the Clinical Target Volume (CTV) by the prescription dose. Therefore, robustness evaluation should be carried out routinely in proton planning. Conventionally robustness of the proton plans was assessed using DVHs. However, DVHs are not a true representation of the actual treatment outcomes. Although current version of TPS support radiobiological evolution from nominal plans, it cannot calculate multiple perturbed scenarios and evaluate it radio biologically. The aim of our study is to develop an in-house script for treatment plan robustness evaluation using both physical and radiobiological parameters. Materials and Methods: CT datasets of four mock patients, comprising of brain, prostate, head and neck (HN) and medulloblastoma were chosen for this study. For each patient Intensity Modulated Proton Therapy (IMPT) plans were generated using single filed optimized (SFO) technique on RayStation (v7) TPS modelled for Proteus Plus proton therapy system. Proteus Plus can deliver proton pencil beam size of minimum 3 mm sigma in air for 230 MeV proton in scanning mode. It can modulate the energy from 4.1 to 32 g/cm2 without any range shifter. An in-house script was written in Iron python 2.7 and implemented through the scripting module in the Ray station TPS. Using this script, all the possible perturbed scenarios due to set-up error of ±3 mm in A-P, S-I and R-L, and range uncertainty of ±3.5% were simulated and the corresponding perturbed DVHs of CTV, PTV and OARs were obtained. For each nominal and perturbed scenarios, TCP and NTCP were calculated based on Niemierko model in-build in the scripts. For each plan, 24 sets of perturbed dose distributions were created, resulting in 96 perturbed dose plans. From the resulting perturbed dose distribution, worst case decrease in TCP for CTV and worst case increase in NTCP for the OARs were evaluated and validated using AAPM TG166 protocol. Results: The in-house python script programing allows easy and quick simulation and evaluation of perturbed dose distribution both in physical and radiobiological parameters. The validation of our in-house python script using AAPM TG166 protocol resulted maximum variation of ±1% in EUD, TCP and NTCP. Although worst case scenario leads to a large difference in physical dose distribution compare to nominal plan, radiobiological evaluation of the same showed similar EUD, TCP and NTCP. In comparison to the nominal plans of each clinical sites, worst case scenario plans reduce the EUD and TCP to CTV by a maximum of 2% and 1% respectively. The worst case increase in NTCP for brain, H&N, Prostate and medulloblastoma were 2.5% in optic chiasm, 1.1% in parotid, 0.3% in rectum and 4.2% in lens. Conclusion: We have developed, validated and successfully implemented the in-house python script for plan robustness evaluation. The in-house python script allows easy and quick evaluation of perturbed dose distribution both in physical and radiobiological parameters. All SFO IMPT plans showed sensitivity to uncertainties in physical dose evaluation whereas it is less appreciable in radiobiological model. Use of radiobiological model as a supplement to robust evaluation will help in making a proper clinical judgement of the IMPT treatment plan.

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Clinical application of DIBH amplitude gated technique for stereotactic body radiotherapy (SBRT) lung and liver oligometastases

C. P. Bhatt, Irfan Ahmad, Kundan S. Chufal, Arvind Kumar

Aim: While DIBH is an established technique for irradiation of left-sided breast cancers, its application in SBRT for liver and lung oligometastases remains unexplored. The main challenge in planning is to minimize the internal organ motion to reduce the PTV size and hence dose to surrounding normal tissue. Diaphragmatic motion is a good internal surrogate and closely correlates with the motion of the external surrogate marker used in the RPM gating. The purpose of this analysis is to report the clinical implementation and our initial experience with Deep Inspiration Breath Hold (DIBH) amplitude based motion management for Stereotactic Body Radiotherapy Technique (SBRT) in patients with lung and liver oligometastases. Materials and Methods: Eight consecutive patients treated with DIBH-Amplitude-based SBRT were included in this prospective study [Table 1]. Varian respiratory gating system (RPM) (Varian Medical System, Palo Alto, CA) was used for respiratory motion monitoring. All patients were coached for 3-4 days in order to achieve a reproducible breath hold in terms of amplitude and duration. Simulation CT scans were acquired in free breathing and DIBH phases on Siemens Somatom Sensation Open (Siemens Healthineers, Erlangen). A pretreatment 4D-CBCT in free-breathing phase was acquired and orthogonal fluoroscopy images of the treatment site were also acquired in free breathing and DIBH phases on Varian TrueBeam 2.5. Acquired imaging data was imported into Eclipse v13.5 and target delineation was performed in accordance with the ongoing RTOG BR001 protocol. Treatment planning was performed with co-planar 6MV FFF 2-3 Arc VMAT technique and evaluated as per acceptance criteria of RTOG BR001. The dose was escalated in consecutive patients from BED10 of 75 Gy10 up to 132 Gy10. Patient-specific pre-treatment QA was performed for all patients. Pre-treatment and intra-treatment positioning verification was performed with DIBH CBCT for all patients for each treatment fraction and corrected on a 6D treatment couch. Results: Breath-hold for across all patients varied from 25-45 seconds. Maximum Tumor motion measured during fluoroscopy in free-breathing varied from 8 mm to 15 mm and in DIBH it came down to 1-3 mm, which allowed a reduction in PTV margins from 5mm to 3mm. The Dose gradient index for all DIBH SBRT plans varied from 0.73 to 1.4 cm. Total MU's varied from 1794 to 4765, with total treatment time per session varying from 11.33 minutes to 41.28 minutes and beam-on time varying from 120 seconds to 537 seconds. Conclusions: DIBH Amplitude Gated SBRT reduced the target motion by freezing the target in the DIBH amplitude phase, which allowed a reduction in the PTV margin. DIBH amplitude based SBRT is a precise and reliable motion management technique for SBRT lung and liver. The Treatment time of DIBH gated SBRT can be reduced with treatment techniques like FFF and Gated VMAT/IMRT for moving tumours in Liver and Lung. The main limitation of DIBH Amplitude gated SBRT is the breath-hold time of the patient. The DIBH process is time-consuming and dependent on specialized training as compared to free breath SBRT.
Table 1

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Preliminary clinical experience with indigenously developed tissue equivalent bolus in telecobalt, megavoltage photon and electron radiation therapy

S. Senthilkumar, G. Kesavan1

Department of Radiotherapy, Madurai Medical College and Government Rajaji Hospital,1 Department of Radiotherapy, Vadamalayan Hospitals Pvt. Ltd., Madurai, Tamil Nadu, India

Objectives: Frequently, in external beam therapy one must treat superficial lesions on cancer patients; these are at or adjacent to the skin. Telecobalt, Megavoltage photon radiotherapy penetrates through the skin to irradiate deep seated tumors, with skin sparing property. Hence, to treat superficial lesions, one must use a layer of scattering material to simulate as the skin surface. Although megavoltage electron beams are used for superficial treatments, one occasionally needs to enhance the dose near the surface. Such is the function of a bolus, a natural or synthetically developed material that acts as a layer of tissue to provide a more effective treatment to the superficial lesions. Materials used as bolus vary from simple water to metal and include various mixtures and compounds. Even with the modernization of the technology for radiotherapy and the emergence of various commercial boluses, the preparation and utilization of a bolus in clinical radiotherapy remains an art. The main objective of this study was to present our preliminary clinical experience with indigenously developed tissue equivalent bolus in telecobalt, megavoltage photon and electron radiation therapy and assess the dosimetric properties in LINAC machine for photon and electron beams and also in telecobat machine. Materials and Methods: The new Senflab bolus has been fabricated successfully with the size of 30 x 30 cm2 in different thicknesses like 0.5cm, 1.0cm, 1.5cm and 2.0cm. Ionization measurements have been made in the Varian clinac iX machine for 6, 9,12,15, 18 MeV electron beams and for 6 and 15 MV photon beams and also in 1.25 MeV Gamma Beam. The Senflab were place over the RW3 plastic water phantom with and without bolus material on the surface. The same procedure were repeated for 1.0cm, 1.5cm, 2.0cm and also for the imported bolus for the dosimetric comparison. The stability of the bolus material was investigated over a time duration typical of patient treatment by weekly measurements of ionization at depth 1 cm in plastic water phantom under a 1.0 cm thick slab of fabricated bolus in air and in water. Results: The dosimetric properties of bolus material were determined by comparison with imported bolus of various thicknesses, using Gamma Photon, X-ray photon and electron beams of various energies. Since the Senflab has both an electron build-up characteristic and a density closer to that of water than imported bolus, it is anticipated that this flexible tissue substitute will find wide acceptance in radiotherapy. Conclusion: Newly developed Senflab Bolus material does not suffer inelastic strain from normal stresses, it does not have to be bagged or wrapped in plastic film to maintain its shape. Senflab bolus are nontoxic, flexible and soft. It does conform nicely to patient's contour while maintaining good uniformity to thickness.

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The Impact of Multi Criteria Optimization (MCO) on Flattening Filter Free (FFF) Volumetric Modulated Arc Therapy (VMAT) in Monaco™ treatment planning system (TPS) for Craniospinal Irradiation (CSI)

P. Mohandas, D. Khanna, T. Thiyagaraj, C. Saravanan, Narendra Bhalla, Abhishek Puri

Aim: To study the impact of multicriteria optimization (MCO) on flattening filter free (FFF) volumetric modulated arc therapy (VMAT) in Monaco™ treatment planning system (TPS) for craniospinal irradiation (CSI). Materials and Methods: Five CSI patients treated with 23.4Gy/13 fractions followed by boost dose using 6MV-FFF photon beam were chosen for this study. For each case, dual partial arcs were used for cranium (50°– 180°&180°–310°), upper spine (125°–180°& 180°–235°) and lower spine (110°–180°&180°–250°). Conventional VMAT (c-VMAT) plans were generated with Monaco™ V5.10 TPS for Elekta Versa HD™ linear accelerator with 0.5cm leaf width at isocenter. Keeping all other parameters constant, c-VMAT combined MCO (VMAT-MCO) plans were generated. Evaluation of VMAT-MCO method was done by direct comparison with c-VMAT which was a benchmark plan. For plan comparison, conformity index (CI), homogeneity index (HI) to planning target volume (PTV), dose coverage to PTV (D98%) and maximum dose to PTV were compared. For organ at risk (OAR), mean dose and dose volume received by left lung, right lung, left eye, right eye, left parotid, right parotid, left kidney, right kidney, heart, and liver were analyzed. In addition, max dose to left lens, right lens, and small bowel was analyzed. The normal tissue volume receiving dose ≥5Gy & ≥10Gy and normal tissue integral dose (NTID) (patient volume-PTV), total monitor unit (MU) calculation time (mins) and delivery time (mins) were compared. Results: The CI and HI slightly improved in VMAT-MCO plan as compared to c-VMAT plan. No significant difference was observed (P>0.05). Similarly, no significant dose difference was observed in Dmean and D98% to PTV, normal tissue volume receiving dose ≥5Gy & ≥10Gy and NTID (P>0.05). The slight increase of maximum dose to PTV was found in VMAT-MCO plan as compared to c-VMAT plan. However, no significant dose difference was observed (P>0.05). The mean dose, max dose and dose volume received by left lung, right lung, left eye, right eye, left parotid, right parotid, left kidney, right kidney, heart, liver, left lens, right lens and small bowel showed significantly less dose in VMAT-MCO plan as compared to c-VMAT plan (P < 0.05). An increase in MU and delivery time was noticed in VMAT-MCO plan when compared with a c-VMAT plan (P < 0.05). On comparison of both optimization plans, it was seen that there was a significant difference in calculation time (P < 0.05). Also, the VMAT-MCO plan was faster in dose calculation as compared to c-VMAT plan. Conclusion: The MCO-VMAT was more efficient and able to generate a better quality plan. For craniospinal irradiation, the MCO-VMAT plan could be used without compromising target coverage, reduced OAR dose and calculation time as compare to c-VMAT plan.

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Testing the IAEA code of practice TRS-483 and application for small field dosimetry with 10MV WFF beam with truebeam linear accelerator - Two years dosimetry experience

Rajesh Kinhikar, Vinay Saini, Suryakant Kaushik, Sudarshan Kadam, Chandrashekhar Tambe, A. Sutar, Rituraj Upreti, Rajesh A. Kinhikar, Deepak Deshpande, Karen Christaki1, Saiful Huq2

Department of Medical Physics, Tata Memorial Hospital, Mumbai, Maharashtra, India,1 Department of Nuclear Sciences and Applications, IAEA, Vienna, Austria,2 Department of Radiation Oncology, UPMC Hillman Cancer Center, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania, USA

Aim: To measure field output factors(OF) for range of small field sizes(FS) in water and virtual water using three different detectors for 10MV WFF X-ray beam and test the feasibility of fields under study defined by FWHM for clinical use. Materials and Methods: Five small volume ion chambers were used to measure beam quality in water for 6x6cm2 & 4x4cm2 FS for 10MV WFF photons on Varian TrueBeam Linac using SNC 3D Scanner RFA and in virtual water. On basis of measured beam quality & chamber physical dimensions, two chambers PTW Pinpoint & IBA CC01 were selected for relative dosimetry based on lateral charge particle equilibrium condition(rLCP) upto 4x4cm2 FS. One diode detector passing (rLCP) condition was also selected for relative dosimetry. The profiles & OF were measured for range of FS from 0.5x0.5 cm2 to 10x10cm2. The corrections to OF were applied from TRS 483 based on measured FHWM values for each FS. Field size for which correction factors were not available in TRS 483, daisy chaining method was applied to get corrected OF. The absorbed dose to water in both SSD and SAD setup and Cross calibration against 0.65cc chamber was also performed. All measurements were performed at 10cm depth & normalized for the reference FS10x10cm2, the OF measured with three detectors was compared. In addition, the uncertainty budget was also estimated and reported. All these measurements were also performed, last year with CC13, Pinpoint and EFD detector in water with SSD setup. Results: The Maximum deviation in beam quality was found 0.67% in water at FS 4x4cm2. The Absorbed dose to water measurement among detectors was found within 3%. The ND, W,10MV differs not more than 1.3% from ND, W, Co-60, and maximum deviation between SSD and SAD setup was found 0.7%. In profile measurement, FWHM of 0.5x0.5cm2 had the maximum deviation from geometrical FS. The Corrected O.F for FS 0.5x0.5cm2 for all chambers was found higher in virtual water phantoms than in water. Uncertainty (k=2) in Absorbed dose to water was 2.5% for FC-65G chamber and 3.1% for all other chambers. Uncertainty (k=2) in Corrected OF was highest for EFD detector 4.05% in SSD setup & 3.48% and 3.48% respectively for CC01 and Pinpoint in SAD water setup. Conclusion: IAEA code of practice TRS483 was tested successfully and field output factor were determined. The corrected field OF for three detectors were found to be in close agreement (<3%) for all FS except 0.5x0.5cm2 smallest FS where maximum uncertainty was upto 4%, due to larger contribution from correction factor uncertainty. These results were also compared with last year measurement and found in close agreement upto 3x3cm2. We also found that Palmans equation can be used to measure TPR20,10(10) within 1% uncertainty. In absorbed dose to water measurement, both CC01 and Pinpoint ion chamber over responded compared to FC-65G because of steel central electrode. In profile measurement, EFD was best detector among CC01 and Pinpoint ion chamber except EFD had some higher positional uncertainty compared to other chambers because of small sensitive volume.

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Calculation of rotational patient positional error corrected setup margin in frameless stereotactic radiosurgery and radiotherapy

Biplab Sarkar, Anusheel Munshi, T. Ganesh, A. Manikandan, B. K. Mohanti

Manipal Hospitals, Dwarka, New Delhi, India

Aim: To calculate the rotational positional error corrected setup margin in frameless stereotactic radiotherapy (SRT) and radiosurgery (SRS). Materials and Methods: A total of 79 patients of SRS/SRT each received >1 fraction (3-6 fractions) incorporated in this study. Two cone-beam CT scans were acquired for each session of treatment, before any patient position correction and after the positional correction using a robotic couch. Six dimensional robotic couch was used to correct the patient positional setup error obtained by first CBCT. Cone beam CT (CBCT) obtained patient rotational (rolls, pitch, and yaw) positional errors were reduced to equivalent translational shifts (lateral, longitudinal, and vertical) using a mathematical formulation based on the Wolfram MathematicaV10.0 (Champaign, IL) software platform and described below

The post-positional correction setup margin was calculated using the van Hark formula. Further, a PTV_R (PTV with rotational correction) and PTV_NR (PTV without rotational correction) were calculated by applying the rotation corrected and uncorrected setup margins on the GTVs. Results: A total of 380 sessions of pre (190) and post (190) table positional correction CBCT data was analyzed. The pre-table position correction mean positional error for lateral, longitudinal, and vertical translational and rotational shifts were (x) 0.08±0.09 cm, (y) 0.04±0.21 cm, (z) -0.12±0.2 cm, and (α) 0.15±1.1°, (β) 0.38±0.94 °, (γ) -0.05±1.1° respectively. The post patient positional correction error (in same sequence) was -0.01±0.05 cm, -0.02±0.05 cm, 0.0±0.05 cm and 0.04±0.34°, 0.13±0.44°, 0.02±0.44° respectively. The GTV volumes show a range of 0.13 cc to 39.56 cc, with a mean volume of 6.35±8.65 cc. Rotational correction incorporated post-positional correction setup margin the in lateral (x), longitudinal (y), and vertical (z) direction were 0.05 cm, 0.12 cm, and 0.1 cm, respectively. PTV_R ranges from 0.27 cc-44.7 cc, with a mean volume of 7.7±9.8 cc. PTV_NR ranges from 0.32 cc-46.0 cc, with a mean volume of 8.1±10.1 cc. As a corollary we found rotational corrections become ineffective for a tumour of radius ≥4cm. Conclusion: Historically, the setup margin for stereotaxy was considered as 1 mm, which was empirically deduced from the gold standard invasive, frame-based stereotaxy. Frameless stereotaxy can achieve the same margin if a good imaging protocol and appropriate positional corrections are incorporated. This study is the first to incorporate the rotational patient positional error in the setup margin calculation. Although this investigation is performed for cranial stereotactic cases, the formulation and hence the setup margin calculation is valid for all sites. In subsequent studies, we will report on the rotational-error-incorporated setup margin for other sites using cone beam imaging data.

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In pursuit of enhancing the MAGAT polymer gel dosimeter with nanoparticles for X-ray CT-based readout for quality assurance in radiotherapy

S. Ebenezer Suman Babu, B. S. Timothy Peace, K. Mohamathu Rafic, E. Winfred Michael Raj, J. Sujith Christopher, B. Paul Ravindran

Aims/Objectives: The aim of this study was to investigate the effect of bismuth nanoparticles incorporated in the MAGAT polymer gel dosimeter for enhanced spatial dose information for X-ray CT. Materials and Methods: Polymer gel dosimeters work on the principle of radiation induced polymerization that is reflected in terms of change in the Hounsfield units (HU), also called the CT numbers. MAGAT polymer gel belong to the category of normoxic polymer gels that can be prepared at normal atmospheric conditions. The MAGAT recipe taken as reference in this study consists of 5% gelatin, 6% Methacrylic acid, 10 Mm Tetrakis Hydroxy Phosphonium Chloride and 89% distilled water. Though the X-ray CT can be used as a readout technique for extracting dose information from gel with a dose sensitivity of 0.6177 HGy-1 (reported in the literature), this study focusses on the investigation of the effect of Bismuth nanoparticles on the dose response of MAGAT gel for X-ray CT and cone beam computed tomography (CBCT). Various concentrations of Bismuth nanoparticles were incorporated in the reference MAGAT gel. The gel solutions were poured into cuvettes and acrylic cylinders of 2 cm inner diameter and 16 cm long. All the gel dosimeters were irradiated using a 6 MV photon beam from a linear accelerator (Truebeam, Varian) to doses of 0, 1, 3, 5, 7, 10 and 15 Gy in a water tank of dimensions 30 × 30 × 30 cm3. A parallel-opposed field of 30 × 30 cm2 at the isocentre and at a source to axis distance (SAD) of 100 cm was used for the irradiation. The dose response of the nanoparticle-based MAGAT gel was evaluated using spectrophotometer and compared with the reference MAGAT gel. Similarly, the HU values of the gels in cylindrical containers were evaluated with diagnostic X-ray CT (Biograph, Siemens) and CBCT attached to a linear accelerator (Truebeam STX, Varian). Three cuvettes from each (Reference, nanoparticle MAGAT) were kept unirradiated to account for the background measurements. Results: Visible changes as well as spectrophotometric measurements were found suggesting dose enhancement in the MAGAT gel dosimeter. The regression analysis of the dose response data showed linearity with R2 value of 0.993 and 0.990 for 0.5 and 0.3 mM Bismuth nanoparticles respectively. HU values obtained from the images of the gel cylinders were found to be 0.931 HGy-1 for the 0.5 mM Bismuth nanoparticles incorporated MAGAT gel when compared to the value of 0.684 HGy-1 reported for the MAGAT without nanoparticles. This is a 36% increase in the HU values compared to the reference MAGAT gel. Conclusions: Dose enhancement effect of the bismuth nanoparticles on the MAGAT gel dosimeter has been investigated successfully. It is concluded that nanoparticle incorporated MAGAT can be used to obtain an enhanced spatial dose information from the gel dosimeter when X-ray CT is used as a dose extraction tool.

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Helical tomotherapy v/s step n shoot IMRT in concurrent chemoradiation for locally advanced carcinoma cervix: An analysis of dose distribution

Tasneem Lilamwala, Alluri K. Raju, Vinoth Kumar

Aim: Both IMRT & helical tomotherapy[HT] have been adopted for RT for whole pelvic RT in carcinoma cervix due to better dose homogeneity, conformality and OAR sparing. The purpose of our study is to compare dose distribution and acute toxicities for both conformal techniques-IMRT and helical tomotherapy. Materials and Methods: This is a retrospective analysis from October 2017 to October 2018.14 patients with pathologically provenlocally advanced carcinoma cervix were included in this study-eight patients were in the IMRT group and 6 patients in tomotherapy. Clinical FIGO stage IIB –IVA were only included. The treatment plan was concurrent chemoradiation which includes 50 GY of whole pelvic RT in either 25/28 fractions along with 3 doses of intracavitary radiation [total dose 21Gy,7 Gy per fraction ]and weekly low dose cisplatin based chemotherapy.IMRT and HT plans were evaluated for each patient with the same planning objective. OAR doses were compared using Dose Volume Histograms and plan reports. Results: The bowel parameters including Dmean, V10, V30 & V40 were reduced in the tomotherapy plans when compared to IMRT plans. Dmean for tomotherapy reduced by 5.9Gy(Dmean for HT[21.77Gy: Dmean for IMRT [27.67Gy].V10HT[85.17%] was lower than V10IMRT[81.67%].V30 for tomotherapy was reduced by 19.6Gy –[V30 HT-23.73; V30IMRT-43.33%] &V40HT[6.75] was also lower than V40IMRT[21.89%]. Dmean for rectum was also reduced in HT plans from 31.58Gy to 40.14Gy in IMRT plans.however, mean HI was better using IMRT than tomo therapy. The monitor units[ MU] used for mean IMRT plans were significantly lower[1020.24MUs] compared to HT [4556.5MUs]. The HT plans consistently demonstrated that the dose received by the bowel and rectum were lower than the IMRT. However the bladder and femur head doses on the HT and IMRT plans were almost similar. Conclusion: This study demonstrated that HT is definitely a better modality for bowel sparing with V30 reduced by 19.6Gy, but at the cost of increased MUs which is 4 times more with tomotherapy. On rectum the mean dose was reduced around 8.5 Gy.

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Design and study of multi rotational human equivalent phantom for VMAT dosimetry using ion chamber, EBT3 film and RPL glass dosimeter

S. Senthilkumar, G. Kesavan1

Department of Radiotherapy, Madurai Medical College and Government Rajaji Hospital,1 Department of Radiotherapy, Vadamalayan Integrated Cancer Center, Madurai, Tamil Nadu, India

E-mail: drsenthilgh@gmail.com

Objectives: The main objective of the present study was to fabricate indigenously multi purpose multi rotational human equivalent phantom. Secondary objective of the study was to use the developed phantom to implement for the clinical application of pretreatment patient specific quality assurance for Volumetric Modulated Arc Therapy (VMAT). And also to analyze the dosimetric verification using ion chamber, EBT3 film and RPL glass dosimeter. Materials and Methods: Multi rotational human equivalent phantom was made up of PMMA, which has the density of 1.18g/cm3. The phantom consists of 35 slices and each slice has a dimension of 1cm thickness, 35cm length, and 20cm height. The multi rotation phantom has been design for thoracic part with multiple provisions in both the Lungs (L1, L2, L3, L4), Heart (H1, H2), chest wall(C1, C2), spine and target region. The unique manual rotation system provided on both the lungs as well in the heart region. The rotation almost covers the whole lung and heart, which allows to measuring point dose on various positions. The rotation was accurately moved manually with the help of labeling on the surface of the phantom. The phantom has the provision for Semi-flexible chamber, EBT3 film and RPL glass detector for point dose measurement. The phantom has been used to measure the point dose in the patient specific quality assurance of VMAT using three different detector. In this study, measurements were performed using rotational phantom with Semi-flexible chamber, EBT3 film and RPL glass detector. In order to execute the patient specific QA the phantom was scanned by a 16 slice CT scanner. VMAT patient verification plans were generated by the Varian Eclipse treatment planning system (TPS) for different types of thoracic region cancers. The measurement was carried out on Varian Clinac iX for all the 3 detectors and the point dose was measured for all the three detector and compared with each with TPS calculated dose. Results: Our result demonstrates that a strong correlation between the calculated dose in TPS and the dose measured in the phantom using semi-flexible chamber, EBT3 and RPL. The percentage variation between dose calculated in TPS and dose measured in the phantom is less than ±2% for most of the points. The study confirmed that the observed deviations were well within the limits of international standards and ensured the accuracy and quality of the treatments delivered at the authors'oncology Centre. Conclusion: The fabricated PMMA phantom was used in the pretreatment patient-specific QA of VMAT was validated and accepted for the dosimetric purpose, since all the measurements carried out with it passed the deviations were well within the acceptable limits. This study conclude that the thorax multi rotational phantom can be used in patient specific QA measurements for VMAT. The results revealed that all the three detector are suitable for the patient specific QA of RapidArc treatment pretreatment patient-specific QA.

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Do we really need the flattening filter free beam, higher energy and dose rate: Dosimetric comparison of FF and FFF beams for Lung Stereotactic Body Radiation Therapy

Priyanka Agarwal, Rajesh A. Kinhikar1, Rakhi Barman1, Sangeeta Hazarika1, Naveen Mummudi2, Anil Tibdewal2, J. P. Agarwal2

Homi Bhabha Cancer Hospital, Varanasi, Uttar Pradesh, Departments of2 Radiation Oncology and1 Medical Physics, Tata Memorial Center, Mumbai, Maharashtra, India

Introduction: Stereotactic Body Radiation Therapy (SBRT) has become a preferred choice to treat patients with Lung Cancer as seen in its clinical advantages over other modalities. SBRT is conventionally planned using 6 MV photons generated by Linac with flattening filter. With advent of Linac capable of delivering FFF beams, SBRT is being planned using FFF beams. In this study, we study we compared the dosimetric differences of energy in Lung SBRT. Materials and Methods: Eleven patients treated with SBRT using 6 MV FF photons (5 left lung and 6 right lung) having volume planning target volume (PTV) from 63.3cc to 240cc were retrospectively selected for this study. The prescribed dose was 60 Gy in 8 Fractions & planned with two partial arcs on Eclipse treatment planning system (version 13.5) & calculated using Acuros Algorithm. The clinical acceptance of plan was set using RTOG guidelines 0813 & 0913. The plan with 6 MVFF energy were labeled as Plan A, the treatment plans for 6XFFF and 10XFFF energies were generated keeping same optimization constraints as that of Plan A and labeled as Plan B & plan C. The highest dose rates were used for plans with FFF energies [1400 MU/Min for 6X_FFF & 2400 Mu/Min for 10X_FFF]. The three plans were analyzed qualitatively and quantitatively for PTV and organ at risk (OAR) doses. Results: The mean CI (coverage Index) for Plan A, Plan B were 96% ±0.008 and for PlanC were 94%±0.012. The mean COIN (conformity Index) for Plan A, PlanB and PlanC were 0.956±0.036, 0.957±0.037 and 0.936±0.043. The average treatment time (TT) for Plan A, PlanB and PlanC were 3.7±0.41, 1.55±0.21 and 1.13±0.13 minutes. The high Gradient Index (GI) for PlanA, PlanB and PlanC were 2.91±0.26, 2.88±0.27 and 2.87±0.26 respectively. For Lung-PTV, V5 and V20, PlanB and PlanC were reduced 0.93, 1.01 and 0.86 and 0.9 times than PlanA resepectively. The mean Lung-PTV doses were also less 0.86 and 0.9 times in PlanB and PlanC than PlanA. The mean heart doses were comparable among three plans, V5 for heart for PlanB were 0.946 times less, but 1.036 times more for PlanC than PlanA. For Spine (V0.5cc), PlanB and PlanC were 1.011 and 1.057 times more than PlanA. For Esophagus (V5cc), PlanB and PlanC were 1.031 and 1.107 times more than PlanA. The mean intergral dose of body for PlanB and PlanC were 0.98 and 0.99 times than PlanA respectively. Conclusion: As per study, FFF beams were beneficial to reduce treatment time. The optimal plan can be obtained using energy 6XFFF and mean delivered dose rate of 1000-1200 Mu/min with no compromise with coverage index, conformity index, integral dose of body and OAR doses. The long terms clinical outcomes are required for studies.

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Critical Appraisal of Robustly Optimized Intensity Modulated Proton Therapy(IMPT) over photon based Volumetric Arc Therapy (VMAT) and Helical Tomotherapy (HT) in AAPMTG244

C. H. Kartikeswar Patro, S. Dayananda, A. Manikandan, M. P. Noufal, K. Ganapathy, T. Rajesh, C. Srinivas, R. Jalali

Aim and Objective: Proton treatment planning approach and techniques differ largely from photon due to its integrated depth dose characteristics, delivery techniques and susceptibility to uncertainties. The aim of this study is to design site specific robust Intensity Modulated Proton Therapy (IMPT) plan and compare the dosimetric outcome to photon based Volumetric Modulated Arc Therapy (VMAT) and Helical TomoTherapy (HT) plans. Materials and Methods: The CT and contoured structure datasets of four clinical sites comprising of anal canal, head and neck, lung and abdomen were selected from the database of AAPM TG244. These sites represent some of the most complicated cases in any busy radiotherapy center. All the proton (IMPT) and photon (VMAT and HT) plans were created in RayStation (Version7) Treatment Planning System (TPS), which is commissioned for ProteusPlus proton therapy system, TruebeamLA and RadiXactX9Tomotherapy. For every clinical site except lung, four simultaneously integrated boost (SIB) treatment plans were created. The primary goal of each treatment plan was to cover at least 99% of CTV (D99%CTV) & 95% of PTV (D95%PTV) by 100% of the prescribe dose while limiting the doses to OARs within the tolerance limit recommended in AAPM-TG244. Proton plans were created for ProteusPlus using pencil beam scanning technique with energy modulation ranging from 228 to 70 MeV without range shifter. For each case, two IMPT plans were created, one optimized directly on PTVs and second optimized to PTVs with robustness applied to CTVs. Robustness was accounted for ±3mm setup and ±3.5% of range uncertainty. VMAT plans were created using two full arcs whereas HT plans were generated using 2.5 cm field width, 0.3 pitch and 2-2.5 Modulation factor. Dosimetric outcome from all the plans were compare using standard dose volume indices considering VMAT as reference. Moreover, normal tissue (any tissue outside PTV) volume receiving high (≥70%), intermediate (<70% and ≥40%) and low (<40% and ≥10%) dose were evaluated for each clinical sites from each plan. Results: In all four clinical sites, competing plans resulted comparable D99%CTV and D95%PTV (both for low and high risk target) within a Standard deviation of 0.65 CoEGy. Although dose to all OARs were within the prescribed tolerance limit of AAPM-TG244, OAR sparing was much better with proton compare to photon. In case of anal canal, both proton plans reduces V35Gy to bladder and V30Gy to rectum by 89% and 72% respectively. The corresponding decrease from HT plan was 67% and 58%. V45Gy to bowel was within 3cc in Proton and 7cc in photon plans. The mean dose to genitalia was <1Gy from both proton plans while it was 14.4Gy and 7.7Gy from VMAT and HT. The mean of V30Gy to femoral heads were 0%, 5%, and 20% from both proton plans, VMAT and HT. For abdominal, V18Gy to kidneys were 0% from proton plans which increases to 6% in HT and 14% in VMAT. The mean dose to liver and D25% to stomach were <3Gy and <0.6Gy from proton plans, whereas VMAT and HT plan shows similar results with <6Gy and <12 Gy. The maximum dose to spinal cord were slightly higher in proton plans with 34.4, 38.7 Gy compare to 34.3, 32.2 Gy from VMAT and HT. In head and neck case, D0.03cc to brainstem and spinal cord were around 32Gy and 39Gy from proton plans, 42.7Gy from VMAT and 35.5 and 32.5Gy from HT. V30Gy to lips were least at 1.41% in IMPT without robustness which increases to 15.8% with robust optimization. The corresponding values from VMAT and HT were 66.3% and 87.1%. The mean dose to right parotid were around 17Gy from IMPT and 25Gy from photon. The V70Gy to mandible and mean dose to larynx were similar in all plans. For Lung case, V10Gy to contralateral lung were zero from IMPT compare to 16% in VMAT and HT. In comparison to VMAT plan, IMPT plans reduces V20Gy,V10Gy, and mean dose to total normal lung to around 36%, 43% and 42% respectively. HT plan marginally increase all these parameters with a maximum of 3%. Both IMPT plans resulted V50Gy to heart of 1% and mean dose to esophagus of 17Gy. The corresponding results from VMAT were 5.6%, 20.2Gy and HT were 3.2% and 19Gy respectively. In comparison to VMAT plan, IMPT reduces high, intermediate and low dose volumes to normal tissue by almost 50%, 12%, 60% for anal canal, 15%, 50%, 60% for abdomen,3%, 26%, 59% for head and neck, 17%, 52%, 68% for lung respectively. Conclusion: IMPT plans with or without robust optimization shows a clear dosimetric advantage over VMAT and HT plans in terms of reduction of dose to OAR and normal tissue doses exposed to various dose level. Although IMPT plans with or without robust optimization shows similar dosimetric results, robustly optimized plan must be consider for clinical implementation in cases where the uncertainty are expected to be high.

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IGRT android app – A daily alert tool for physician

Kanhu Charan Patro, Partha Sarathi Bhattacharyya, Chitta Ranjan Kundu, Venkata Krishna Reddy, Madhuri Palla, A. Priyanka Chandra, A. C. Prabu1, Subhra Das1, A. Srinu1, A. Anil Kumar1

Departments of Radiation Oncology and1 Radiation Physics, Mahatma Gandhi Cancer Hospital and Research Institute, Visakhapatnam, Andhra Pradesh, India

Introduction: Image guidance in radiation oncology practices is an essential tool to verify the treatment procedures in day to day practices. In most of the radiotherapy facilities radiotherapy technologists do the job of image verification as consultants hardly could make it because of heavy patient load. In a busy schedule consultant find it very difficult to verify the images daily online before treatment. Hence some follow offline practices and some take the assistance from residents or else otherwise they have to believe in radiotherapy technicians during entire treatment period. The dependence on radiotherapy technologists mandates thatthey should be adequately trained for both on site and off site image verificationso that they can handle the situation in a more efficient manner. Aim and Objective: To make the things easy and handy, we developed an android based app which can be downloaded free from Google play store and which can be set to keep an watch on what is going on in treatment room as regards to image guidance. The technologists can also communicate difficulties in image verification through this app. Materials and Methods: In this app any number of doctors can sign up and their technologists can update about IGRT errors to the consultants for further action if needed. The technologists can add patient details at new entry and deactivate the same after completion of treatment. During image matching, for each patient, technologist has to follow three traffic light signs: a Green button signal that everything is OK that technically means GTV is inside GTV; CTV is inside CTV and can go ahead with treatment. Orange button is meant for guarded treatment, i.e. GTV inside at least in CTV and CTV inside PTV and to put a cautionary note for doctor and can go ahead with scheduled treatment. Red means there is gross error can not be matched and it needs physician intervention before treatment. If there is any OAR discrepancy they have to click on traffic person. For security access each doctor and technologist will have different password and there is no chance of trespassing the directory of other doctors. At completion of the treatment the consultant can download the IGRT matching data in excel sheet for each patient. Conclusion: The aim of this app is to decrease the daily errors during treatment to act as an watch dog tool for the consultants. In second phase we are planning to have an app for auto calculation of systematic and random errors with individualized auto PTV calculation, individual wise, institution wise and site wise.

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Stereotactic radiosurgery on helical tomotherapy - An institutional experience

Vijay Palwe, Prakash Pandit, Roshan Patil, Raj Nagarkar, Meghraj Borade, Supriya Borade

Aims and Objectives: To determine the feasibility and safety of Stereotactic radiosurgery on Helical Tomotherapy. Materials and Methods: Total 8 eligible patients were identified prospectively from August 2018 till August 2019. 5 patient were solitary brain mets, one patient with dorsal spinal metastasis, one patient with acoustic schwannoma and one with recurrent meningioma. Helical TomoTherapy is designed to perform intensity-modulated radiation therapy (IMRT) built on the ring-based gantry has tight machine tolerances required for radiosurgery. A frameless system of thermoplastic mask were used for patient immobilization. CT image dataset is taken at 1.25 mm slice thickness MRI scan in the treatment position. Both the CT and MRI image datasets fused in Eclipse Planning System for target volume, OAR volumes, and pseudo-structures contouring. The image dataset and the structure set exported to TomoHD TPS. The acceptability of the treatment plans evaluated to ensure that plan meets the target conformity index and normal tissue dose-volume constraints. After plan is accepted, a delivery quality assurance (DQA) plan is created. The image-guidance system on the Helical TomoTherapy was used for patient localization. acquisition of the MVCT image dataset at the fine level (2 mm slice) and co-registered against the CT-simulation image dataset. Co-registration is done primarily on the bony structures of the skull with emphasis near the treatment region by the radiation therapists and with the approval of the radiation oncologist. After the approval, the treatment commences. After the first treatment session is completed, another MVCT image dataset is immediately collected and co-registration is performed to evaluate and confirm the patient position. Results: All the 8 patients tolerated SRS on Tomotherapy well without any side effects. Post SRS response assessment scans showed significant reductions in size and enhancement in lesions. The treatment time about 10 minutes is comparable to that for IMRT patients. The same patient specific quality assurance for IMRT is used for radiosurgery. As demonstrated, SRS using Helical Tomotherapy is not a whole day event unlike SRS using other dose deliver system or performed in the past. Patient is discharged promptly after the CT-simulation and thin slice MRI scan and called on the treatment day after planning is done. Standard patient specific QA for the IMRT patients are used for the SRS patients. Overall, the workflow for radiosurgery follows a similar procedure to that of IMRT patients. Unlike SRS using other dose delivery systems, or performed in the past, SRS treatment using Helical Tomotherapy is not a whole day event but just like a typical IMRT procedure. Conclusion: Our clinical experience indicates that the implementation of radiosurgery on the Helical Tomotherapy unit can be streamlined. A thin-cut MRI scan is performed, and this dataset is fused with the CT simulation dataset. Otherwise, the clinical setup for radiosurgery is similar to that of IMRT patients. This radiosurgery procedure is safe. The treatment time is short about 10-15 minutes comparable to the treatment times for IMRT patients which is more convenient for patients.


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