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ORIGINAL ARTICLE
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Multi-institutional dose audit in radiotherapy facilities using in-house developed optically stimulated luminescence disc dosimeters


1 Department of Physics, National Institute of Technology, Raipur, Chhattisgarh; Directorate of Regulatory Inspection, Atomic Energy Regulatory Board, Mumbai, India
2 Radiological Physics and Advisory Division, Bhabha Atomic Research Centre, Mumbai; Homi Bhabha National Institute, Mumbai, India
3 Department of Radiation Oncology, Apollo Hospitals, Navi Mumbai, Maharashtra, India
4 Department of Radiotherapy, Pt. J.N.M.Medical College and Hospital, Raipur, Chhattisgarh, India
5 Directorate of Regulatory Inspection, Atomic Energy Regulatory Board, Mumbai, India
6 Department of Physics, National Institute of Technology, Raipur, Chhattisgarh, India

Date of Submission08-May-2021
Date of Acceptance02-Jul-2021
Date of Web Publication15-Feb-2022

Correspondence Address:
Sadhana Agrawal,
Department of Physics, National Institute of Technology, G.E. Road, Raipur - 492 010, Chhattisgarh
India
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jcrt.jcrt_753_21

 > Abstract 


Aim: The aim of this study was to carried out the audit of radiotherapy centers practicing conformal radiotherapy techniques and demonstrate the suitability of this indigenous optically stimulated luminescence (OSL) disc dosimeters in beam quality audit and verification of patient-specific dosimetry in conventional and conformal treatments in radiotherapy.
Materials and Methods: Dose audit in conventional and conformal (intensity-modulated radiotherapy and volumetric-modulated arc therapy) radiotherapy techniques was conducted using in-house developed Al2O3:C-based OSL disc dosimeter and commercially available Gafchromic EBT3 film in 6 MV (flat and unflat) photon and 6 and 15 MeV electron beams. OSL disc dosimeter and Gafchromic EBT3 film measured dose values were verified using the ionization chamber measurements.
Results: Percentage variations of doses measured by OSL disc dosimeters and EBT3 Gafchromic film for conventional radiotherapy technique were in the range of 0.15%–4.6% and 0.40%–5.45%, respectively, with respect to the treatment planning system calculated dose values. For conformal radiotherapy techniques, the percentage variations of OSL disc and EBT3 film measured doses were in the range of 0.1%–4.9% and 0.3%–5.0%, respectively.
Conclusion: The results of this study supported by statistical evidence provided the confidence that indigenously developed Al2O3:C-based OSL disc dosimeters are suitable for dose audit in conventional and advanced radiotherapy techniques.

Keywords: Al2O3:C, dose audit, dosimetry, optically stimulated luminescence, radiotherapy



How to cite this URL:
Kumar P, Sharma SD, Dhabekar B, Rawat NS, Mishra DR, Chaudhari S, Chandola RM, Routh T, Agrawal S. Multi-institutional dose audit in radiotherapy facilities using in-house developed optically stimulated luminescence disc dosimeters. J Can Res Ther [Epub ahead of print] [cited 2022 Dec 8]. Available from: https://www.cancerjournal.net/preprintarticle.asp?id=337716




 > Introduction Top


India is a low- and middle-income country (LMIC) and accounts for 17.7% of the world's population, with 7.8% of the global cancer burden.[1],[2],[3] Cancer ranks as a leading cause of death and reduces the increasing life expectancy in every country of the world.[3] Radiotherapy is one of the treatment modalities which is widely used for the treatment of cancer. Nowadays, majority of radiotherapy treatment is administered using medical electron linear accelerators (LINACs). These LINACs are equipped to carryout modern radiotherapy treatment such as intensity-modulated radiotherapy (IMRT), image-guided radiotherapy (IGRT), and volumetric-modulated arc therapy (VMAT). These treatment techniques allow to deliver radiation dose conforming to complex-shaped target volumes and at the same time efficiently spare the surrounding normal/healthy structure, which is a major advantage over the conventional treatment techniques. Because of this advantage, majority of curative cases are treated by advance radiotherapy techniques. However, the dose delivery through advance treatment techniques is complex in nature, and the major problems associated with these techniques are the possibility of dosimetric error due to failure in delivery of planned fluence and geographical miss due to setup errors and mechanical failures. This necessitates the institution of stringent quality control throughout the treatment tenure.

In India, there are 519 radiotherapy centers where 559 medical LINACs (majority of them are advance medical electron accelerators including Cyber Knife, Tomotherapy, and recent versions of C-arm LINACs) are operational for the treatment of varieties of cancer cases.[4] Radiotherapy facilities in India are growing at a rapid rate, and approximately 15 LINACs are added per annum. Every radiotherapy center is having their own quality assurance (QA) program, and accordingly, they perform both machine-specific and patient-specific QA. During regulatory inspection of radiotherapy centers practicing IMRT and other advance techniques, it was observed that each institution uses its own specific equipment and method for imaging, contouring, planning, delivery, and QA. Considering wide variability in QA and pretreatment dose verification methods employed in IMRT, IGRT, and VMAT, it is highly essential to carry out an in-depth study on QA parameters because preliminary study carried out in the country had revealed some significant findings.[5],[6],[7] The similar studies have also been conducted in developed countries including USA,[8],[9] Korea,[10] and Australia.[11],[12] It has been reported in the literature that local IMRT dose verification results do not necessarily match with independent dosimetry audit data.[13],[14]

International Atomic Energy Agency (IAEA), Vienna is conducting postal dose audit in radiotherapy centers from the last five decades and also supporting its member states in developing the country-specific infrastructure for conducting audit of their centers.[15],[16],[17],[18] Dosimetry audits have improved the accuracy of treatment delivery in many radiotherapy centers over time. In light of this observation, IAEA recommended that all the beams used to treat cancer patients should be verified by an independent national, regional, or international auditing organizations.[16] Accordingly, dosimetry audit should be a periodic exercise, conducted by an external agency, which is expected to standardize and harmonize the dosimetry practices.

In India, Radiation Standard Section, Radiation Safety Systems Division, Bhabha Atomic Research Center (BARC) is involved in developing national infrastructure and conducting postal dose audit in radiotherapy centers in the country.[19],[20],[21] This audit program has its own limitations; unable to audit all centers, prioritising new centers, all beam energies, verification of modern treatment techniques, etc. Further, they are using age old LiF-based thermoluminescent dosimeters (TLDs) for dosimetry audit. In addition, the dosimetry audit is limited to the central axis of the beam which requires specific measurement set up for irradiating the dosimeters. Therefore, there is a need to establish an indigenous dose audit program, which could provide users to participate in the audit in their own set up arrangement, flexibility to participate, easily available, cater the load of the country, and may be used worldwide for the dosimetric applications.

This multi-institutional dose audit was carried out using disc dosimeters made from indigenously developed Al2O3: C optically stimulated luminescence (OSL) phosphor. This phosphor was developed by BARC using melt processing technique in reducing the atmosphere of graphite ambience and this technique was registered as USA patent.[22],[23] OSL dosimetry is gaining popularity over thermoluminescence (TL) dosimetry due to its small size, availability in desired shape, wide measurement dose range, capability to record low dose, and free from cables and applied voltage. OSL dosimeters behave in the similar fashion as TL dosimeters, except that the stimulation process is optical rather than thermal. Although OSL-based dosimeters are having a number of merits over other dosimeters, their sensitivity to room light is thought to be a significant drawback. However, the problem of room light sensitivity could be overcome by packing them in a light-tight packing. Al2O3:C OSL dosimeters are commercially available and it has already been used in radiotherapy dosimetry and multi-institutional beam quality audit in radiotherapy centers in Australia.[11],[24] The aim of this study was to carryout audit of radiotherapy centers practicing conformal radiotherapy techniques and demonstrate the suitability of this indigenous OSL disc dosimeters in beam quality audit and verification of patient-specific dosimetry in conventional and conformal treatments in radiotherapy. As commercially available OSL dosimeter is costly, hence locally made OSL disc dosimeters were used for multi-institutional dosimetry audit. Measurements using Gafchromic EBT3 film were also conducted to verify the data acquired using the locally made OSL disc dosimeters. In some cases, ionization chambers, which are used routinely by the institutions, were also used for acquiring the dosimetric data for comparing with the values obtained by OSL disc dosimeters.


 > Materials And Methods Top


Optically stimulated luminescence disc preparation and readout

The OSL disc dosimeters used in this study were prepared using BARC developed Al2O3:C (Zeff. = 11.28) phosphor.[23] This OSL phosphor is highly sensitive which can record doses as low as a micro-Gray (μGy). Ten minutes after irradiation, 5% fading was observed, and afterward, fading was negligible. To prepare the disc, OSL phosphor powder was uniformly mixed with polytetrafluoroethylene in 1:3 ratio by weight. Afterward, liquid nitrogen is poured into this mixture and stirred continuously to make a free-flowing mixture. The mixture is then homogenized in a ball mill by continuously rotating it for about 30 min. To fabricate dosimetric grade discs, mixture is weighed on the electronic weighing scale and carefully transferred to 20 mm diameter stainless steel die punch set. The die punch set is then applied with 3 tons of pressure by using automatic hydraulic press to make 0.4 mm thick discs. To make the discs rugged and nonbrittle, discs were heated for around 1 h at 380°C temperature. Heating provides surface uniformity, consistency, and reproducibility of discs. Finally, 20 mm discs were cut into the circular discs of 5 mm diameter and 0.4 mm thickness.

To carry out this dosimetric study, around 250 discs were separated after visual and radiometric analysis of entire disc fabricated in a single batch. Prior to its use, discs surface uniformity, consistency, and reproducibility were verified. Those discs, which were having standard deviation <2% were segregated for this work. To carry out the measurement, three discs were packed together in a light tight pouch.

TL/OSL-DA-20 reader system, Riso, Denmark was used for OSL readout of the irradiated disc dosimeters. This reader has fully automated readout process and can read up to 48 discs at a time. OSL readout of the irradiated OSL dosimeters was recorded for 10 s. To estimate the OSL count, the signal was integrated for the first 1 s. For every measurements, background value was accessed and found negligible (<50 counts in comparison to lakhs); therefore, it is ignored.

A comprehensive experimental study was carried out for dosimetric characterization of the OSL disc dosimeters before their use in multi-institutional dose audit.[25],[26],[27] The suitability of OSL discs was also verified in patient-specific dosimetry.[26]

RT institution and treatment units

Dosimetry accuracy depends on a number of factors including machine-related parameters and human factors.[28] Considering this, the spectrum of radiotherapy institutions was short listed for the study to cover various working environment both from technology and capability of LINACs and skill and experience of the medical physicists. In addition, institution with single treatment unit, multiple treatment units, medical physicist of working experience <10 years and more than 10 years, and the institution already participated in IAEA/WHO postal dose audit were included in this study. Accordingly, dosimetry audit was conducted on 12 medical LINACs operational at eight radiotherapy centers. Out of 12 LINACs, 11 were standard accelerators (8 from Varian, 3 from Elekta) and one by specialized accelerator (Hi-Art Tomotherapy unit from Accuray), as shown in [Table 1]. LINACs selected in this study were representing almost all the existing models of operational LINACs in the country.
Table 1: Details of medical electron accelerators used in the dosimetry audit

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Calibration of dosimeters

Prior to their use in determining the delivered dose, the dose response of OSL disc dosimeters and Gafchromic EBT3 film was measured on the same medical electron linear accelerator on which the dosimetry audit was planned to be conducted. Dose response calibration was performed by irradiating the dosimeters and Gafchromic EBT3 film for 200 cGy in source to axis distance set up using slab phantoms for both flat and unflat photon beams. Dose response of these dosimeters was also calibrated in 6 and 15 MeV electron beams.

The readout of irradiated Gafchromic EBT3 film was conducted using Epson Expression 10000XL flatbed scanner in professional mode, and the digital images of the film pieces were analyzed using ImageJ 1.49r software.

Dose audit for conventional treatment

A conventional treatment plan using a pair of parallel opposed beam (anterior-posterior and posterior-anterior) was generated by the treatment planning systems (TPSs) of the institution (Varian eclipse, Elekta Mosaic and Tomo Hi-Art) to deliver 200 cGy from 6MV and 6 MVFFF photon beam energies. Dose to the point of interest was measured using OSL disc dosimeters, Gafchromic EBT3 film, and cylindrical ionization chamber, and the dose values measured from each of these dosimeters were calculated and compared.

Dose audit for conformal treatment

Point dose measurements in IMRT and VMAT were conducted using OSL disc dosimeters and Gafchromic EBT3 film. Ionization chamber-based measurements were also carried out in some cases to obtain additional confidence in measured values. For this IMRT and VMAT plans were generated using the TPS of the institution. Usually, the pretreatment dose verification in such cases is conducted by the institutions using their own methods. We have done the dosimetric measurement using our dosimeters using the dosimetry setups used by the institutions (Slab phantom, I-matrix, Arc-check, PTW 2D array, IBA IMRT phantom and Tomo cheez phantom). Dosimetry audit measurements were conducted for one of the routinely practiced treatment techniques (IMRT or VMAT), and the dosimeters and Gafchromic EBT3 film were irradiated using 6 MV flat photon beam as per the treatment plan generated by the treatment planning system of the hospital. Dose audit measurement was also conducted for an IMRT plan which was generated to be delivered by 6MV FFF beam.

Dose audit for electron beams

Doses from electron beams (6 and 15 MeV) were measured at 1.9 and 5.3 cm depths in a slab phantom using 10 × 10 cm2 applicator. The true beam medical electron accelerator was used in this work. Prior to dose audit measurement, the OSL disc dosimeters and Gafchromic EBT3 film were irradiated for 200 cGy at the reference depths of these electron beams for obtaining a calibration factor for the dosimeter.

Uncertainty analysis

IAEA-TECDOC-1585 was referred for uncertainty analysis in this study.[29] The uncertainty contributing factors were identified, and combined standard uncertainty was determined. Details of uncertainty analysis have already been published elsewhere.[25],[26]


 > Results Top


Dose audit for conventional treatment

[Table 2] presents the percentage variations of measured doses by OSL disc dosimeters and Gafchromic EBT3 film from different units. The percentage variation was calculated taking the TPS planned dose value as reference value. The percentage variation of ionization chamber measured doses with respect to TPS planned dose have been given in the parentheses. These variations are shown for a planned dose value of 200 cGy. It is observed from [Table 2] that the percentage variation of OSL disc dosimeter measured doses is in the range of 0.15%–4.6%. Similarly, the percentage variation of Gafchromic EBT3 film measured doses is in the range of 0.40%–5.45%, and the percentage variation of ionization chamber measured doses is in the range of 0.30%–3.00%. In case of dosimetry audit conducted by TLD, up to 5% variations between planned dose and measured dose are thought to be acceptable.[15],[30] Accordingly, our study indicates that dose delivered to the tumor are within the limit of acceptability. In case of Gafchromic EBT3 film measured doses, the maximum variation is 5.45% which slightly higher than the expected variation of 5%. The reason for this variation could be use of hospital-specific measurement set ups in this work. Further, variation of 5.45% was observed for one of the measurements conducted at 6MV FFF beam (Tomotherapy machine) and it could due to many factors including statistical variation of the data. Considering the uncertainty in measurements, variation observed with Gafchromic EBT3 film is also thought to be acceptable.
Table 2: Dose measured in conventional treatment technique (anterior-posterior/posterior-anterior) from 6MV and 6 MV FFF photon beams(6FFF: Means dose data are from 6 MV FFF photon beam)

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Dose audit for conformal treatment

Percentage variations of planned and measured doses for IMRT and VMAT of different cancer cases are shown in [Table 3]. The percentage variation of ionization chamber measured doses with respect to TPS planned dose has been given in the parentheses. The data are collected from eight different institutions for 6 MV flat photon beam. For completeness of the study, measurements were also conducted using 6MV unflat photon beam (6FFF), and the percentage variations of planned and measured doses are included in this table. As has been mentioned earlier that dosimetry measurements using OSL disc dosimeters and Gafchromic EBT3 film were carried out using the same dosimetry set up which is routinely used by the radiotherapy institution, it was not possible to conduct OSL/film-based measurements in some situation where Arc-check and 2D array were used. Accordingly, OSL discs dosimeters and Gafchromic EBT3 film measured dose data are not available for such situations. It is observed from [Table 3] that the percentage variation of OSL disc dosimeter measured doses is in the range of 0.1%–4.9%. Similarly, the percentage variation of Gafchromic EBT3 film measured doses is in the range of 0.3%–5.0%. This indicates that dose values measured by OSL disc dosimeters were within the acceptable limit.
Table 3: Dose audit in intensity-modulated radiotherapy and volumetric-modulated arc therapy techniques

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Dose audit for electron beams

[Table 4] presents the percentage variation of doses measured for 6 and 15 MeV electron beams with respect to planned dose of 200 cGy. It is observed that the percentage variation of OSL disc dosimeter measured dose is 2.65% and 4.9% for Gafchromic EBT3 film in 6 MeV and 1.0% for OSL disc dosimeter and 1.9% for Gafchromic EBT3 film in 15 MeV.
Table 4: Dose audit data of 6 and 15 MeV electron beams

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Statistical analysis

A statistical t-test of the difference in average measurement deviations observed in the two datasets of conventional therapy measured data as mentioned in [Table 2] is carried out with the null hypothesis as “Deviations in measurement by OSL dosimeter and Gafchromic film are same.”



The analysis indicates that the calculated t-value is 0.18 as against the tabulated t-value of 2.056[31] at 95% confidence for 26 degrees of freedom, and therefore, do not offer sufficient statistical evidence to conclude against the null hypothesis.

Similarly, the analysis carried out on conformal therapy measured data tabulated in [Table 3], indicates that the calculated t-value is 0.01 as against the tabulated t-value of 2.021.

The mean deviations observed in the two datasets for conventional and conformal therapies are not significantly different at 95% level of significance.

Since we are more concerned with the precise measurement of dose, from the range and average, it is seen that percentage deviation values are lower in OSL dosimeter measurements as compared to that of Gafchromic film. Further, all the deviations observed with OSL dosimeter are within the two sigma deviations (95% confidence) whereas those from Gafchromic film are not. Therefore, it can be concluded that the indigenously developed OSL disc dosimeter is suitable for dose audit in conventional and conformal radiotherapy techniques.


 > Discussion Top


The results of this study indicate that Al2O3:C OSL disc dosimeters and Gafchromic EBT3 film measured dose values for photon (flat and unflat) and electron beams are within the acceptable variation of 5% both in case of conventional and conformal treatment techniques. It is important to highlight that these OSL discs dosimeters were used first time in dosimetry audit in radiotherapy rather than idealized laboratory conditions in the past and have demonstrated their promise of suitable dosimeters for dosimetry audit.

IAEA recommends that every radiotherapy center should participate in periodic dosimetry audit to ensure the good quality of dosimetry practice which is one of the basic requirements to fulfil the objectives of radiotherapy.[16] The dosimetry audit is also important to assure the patient that the dose delivered during the treatment has been verified and ascertained by an independent agency. Accuracy and precision in radiotherapy are demanding because the window for the success and failure of the treatment are very narrow. Hence, dosimetry audit is expected to provide the support in meeting this goal. Skill of the professional is always linked with the quality of services rendered, and hence, the services of experienced professionals are always expected to be up to the mark. The variations among measured and planned doses in almost all the cases are within 5% which is also a reflection of the skill of experienced professional as we have conducted the audit at those centers where concerned professionals were of about 10 years of experience.

Being a LMIC having the second largest population in the world with high cancer burden, the current understanding of the incidence and types of radiotherapy error or causality relationship in the background of a low resource, high throughput setting is mostly unknown.[32] It would be more appropriate to develop an indigenous system of learning and formulations of guidelines, keeping in mind the local needs, than overlaying recommendations directly from a very different work environment of developed countries. To understand those local needs, a dose audit of the current situation regarding dosimetry errors would be ideal. It will be valuable to understand our strengths and challenges for the improvement of existing radiotherapy practises.


 > Conclusions Top


The efficacy of OSL disc dosimeters has been evaluated in 6 MV (flat and unflat) photon and 6 and 15 MeV electron beams for multi-institutional dose audit in radiotherapy. It is observed that the indigenously developed Al2O3:C OSL disc dosimeters are having potential for the application in radiotherapy dosimetry and dose audit. The results of dose audit were found within the acceptable limit of 5% which provide the confidence that dosimetry practices of the institutions involved in this study are satisfactory.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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    Tables

  [Table 1], [Table 2], [Table 3], [Table 4]



 

 
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