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ORIGINAL ARTICLE
Year : 2021  |  Volume : 17  |  Issue : 4  |  Page : 859-864

Validation of absolute point dosimetry by the analytical anisotropic algorithm and Acuros XB algorithm employing intensity-modulated radiotherapy technique on an in-house develop cost-effective heterogeneous thorax phantom


School of Studies in Physics, Vikram University, Ujjain, Madhya Pradesh, India

Date of Submission06-Dec-2019
Date of Decision26-May-2020
Date of Acceptance05-Aug-2020
Date of Web Publication30-Jul-2021

Correspondence Address:
Priyusha Bagdare
Research Scholar, School of Studies in Physics, Vikram University, Ujjain, Madhya Pradesh 456010
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jcrt.JCRT_1072_19

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 > Abstract 


Introduction: Dose validation inside the human body needs a medium which can simulate the actual heterogeneities of a specific body site. The aim of the present work is to study the properties of a cost-effective heterogeneous thorax phantom (HTP) developed in-house by the author and its application for the evaluation of patient-specific absolute point dosimetry by employing analytic anisotropic algorithm (AAA) and Acuros XB (AXB) algorithm.
Materials and Methods: HTP was made from the dust of porous pinewood, rib cage, and honeybee's wax. Density and central axis isodose depth distribution was measured on computed tomography images of actual patient and on HTP. Absolute point dose verification of 35 patients was done using AAA and AXB algorithm. The difference in the calculated dose by AAA and AXB was compared using the Wilcoxon signed-rank test.
Results: Density distribution and central axis depth dose inside the HTP compare well with that of an actual patient. The mean percentage variation between the planned and the measured doses inside the HTP was found to be 4.85 (standard deviation [SD] = 3.38) and 1.3 (SD = 2.93), respectively, using AAA and AXB algorithm. The difference in the measured dose and the planned dose was found to be significant for AAA with the significance level of 0.01 (p-value < 0.00001), whereas it was found to be insignificant (p-value < 0.00001) for AXB.
Conclusion: The results of this study showed that the HTP is resembled with the human thorax in terms of its heterogeneities and radiological properties and can be used for pretreatment plan verification.

Keywords: absolute point dosimetry, acuros XB algorithm, analytic anisotropic algorithm, heterogeneous phantom


How to cite this article:
Bagdare P, Dubey S, Ghosh S. Validation of absolute point dosimetry by the analytical anisotropic algorithm and Acuros XB algorithm employing intensity-modulated radiotherapy technique on an in-house develop cost-effective heterogeneous thorax phantom. J Can Res Ther 2021;17:859-64

How to cite this URL:
Bagdare P, Dubey S, Ghosh S. Validation of absolute point dosimetry by the analytical anisotropic algorithm and Acuros XB algorithm employing intensity-modulated radiotherapy technique on an in-house develop cost-effective heterogeneous thorax phantom. J Can Res Ther [serial online] 2021 [cited 2021 Nov 28];17:859-64. Available from: https://www.cancerjournal.net/text.asp?2021/17/4/859/322705




 > Introduction Top


Dose calculation inside human body is quite challenging due to various inhomogeneities present in it. Different sites in the human body possess different radiological properties due to differences in their density and are, therefore, quite challenging to calculate the exact dose, especially at the boundary regions.[1],[2],[3] To date, an algorithm based on Monte Carlo code can be considered as the most accurate one when precise dose estimation is needed across the boundary region.[4],[5] It uses the Linear Boltzmann Transport Equation (LBTE) solvers for the dose calculation but requires longer computer processing time as compared to other commercially available algorithms. A new photon dose calculation algorithm, known as Acuros XB (AXB), has recently been used by the treatment planning system (TPS) which uses the same LBTE solvers as are being used by MC-based algorithms for dose calculation but requires shorter calculation time.[6],[7] Apart from the MC code, all other algorithms are based either on equivalent path length (EPL) or on convolution technique for inhomogeneity correction. Both the models have their own shortcomings for calculating the dose inside a heterogeneous medium.[8]

In accordance with the recommendations of International Commission on Radiation Units and Measurements (ICRU), an overall 5% uncertainty in dose calculation can be considered to result from the patient set-up, machine calibration, and dose calculation algorithm employed.[9] Uncertainty due to the algorithm used can be restricted to within 3% using a high-end algorithm. To validate the dose calculated by employing a algorithm, phantoms are needed.[10] Phantoms are tissue-equivalent materials used for the purpose of pretreatment dose verification instead of an actual human body. Accuracy of a given algorithm can also be verified using an appropriate phantom.[11],[12] Phantoms which are being currently used in radiotherapy for quality assurance (QA) purpose are mostly of homogeneous and water equivalent type. Indirect dose verification is performed by comparing the planned dose with the measured dose using such phantoms. Evaluation of the results obtained from verification techniques is of prime importance in arriving at the treatment option to be employed. However, the results and dosimetric errors obtained using homogeneous phantoms have some limitations.[13] As human body is heterogeneous in nature, bones, soft tissue, and air pose different amounts of attenuation, absorption, and scattering for a photon beam because of differences in their densities.[14] Different boundaries, such as air–soft tissue and soft tissue–bone exist inside body which significantly affect the dose deposition pattern across the various interface regions.[1],[15] Hence, the accuracy of dose predication is greatly affected by the design of the phantom employed. Due to the discrepancy associated with homogeneous phantom, researchers keep on designing their own phantom suitable to fulfill their study needs.[16],[17]

Constructing a heterogeneous phantom is a challenging job as it is expected to have the same anatomy as well as physical and radiobiological properties as those of an actual patient. The anatomic distribution of various tissue layers in the phantom has to be accurate to enable an analysis of the actual scenario. This is especially important if the application is heavily dependent on the constitution of the body part, particularly its density. Physical and radiobiological properties of phantoms must be similar to those of an actual patient to simulate it exactly. A high-quality phantom should replicate as closely as possible the actual tissue present inside human body.

Looking into present scenario, the aim ofthe present work is to study the physical and radiological properties of a cost-effective heterogeneous thorax phantom (HTP) that has been developed in-house by the author. Designed phantom is used for pretreatment plan verification of 35 patients treated with intensity-modulated radiotherapy (IMRT) technique using the analytic anisotropic algorithm (AAA) and AXB algorithm.


 > Materials and Methods Top


Standard human rib cage made from a material with the same density as that of the actual human rib was taken as the base for designing the HTP. The coating of honeybee's wax is used to cover the rib cage to replicate the tissue-equivalent material. To mimic the lung region, the hollow part inside the rib cage was filled with the porous pinewood sawdust, as shown in [Figure 1].
Figure 1: In-house develop heterogeneous thorax phantom

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Computed tomography (CT) images of the designed HTP as well as of thoracic region of an actual patient with 3.0 mm slice thickness were taken on a Siemens SOMATOM Definition Adaptive Scanner (Siemens Medical systems, Germany). The CT images of the above media were fed to the TPS eclipse version 13.7 (Varian Medical Systems, Palo Alto, CA, USA).

Average dimensions of thorax regions of 40 adult patients irrespective of their gender and age distribution were taken for designing the HTP.[18] The thickness of the chest wall was taken as 5 cm, that of lungs was taken as 17 cm, and for soft tissues behind the lungs was taken as 10 cm for designing the HTP. Hounsfield units (HU) measured on TPS were used to calculate the mean density of the chest walls, lungs, and soft tissues behind the lung in the patient as well as in HTP[19] using the formula:

H = 1000 ([ρ/ρw] −1.0)

Where (ρ) represents the density of any given medium, and ρw is the density of water.

To measure the depth of central axis isodose curves on TPS, plans were created on the CT scan of the patients as well on HTP. All plans were created for 100 cm source to surface distance, keeping plan normalization 100%. 10 MV photon beam was used with the field of view of 10 cm × 10 cm size and with zero gantry angle. AXB algorithm version 13.6.23 with the grid size of 0.25 mm was used for calculation. Depth of isodose curves for 100%, 95%, 90%, 85%, 80%, 75%, 70%, 65%, 60%, and 55% were measured on CT images of 40 patients as well as on HTP. The depth variation of central axis isodose curves on CT scan of one of the patients as well as on HTP is shown in [Figure 2]a and [Figure 2]b.
Figure 2: Isodose depth dose distribution for 10 MV in the computed tomography slice (a) inside the actual patient (b) inside the heterogeneous thorax phantom

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Patient-specific absolute dosimetry was performed for IMRT plans of 35 randomly selected thorax patients. The prescribed dose was in the range of 2–2.5 Gy per fraction with 5 fractions a week. Plans were optimized with a grid size of 0.25 mm on TPS using AAA (version 13.6.23) and AXB algorithm, with the aim of covering 95% of PTV with the 95% isodose. QA plans were generated on CT image of HTP, keeping the gantry, couch, and collimator position the same as that of an actual patient plan. Absolute point dose calculation was performed for a single fraction at the reference point inside the chamber on TPS for both AAA and AXB algorithm.

Subsequently, to verify the absolute point dose obtained on TPS, various plans were exposed on Clinac DMX accelerator (Varian Medical Systems, Palo Alto, CA, USA). Each IMRT plan was executed on HTP phantom. Measurement of point dose at a reference point was done inside the HTP using 0.13 cc ion chamber and DOSE1 electrometer. The position of ion chamber inside the phantom as shown in [Figure 3] was verified by its CT images and matching them with the primary CT images obtained by means of the Eclipse TPS. The percentage variation between the absolute point dose planned on TPS and measured on Clinac was calculated using the following relation:
Figure 3: Position of ion chamber inside the heterogeneous thorax phantom

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Percentage variation = [{(planned dose − measured dose)/measured dose} × 100]


 > Results Top


In this study, we have compared the density and isodose depth variation on CT images of HTP with that of the CT images of an actual patient. Density measurement in the designed HTP using the HU was found to be similar to that of an actual patient. The mean density of the chest wall, lung, and soft tissues was found to be 1.78 g/cc, 0.2 g/cc, and 0.89 g/cc for an actual patient and 1.84 g/cc, 0.24 g/cc, and 0.86 g/cc in the HTP. Density variations inside the HTP follow exactly the same pattern as in the case of human thorax.[20]

[Table 1] represents the mean central axis isodose depth distribution for 100%, 95%, 90%, 85%, 80%, 75%, 70%, 65%, 60%, and 55% isodose curves inside the CT image of 40 patients as well as inside the designed HTP. As can be seen, the depths of isodose match quite well in the two media which can result the same radiological properties in both the media.
Table 1: Mean isodose depths in computed tomography images of the actual patient and the heterogeneous thorax phantom

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Verification of absolute point dose measurement was done inside HTP. [Table 2] gives the mean percentage variation between the planned and measured doses for all IMRT QA plans in the designed HTP phantom using AAA and AXB algorithm. It was found to be 4.85 (SD = 3.38) and 1.3 (SD = 2.93), respectively, for AAA and AXB algorithm. The difference in the planned dose and the measured dose was found to be significant for AAA with the significance level of 0.01 (p-value < 0.00001), whereas it was found to be insignificant (p-value < 0.00001) for AXB, as shown in [Figure 4] and [Figure 5], respectively.
Figure 4: Variation between the planned dose and the measured dose for analytic anisotropic algorithm

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Figure 5: Variation between the planned dose and the measured dose for Acuros XB

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Table 2: Percentage variation between measured dose on Clinac the planned dose on treatment planning system and using heterogeneous thorax phantom

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 > Discussion Top


Based on different dosimetric guidelines such as ICRU 83, AAPM Task Group Report 58, and TRS 398,[21],[22],[23] the phantom suggested for pretreatment plan verification is of water or a water equivalent medium. As the human body contains almost 65% water, a homogeneous water/water equivalent medium was considered to be a good choice for dose verification purposes.

However, in reality, the human body is heterogeneous in nature, and therefore, to get accurate dosimetric results, we need to have a phantom which can replicate the exact heterogeneities which are present inside the human body. In the case of megavoltage beams, the density of the inhomogeneity and the scattered dose are important parameters in determining the overall dose deposition inside a given medium. For high-/low-density inhomogeneities, there is increased production of electrons in a high-density material relative to a low-density material which leads to an increase in dose on the low-density side due to larger number of electrons entering the tissue from high-density sides and vice versa. In human body, many interfaces of different densities exist, such as air cavity–soft tissue, soft-tissue–bone, and air cavity–bone, and thus, the dose varies significantly at these boundaries.[24],[25] If we compare this dose distribution characteristic with a homogeneous medium of uniform density, we can find a significant difference between the two media.

The present work is based on developing a heterogeneous phantom which can mimic real human thorax in terms of its density and dose deposition characteristic so that the dose prediction accuracy can be improved. The results obtained from the study clearly indicate similarity between the actual human thorax and HTP in terms of its density and radiological properties. Thus, the designed phantom can be used as QA tool for pretreatment plan verification.

The accuracy of algorithm used for dose calculation also plays an important role in dosimetric verification. An algorithm such as AAA does not account for scattering around the interface, especially in the lateral direction, and therefore, underestimates/overestimates the dose at the boundary, depending on the variation of density across the interface.[26] To overcome this limitation, Monte Carlo-based algorithms, such as AXB, can be used which take into account all the aspects of interaction of radiation within the matter as well as across the interface region. Heterogeneity correction factor is automatically taken into consideration while making dose calculation by AXB.[27]

In the present work, dosimetric evaluation of AAA and AXB algorithm was done for patient-specific point dosimetry using the designed HTP. Percentage variation between the planned dose and the measured dose was found to be less for AXB as compared to AAA which indicates that the AXB gives more accurate results when heterogeneities are involved.

In the designed phantom, relative dosimetry can also be done using film for the measurement of dose fluence. There is also the scope of introducing the lung movement inside the phantom, and the author is pursuing further work in this direction. The designed phantom can be used for verification of various algorithms, and the results obtained are quite reproducible.


 > Conclusions Top


In this study, the properties of heterogeneous phantom were studied. Designed phantom is used to perform pretreatment plan verification for IMRT technique. Absolute point dose measure on HTP for two different algorithms is compared, and the results showed that the phantom can be used in routine for patient-specific pretreatment IMRT dose verification as well as it can be used as a tool for the validation of a given algorithm.

Acknowledgment

We would like to express our thanks to Roentgen Oncologic Solutions Pvt. Ltd. and Sri Aurobindo Institute of Medical Sciences, for supporting this research by permitting us to use the departmental resources and equipment.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
 > References Top

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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
 
 
    Tables

  [Table 1], [Table 2]



 

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