Abstract
Purpose: Aim of this study is to clinically implement the COMPASS 3D dosimetry system for pretreatment quality assurance of volumetric modulated arc therapy (VMAT-RapidArc) treatment plans. Methods: For this study, 10 head and neck (H&N) and 10 pelvis VMAT plans dose response from Linac was measured using COMPASS system along with MatriXX Evolution and 3D dose was reconstructed in the patient computed tomography (CT) scan. Dose volume histograms and 3D gamma were used to evaluate the difference between the measured and calculated values. In order to validate the COMPASS system, dose response for open fields were acquired for both homogeneous and inhomogeneous phantoms. Results: The average dose difference between Eclipse treatment planning system (TPS) calculated and COMPASS measured (homogenous medium) in normalization region, inner region, penumbra region and buildup region was less than ±2%. In inhomogeneous phantom, there was a maximum difference of -3.17% in lung, whereas the difference other densities was within ±2%. The systematic increase in the average 3D gamma between the TPS calculated and COMPASS measured for VMAT plans with known dose errors and multi-leaf collimator (MLC) offset errors shows that COMPASS system was sensitive enough to find clinical significant errors. The 3D dose parameters (D95, D1, and average dose) of all H&N and pelvis patients were well within the clinically acceptable tolerance level of ±5%. The average 3D gammas for planning target volumes (PTV) and organ at risks (OAR) of the patients were less than 0.6. Conclusion: The results from this study show that COMPASS along with MatriXX Evolution can be effectively used for pretreatment verification of VMAT plans in the patient anatomy.
Highlights
IntroductionEach patient's treatment plan is customized, to produce high gradient dose distribution to the target and low dose to the critical organs
In advanced radiotherapy techniques, each patient's treatment plan is customized, to produce high gradient dose distribution to the target and low dose to the critical organs
We have evaluated the performance of COMPASS 3D dosimetry system clinically: (i) In open field sizes (10 × 10 cm[2] and 20 × 20 cm2) to evaluate the accuracy of beam modeling both in homogeneous and inhomogeneous media, (ii) To check the accuracy of the system in detecting multi-leaf collimator (MLC) and dose errors and (iii) To implement the system for VMAT patient specific quality assurance (QA)
Summary
Each patient's treatment plan is customized, to produce high gradient dose distribution to the target and low dose to the critical organs. Pretreatment QA’s are performed in a phantom using ion chamber, film, 2D array, and electronic portal imaging device (EPID) Each of these devices has been proven useful but has its own limitations.[5,6,7,8,9,10,11] Position of ion chamber in the high dose gradient area leads to discrepancy between measured and the treatment planning system (TPS) calculated dose. Benjamin et al.[12] showed that there is lack of correlation between gamma passing rates from 2D array and dose differences in critical anatomic regions of interest To address this issue, alternate QA techniques has been developed to verify the 3D dose distribution by measuring fluence at different gantry angle using ion chamber matrix or EPID in a patient computed tomography (CT) scan.[13,14,15,16,17,18] The COMPASS QA system (IBA Dosimetry, Germany) is one such technique which uses MatriXXEvolution along with gantry
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