International Journal of Cancer Therapy and Oncology www.ijcto.org Corresponding author: Shanmugam Thirumalai Swamy; Department of Radiation Oncology, Yashoda Hospitals, Hyderabad, India. Cite this article as: Thirumalai-Swamy S, Anuradha C, Kathirvel M, Arun G, Subramanian S. Pretreatment quality assurance of volumetric modulated arc therapy on patient CT scan using indirect 3D dosimetry system. Int J Cancer Ther Oncol 2014; 2(4):020416. DOI:10.14319/ijcto.0204.16 Pretreatment quality assurance of volumetric modulated arc therapy on patient CT scan using indirect 3D dosimetry system Shanmugam Thirumalai Swamy 1,2 , Chandrasekaran Anuradha 2 , Murugesan Kathirvel 1 , Gandhi Arun 1 , Shanmuga Subramanian 1 1 Department of Radiation Oncology, Yashoda Hospitals, Hyderabad, India 2 School of Advanced Sciences, VIT University, Vellore, India Received August 29, 2014; Revised October 16, 2014; Accepted October 18, 2014; Published Online October 22, 2014 Original Article 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 pelvis VMAT plans dose response from Linac was measured using COMPASS system along with MatriXX Evolution and 3D dose reconstructed in the patient computed tomography (CT) scan. Dose volume histograms and 3D gamma were used to evaluate 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 -3.17% in lung, whereas the difference other densities was within ±2%. The systematic increase in the average 3D gamma be- tween 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. Keywords: COMPASS; VMAT; 3D Gamma; Pretreatment QA Introduction In advanced radiotherapy techniques, each patient's treat- ment plan is customized, to produce high gradient dose dis- tribution to the target and low dose to the critical organs. Volumetric Modulated Arc Therapy (VMAT- RapidArc) produces highly conformal dose distribution by simultane- ously changing multi-leaf collimator (MLC) position, dose rate and gantry speed during patient treatment. 1-3 Complex treatment deliveries demand a comprehensive quality assur- ance (QA) procedure. American Association of Physicists in Medicine Task Group (AAPM TG)-82 4 recommends verifica- tion of intensity modulated treatment plans with an inde- pendent QA method before treatment delivery. Traditionally pretreatment QA’s are performed in a phantom using ion chamber, film, 2D array, and electronic portal imaging de- vice (EPID). Each of these devices has been proven useful but has its own limitations. 5-11 Position of ion chamber in the high dose gradient area leads to discrepancy between meas- ured and the treatment planning system (TPS) calculated dose. Film shows excellent spatial resolution; however, problems like chemical processing, scanner readout, and time delay exists. A 2D array shows the dose distribution immediately after the treatment delivery, but their results have limited resolution. Also, results provided by them can- not be directly used to identify delivery errors in tumor and normal tissues. 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 devel- oped 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. 13-18 The COMPASS QA system (IBA Dosimetry, Germany) is one such technique which uses MatriXX Evolution along with gantry