SU-E-T-83: Validation and Testing of Performance of a Novel Phantom Specially Designed for Volumetric Modulated Arc Therapy MU/dose Verification

Abstract

Purpose: To validate and performance test a novel phantom for Volumetric Modulated Arc Therapy MU/dose verification. Methods: A cylindrical phantom was designed using a 2 litter beverage plastic bottle, having useful length of 20.7cm and average diameter is 9.9cm. A hollow plastic tube, having a stopper in the middle, was inserted along the long axis of the bottle and cemented at both end for a reproducible chamber position. A 0.125cc Semiflex chamber (PTW, Germany) was inserted through the central tube. Reproducibility of the set up was ensured. Axial CT cuts were accrued for 2mm separation. Total 33 VMAT plans were generated on Elekta (Norcross, GA) Monaco V 3.1.01 treatment planning system (TPS) and tested using this phantom. Cone Beam CT or portal images were used for reproducing the phantom position, Phantom was repositioned using 3 dimensional or six dimensional (Hexapod couch) couch movement as per the availability. Results: For dose calculation, chamber volume was contoured and 4 individual points were considered along chamber length. MU/dose were calculated against average point dose and volumetric dose. MU verification was done by calculating % variation of TPS calculated and LINAC measured dose. Mean point dose and volumetric dose variations were 1.05±1.077 % and 1.02±1.22% respectively. Conclusion: We present a novel indigenous phantom for VMAT quality assurance. Imaging verification was essential for reproducibility of the phantom positioning. 6D couch movement was marginally better than 3D movement and does not produce a dosimetrically significant variation. This in-house made phantom cost about 9 USD and gives a very good accuracy for MU/ dose verification. Exactrac (Brainlab) system, using a ±45° X-ray images could not detect the phantom as well as chamber position properly; giving an average longitudinal shift of 11.4 cm.