SU‐E‐T‐439: 3D Dosimetry for Volumetric Modulated Arc Stereotactic Delivery Using Compass

Purpose: To study the feasibility of 3D dosimetry for volumetric modulated arc stereotactic delivery using MatriXX with COMPASS verification software. Methods: Twenty patients CT images and RT structures were taken from Multiplan planning system for this study. All these patients underwent radical stereotactic treatment in Cyberknife. For each patient, linac based VMAT stereotactic plans were generated in Monaco TPS v3.1 using Elekta Beam modulator MLC. Dose prescription was in the range of 5–18Gy per fraction. Target prescription and critical organ constraints were tried to achieve the same dose as per the delivered treatment plans. Plan parameters like arc length, increment, grid size, beamlet width, calculation algorithm, minimum segment width, number of arcs and dose rate were kept common for all plans. Each patient data were exported to COMPASS verification software. Measurements were performed using MatriXX, mounted on gantry with gantry angle sensor. Results: Comparison of COMPASS reconstructed dose from measured fluence and COMPASS computed dose has shown a good agreement with TPS calculated dose. Each plan was evaluated based on dose volume parameters for target volumes such as dose at volume (95%) and average dose. For critical organs D20, D50 and maximum point doses were evaluated. Comparison was carried out using gamma analysis with passing criteria 3mm and 3%. Mean deviation of 1.9±1% was observed for D95 of target volumes, whereas very less difference was noticed for critical organs. However, significant dose difference was noticed in two cases due to the smaller tumor size. Conclusion: Analysis of this study revealed that the COMPASS 3D dosimetry is effective and ease to use as pretreatment patient specific QA for VMAT stereotactic delivery. 3D dosimetric QA with COMPASS provides additional degrees of freedom to check the high dose modulated stereotactic delivery with very high precision on patient CT images.