SU‐E‐T‐441: Sensitivity of a 3D‐Verification System to VMAT Delivery Errors

Purpose: VMAT offers continuous dose delivery in the form of arcs while dynamically changing position of collimators and dose rate. The aim of the present study is to investigate the sensitivity of a 3D‐verification system to different VMAT delivery errors. Methods: Three VMAT plans were generated with MONACO TPS (v2.0, Elekta‐CMS, Maryland Heights, MO) for braintumor paradigms. With in‐house software, we modified the plans introducing the following errors: (i) One set of leaf bank was opened by 1mm, 2mm and individual leaves were randomly opened (0mm–2mm). (ii) Both sets of leaf banks were shifted by +/−1mm and +/− 2mm. (iii) Gantry angle was shifted by +/−0.5°, +/−1°, +/−2° and randomly shifted (−2° to +2°). All these plans (a total of 45) were delivered to a MatriXX‐based COMPASS system (v2.0, IBA‐Dosimetry,Germany). 3D‐dose distribution was reconstructed onto the patient anatomy based on the measured fluences. Dose—volume parameters for target (D95) and critical organs (Davg) were compared and analyzed. Results: For three reference plans, the maximum dose‐difference (MDD) between MONACO and COMPASS was about 10% (Davg‐lens). However, the absolute dose‐difference (ADD) for the same was 4cGy. For plans with errors in one set of leaf bank, the MDD and ADD were 10% and 13cGy for Davg‐optic nerve, respectively. With errors in both leaf banks, the MDD and ADD were 15% and 21cGy for Davg‐optic nerve, respectively. With gantry angle errors, the MDD and ADD were 50% and 20cGy, respectively. For PTV‐D95, the MDD noticed was 6% and the ADD was 16cGy for different kind of errors. Conclusions: Any increase in aperture size (error in one leaf bank) showed systematic deviations. Shifts in both leaf banks up to 2mm and gantry angle difference (2°) showed considerable deviations. Errors greater than 2mm/2° can be detected with COMPASS system in most of the cases.