SU‐E‐T‐561: Small Target Dose Calculation Accuracy for Multiple‐Target Single‐Isocenter VMAT

Purpose: To evaluate accuracy of dose calculation for multiple small targets treated simultaneously using single‐isocenter VMAT. Methods: 3, 5, 7, 10, 15, and two 20 mm diameter targets were contoured on a plane in a 15×20×20 cm3 phantom. Radiosurgery plans using coplanar and non‐coplanar arcs were developed using a 6 MV beam with a 5 mm MLC (6X‐STD) and a 10 MV flattening filter free beam with a 2.5 mm MLC (10FFF‐HD). A uniformity goal was applied to one of the 20 mm targets to create a volume suitable for measurement using an ionization chamber. Dose was calculated on a 1×1×1 mm3 grid using the analytic anisotropic algorithm (AAA). Plans were delivered to a water‐equivalent phantom and the dose measured using an ionization chamber and radiographic film. Results: For the 6X‐STD plans, the chamber measurements were 1.7% less than calculation for both plans. For the remaining targets, the film doses ranged from 1.2% to 4.2% lower than calculation for the targets larger than 3 mm, whereas doses were 8.9% and 7.0% lower than calculation for the 3 mm target for the coplanar and non‐coplanar plans, respectively. For the 10FFF‐HD plans, the chamber measurements were 2.6% and 1.5% higher than calculation for the coplanar and non‐coplanar plans, respectively. The film doses for the remaining targets ranged from 1.1% lower to 3.1% higher than calculation, with the largest differences noted for the 5 mm target. Conclusion: With renormalization, the present results suggest dose accuracy within 3% for targets at least 3 mm in diameter for the 10FFF‐HD beam and for targets of at least 5 mm diameter for the 6X‐MLC beam. Work is ongoing to identify the source of the larger differences observed for the 3 mm target and the 6X‐STD beam. Author has research support from Varian Medical Systems.