Room: AAPM ePoster Library
What PTV characteristics affect the intermediate dose spill in an SRS/SRT treatment? We hypothesize that the PTV Surface Area is a stronger determinant of the intermediate dose spill in SRS/SRT than the PTV volume.
Using the IROC Head Phantom®, a series of PTVs of various volumes and surface areas are created and planned on the Eclipse® TPS using RapidArc 5-hemi-arc plans for delivery on a Varian TrueBeam STx® with 120 leaf HD MLC. All plans are prescribed such that 99% of the PTV volume receives 18 Gy in one fraction. Plans are created for PTVs of volumes from 1.8cc to 16.8cc, with surface areas from 10cm² to 31.7 cm².
We assess the intermediate dose spill using the metric R50% = (Volume of 50% isodose) / (PTV volume). Given the same volume, the higher surface area PTV always has a higher R50%. Given the same surface area, the higher volume has a lower R50%. We find an even better correlation between the true 50% Isodose Volume (PIV50%) and the PTV Surface Area. The graph of PIV50% vs PTV Surface Area has a correlation coefficient, r = 0.985. The PIV50% vs PTV Volume has r = 0.963. The Surface Area graph brings all PTVs of any shape into a single straight line plot, while the Volume graph shows a differentiation between different PTVs shapes
This work demonstrates a strong dependence of the SRS/SRT intermediate dose spill upon the PTV surface area. Further, PTV surface area is a better indicator of intermediate dose spill than PTV volume. The metric PIV50% shows the best correlation with PTV surface area. Because of this strong correlation we encourage all TPS vendors to include surface area as a reported PTV characteristic (like PTV volume) in future versions of their software.