WE‐D‐351‐04: Dosimetric Considerations of Treating Larger Target Volumes with Perfexion

Purpose: To quantify the background absorbed dose to the uninvolved brain as a function of target volume for varying isocenter spacing and collimator sizes using Perfexion™. Method and Materials: Multiple treatment plans were generated for a series of spherical targets, using a sphere‐packing algorithm to automatically determine the isocenter coordinates. Target volumes investigated ranged from 2.1 cm³ (diameter=16 mm) to 33.5 cm³ (diameter=40 mm), with isocenter spacing for each volume ranging between 3.2 mm and 8 mm. The dose was determined for each scenario at fixed distances from 3 cm to 7 cm from the edge of the target volume and expressed as a percentage of the dose to the target edge. Dose was calculated using Gamma Plan (v8.0) and measured using EBT Gafchromic film inserted in a plastic spherical phantom (8 cm radius). Results: No statistically significant difference in background dose was found between isocenter spacing levels. Relative dose at each point from the edge of the target increased with increasing target volume. Using only 4‐mm collimated isocenters, the background dose at 7 cm from the target edge was 0.5% (23 isocenters) and 4.6% (226 isocenters) for 2.1 cm³ and 33.5 cm³ targets, respectively. For a given volume size, background dose was higher when using 16‐mm compared to 4‐mm collimated isocenters. Using only 16‐mm collimated isocenters, the average dose rate at 7 cm from the target edge was 2.3% (1 isocenter) and 6.8% (9 isocenters) for 2.1 cm³ and 33.5 cm³ targets, respectively. Conclusion: Treating larger target volumes with Perfexion™ results in larger background dose to uninvolved brain, irrespective of what spacing is used between isocenters. The use of 4‐mm collimated isocenters yields lower background dose levels than 16‐mm collimated isocenters for equivalent target volumes. Conflict of Interest: Research sponsored by Elekta Instruments AB.