SU‐E‐T‐326: Dosimetric Impact of Beam Energies and Jaw Tracking On Intracranial Tumors Using RapidArc

Abstract

Purpose:To determine the dosimetric impact of jaw tracking and beam energies on dose conformity and normal‐brain‐tissue doses for intracranial tumors using VMAT (RapidArc).Methods:Seven patients with 1–2 and three patients with 4–6 intracranial tumors were planned using RapidArc for Varian TrueBeam STx machine with beam energies 6MV‐FFF (Flattening‐Filter‐Free), 8MV, 10MV, and 10MV‐FFF. The prescription dose ranged from 14–23Gy. Between 2 and 8 arcs were used with the following geometries: 2 full coplanar arcs and the non‐coplanar half arcs. Plans were optimized (jaw tracking ON) with a high priority to Normal‐Tissue‐Objective and normal‐brain‐tissue. Plans were calculated on 1mm grid size using AAA algorithm and then normalized so that 99% of each target volume received the prescription dose. Plans for the 6MV‐FFF were also optimized without jaw tracking (No‐JT) for comparison. Plan quality was assessed by target coverage using Paddick Conformity Index (PCI), sparing of normal‐brain‐tissue through analysis of V4Gy, V8Gy and V12Gy, and integral dose.Results:The average PCI ± standard deviation was 0.76±0.21 and 0.76±0.22 for 6MV‐FFF and 10 MV‐FFF, respectively. The average ratio in normal brain tissue volume (reported as follows V4,V8,V12) were (1.12±0.07,1.12±0.07,1.14±0.04), (1.12±0.08,1.12±0.09,1.13±0.06), (1.19±0.10,1.18±0.10,1.20±0.04), and (1.04±0.03,1.03±0.03,1.03±0.04) for 8MV/6MV‐FFF, 10MV‐FFF/6MV‐FFF, 10MV/6MV‐FFF, 6MV‐FFF No‐JT/6MV‐FFF, respectively. Statistically significant differences in normal‐brain‐tissue for V4, V8, and V12 were observed in all cases for the different energies (p‐values <0.05). V4 data shows significant differences in JT vs. No‐JT (p=0.04), however no difference was found for V8 and V12. Brain tissue sparing from best to worst occurred in this order 6MV‐FFF, 6MV‐FFF no‐JT, 10MV‐FFF, 8MV, and 10MV. The average ratio of integral brain dose was 1.05±0.04 (p=0.21), 1.04±0.05 (p=0.33), 1.09±0.06 (p=0.04), and 1.02±0.06 (p=0.61) for 8MV/6MV‐FFF, 10MV‐FFF/6MV‐FFF, 10MV/6MV‐FFF, and 6MV‐FFF No‐JT/6MV‐FFF, respectively.Conclusion:Normal brain tissue and integral dose improved using the lower energy and FFF beams, though plan conformity showed energy independence.