Investigation of two linear accelerator head designs for treating brain metastases with hypofractionated volumetric-modulated arc radiotherapy.

Abstract

To investigate brain radiation dose in complex cases arising from two stereotactic linear accelerator designs and to present a method for comparing brain dose to published data. Two head designs were considered: Beam Modulator (BM) and Agility (AG). 12 patients treated on BM were replanned with AG. Planning objectives were: minimize brain dose and satisfy target coverage and organs at risk dose constraints. Each of the 36 targets was analyzed for conformality index (CI Paddick), gradient index (GI) and homogeneity index (HI). Total volume of tissue receiving 80% (V80) of the prescription dose down to 25% (V25) was evaluated. Similarly the volume of brain minus planning target volume receiving 80% (BMP80) down to 25% (BMP25) was evaluated. The mean brain dose and BMP dose were also evaluated. System differences were statistically evaluated using Wilcoxon signed-rank test. Power-law models for total volume (V) and brain minus planning target volumes (BMP) were generated based on BM data. The median CI Paddick was 0.74 and 0.76 for BM and AG, respectively (p = 0.04). The median GI was 5.5 and 6.1 (p < 0.01) and the median HI was 1.17 and 1.16 for BM and AG, respectively (p < 0.01). Neither V or BMP receiving doses of 80% down to 40% exhibited statistically significant difference between the two systems, whereas the volume of brain minus PTV receiving 25% (BMP25) was weakly different (p = 0.02). AG exhibited a lower mean BMP dose (4.1 Gy) than BM (4.6 Gy) (p < 0.01). Power-law models for V/BMP showed excellent (R(2) > 0.80) agreement for the dose levels studied and comparable results with published data. Treatment plans of equivalent quality were attained with AG compared with BM. The AG system involves a novel collimation design. The present article demonstrates equivalent or improved brain dose for complex, multitarget cases using AG vs an older stereotactic system.