Objective: 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. Methods: 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. Powerlaw models for total volume (V) and brain minus planning target volumes (BMP) were generated based on BM data.Results: The median CI Paddick was 0.74 and 0.76 for BM and AG, respectively (p 5 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 5 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. Conclusion: Treatment plans of equivalent quality were attained with AG compared with BM. Advances in knowledge: 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.
INTRODUCTIONThe use of intensity-modulated radiotherapy or volumetricmodulated arc radiotherapy (VMAT) using multileaf collimators (MLCs) for stereotactic radiosurgery and radiotherapy is becoming increasingly popular, as this technique affords efficient delivery with a high degree of target conformality. [1][2][3] In particular, for multiple targets, the use of VMAT has been implicated in yielding shorter treatment times while providing comparable plan quality to intensity-modulated radiotherapy and three-dimensional conformal treatments. 1 Initial experiences with VMAT were with multi-isocentre plans, 4,5 followed by a series of studies investigating the feasibility of a singleisocentre approach. 2,6