Purpose: Applying SIMT-SRS to multiple brain lesions has improved treatment efficiency, patient compliance, patient comfort, and clinic workflow. When targeting multiple brain lesions using highly modulated SIMT-VMAT, the inherent risk of small field dosimetry errors can compromise the treatment quality. Herein, an MLC aperture-shape controlled method is proposed to mitigate the effects of small field dosimetry errors.
Methods: Seven complex SRS patients, each with 2-8 brain lesions, were selected with an average target size of 13.04 ± 8.83 cc. Treatment plans were developed for each patient using single-isocenter HyperArc VMAT with Encompass device and a 10X-FFF beam, prescribing 20 Gy to each lesion. Alternatively, patients were re-planned using modified HyperArc VMAT, implementing a DCA-based dose and a high priority MLC aperture-shape controller prior to optimization. Identical HyperArc geometry, isocenter location, planning parameters, and objectives were used.
Results: For similar target coverage, conformity, and dose gradient, DCA-VMAT increased average dose to gross tumor volume by 1.5 Gy. It showed slightly higher V8Gy and V12Gy values, increasing by 4.2 cc and 0.3 cc on average, respectively. However, critical organ (optic apparatus, brainstem, and hippocampi) maximal doses and mean brain doses were decreased with DCA-VMAT. Moreover, the DCA-VMAT plan decreased the total monitor units, beam modulation factor, and beam-on time by an average of 1846 MU, 0.92, and 1.37 min, respectively. Independent dose verification through Monte Carlo (MC) code showed treatment planning system agreement for DCA-VMAT within 3%, decreased from the 6% of HyperArc VMAT.
Conclusion: Modified HyperArc VMAT, utilizing DCA-based dose and MLC aperture-shape controller, provided similar plan quality with substantially decreased total monitor units, beam modulation, and beam-on time. This arguably suggests that the modifications further improve SIMT HyperArc treatments for multiple brain lesions. Independent MC calculation showed reduction of small field dosimetry errors and likely patient setup uncertainties via DCA-VMAT.