Monitor unit restrained hybrid approach using conformal arcs for radiosurgery of solitary brain metastasis

Abstract

PurposeThis study appraises the efficacy of monitor unit (MU) restrained hybrid planning approach for solitary brain metastasis combining dynamic conformal and volumetric modulated arcs. The optimal level of monitor unit restrainment is ascertained by plan metric analysis. Besides, dependence of efficiency index (η50%) on beam modulation (BM) and association of normal brain receiving 12Gy (V12) with gradient index (GI) under the applied MU constraints is assessed. MethodsSixteen cases were planned to a dose of 20 Gy using 2–3 non-coplanar DCAT arcs and a coplanar VMAT arc, combined in a 7:3 dose ratio. All VMAT optimizations were based on DCA dose. A baseline VMAT plan was generated without using the MU-tool. Then MUs of the baseline plan were reduced using the MU-tool by 10%, 20%, 30%, and 40% to develop alternate plan types. Alternate plans were compared to baseline plan using the mean difference of plan metrics. The relationship of η50% with BM and that of V12 with GI was assessed using Pearson's r. ResultsEfficiency index, η50% reduced significantly across all plan types except MU-10. CI increased from 90.8% to 91.4% on reducing MU restraint. GI did not deteriorate for MU-10 and MU-20 but increased for MU-30 and MU-40, P < 0.006. Brain receiving 12Gy, V12 increased about 0.1 cm3 for MU-40, P = 0.001. Also, the probability of symptomatic necrosis, S-NEC(V12), increased by 0.1% for MU-40 and MU-30, P < 0.012. BM reduced from MU-20 through MU-40 by at least 5.7% (P < 0.001). Beam-on-time and MU factor reduced maximally by 0.6 min and 15.7%, for MU-40. For all plan types, η50% correlated directly with BM, whereas V12 showed an inverse relationship with GI. ConclusionsModerately constrained, MU-20 type hybrid plans demonstrated optimal overall efficiency, achieving plan objectives with reasonable delivery simplicity. Efficiency index reduced gradually with decreasing beam modulation.