Dosimetric comparison between non-coplanar volumetric modulated arc therapy using flattening filter and flattening filter-free beams during stereotactic radiosurgery for brain tumors

Objective To compare relevant dosimetric parameters of non-coplanar volumetric modulated arc therapy (VMAT) in treating brain tumors in conventional flattening filter (FF) or flattening filter-free (FFF) delivery mode, aiming to explore the appropriate evaluation method of accelerator for stereotactic radiosurgery (SRS). Methods Clinical data of 10 patients with single cranial tumor were retrospectively analyzed. All patients received non-coplanar VMAT at a prescription dose of 25 Gy in 1 fraction. Dosimetric parameters including conformity index (CI), heterogeneity index (HI), gradient index (GI50, GI25), gradient, volume of the brain tissue receiving larger than 10 Gy and 12 Gy(V10 and V12) and beam-on time were statistically compared between two treatment plans by paired sample t-test. Results When FFF-VMAT was compared with FF-VMAT in SRS for intracranial tumors, Paddick gradient index GI50 was 2.91±0.34 vs.3.07±0.35, 6.91±0.28 vs.7.35±0.27 for GI25, (0.57±0.07) cm vs.(0.61±0.08) cm for gradient, respectively (all P 0.05). For the normal brain tissues, the average dose was (160.64±43.64) cGy vs.(174.27±53.98) cGy, (45.35±30.32)% vs.(48.37±30.88)% for V10 and (36.69±25.15)% vs.(39.48±25.37)% for V12, respectively (all P<0.05). Conclusions Non-coplanar VMAT in FFF delivery mode can improve dose distribution and normal brain tissue sparing in the treatment of intracranial single tumors. Meanwhile, supplement of GI index and gradient index can implement comprehensive evaluation of the SRS planning. Key words: Brain neoplasm/stereotactic radiosurgery; Flattening filter-free beam; Dosimetry