To compare the dosimetric performance of flattening filter-free (FFF) beam and flattened beams (FBs) utilising volumetric-modulated arc therapy (VMAT) for craniospinal irradiation (CSI) planning.

Five medulloblastoma patients were randomly selected retrospectively and 40 plans were generated. The dose prescription to the planning target volume (PTV) was 36 Gy in 20 fractions. VMAT plans were created using 6 MV and 10 MV FB and FFF beam. Final dose calculations were performed using Acuros XB (AXB) and analytical anisotropic algorithm (AAA). Dosimetric parameters such as D98%, D95%, D50%, V110%, conformity index (CI), homogeneity index (HI), low-grade dose index, high-grade dose index, dose to the organ at risks (OARs) and normal tissue mean dose were noted. The effect of low-dose volume on normal tissue was also analysed.

The 6 MV and 10 MV flattened and FFF beam plan generates similar target coverage, and a significant difference was observed in the HI and CI. FFF beam plan produces lower doses in some of the OARs as compared to FB. Significant differences were also noted in monitor unit (MU), body-PTV mean dose and low-dose spillage regions (1–10 Gy) outside the PTV. In our study, 6 MV and 10 MV FFF beam beams need 23–25% more MUs to achieve planning goals when compared to FBs. The increased MUs in FFF plan decreases the body-PTV mean dose (0·07–0·09 Gy in 6 MV FFF and 0·31 Gy in 10 MV FFF in both algorithms) when compared to FB plans.

FFF beams generate a highly conformal and homogenous plan in CSI cases. FFF beam plan reduced the non-tumour dose and will aid in reducing the probability of second malignancies.