Dosimetric Comparison of Hypofractionated Multi-Beam Intensity-Modulated Radiation Therapy and Volumetric Modulated Arc Therapy With Flattened Beam and Flattening-Filter-Free Beam for Skull Base Meningioma Adjacent to Optic Pathways

Background Since the optic pathways are the most vulnerable to radiation, the treatment of skull base tumors involving them is challenging. In this study simulation plans by multi-beam (MB) intensity-modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT), both with the flattened beam (FB) and flattening-filter-free beam (FFF), were compared in terms of covering of the target and sparing of the optic pathways. Materials and methods Treatment planning was simulated by MB-IMRT with FB and FFF and by 2-rotational VMAT with FB and FFF in three cases of skull base meningioma [volume of the planned target volume (PTV; PTV margin=2 mm except for overlapping area with optic pathways or brainstem): 8.6 ml, 34.6 ml, and 55.3 ml respectively], which were treated previously by multi-fractionated MB-IMRT [45 Gy/18 fx. (fraction) with 7-, 6-, and 5-beam] using a conventional Novalis (BrainLAB, Tokyo, Japan) planned by iPlan (BrainLAB, Tokyo, Japan). In all three cases, the optic pathways were adjacent to the lesion. The reference CT with contouring data set of target volumes [gross tumor volume (GTV) and PTV] and OARs (organs at risk) was transferred from iPlan to Eclipse (Varian Medical Systems, Tokyo, Japan). In this study, hypofractionated radiation therapy by 30 Gy/5 fx. was designed; 95% dose (28.5 Gy/5 fx.) was prescribed to D95 (dose to 95% volume of PTV). Conformity index (CI), homogeneity index (HI, D5/D95), D[0.1 ml] (dose to 0.1 ml) for optic pathways, and D[1 ml] for brainstem and eyes, and V[20 Gy] (volume delivered with 20 Gy or more/5 fx.) of the whole brain were evaluated. Results The indices did not differ between FB and FFF, in either MB-IMRT or VMAT. Between MB-IMRT and VMAT, the indices were similar. The mean dose of PTV and HI was a little larger with MB-IMRT than with VMAT. D[0.1 ml] of the optic pathways and D[1 ml] of the ipsilateral eye were smaller with VMAT in all three cases. D[1 ml] of the brainstem was smaller with VMAT in two cases, though it was similar in one case. Conclusion Based on our findings, VMAT with FFF might be the optimal method to treat cases of skull base meningioma involving optic pathways. However, further studies involving more cases are required to arrive at a conclusive verdict.