Efficacy of tangential irradiation with volumetric modulated arc therapy on scalp angiosarcoma using medical linac

Purpose: To increase the superficial dose and reduce the brain dose for radiotherapy of scalp angiosarcoma, we propose a novel irradiation technique of tangential irradiation volumetric modulated arc therapy (TI-VMAT). Methods: TI-VMAT and the conventional VMAT treatment plans for thirteen scalp angiosarcoma patients were created with a prescribed dose of 70 Gy. Each treatment was normalized to cover 95% of the planning target volume (PTV) with its prescribed dose. To realize TI-VMAT, an avoidance structure (AS) function was applied. AS was defined as a contour subtracted PTV by a certain space from the brain contour. TI-VMAT treatment plans for six different spaces between PTV and AS were developed and compared with the conventional VMAT treatment plan with respect to the following dosimetric parameters: homogeneity index (HI) and conformity index (CI) of the PTV, mean brain dose, and brain volume irradiated with 20% (V-20% [cc]), 40% (V-40% [cc]), 60% (V-60% [cc]), 80% (V-80% [cc]), and 100% (V-100% [cc]) of the prescribed dose. Results: HI and CI were comparable between TI-VMAT and the conventional VMAT, the mean brain dose for TI-VMAT with AS defined by a space of 2.0 cm and jaw tracking was 14.27 Gy, which was significantly lower than that for the conventional VMAT (21.20 Gy). In addition, dosimetric parameters such as V-20% [cc] were significantly suppressed compared to those for high doses. Conclusion: Our proposed irradiation technique TI-VMAT shows the potential to reduce radiation doses in the brain with maintaining higher dose coverage on the PTV.