Dosimetric Comparison of Automated Non-Coplanar Volumetric-Modulated Arc Therapy and Intensity-Modulated Proton Therapy in Angiosarcoma of the Scalp

Abstract

Angiosarcoma of the scalp (AS) is a rare tumor and has often been treated by total scalp irradiation (TSI). TSI is a challenging technique because of the target close to the organs at risks (OARs), located in the skin surface, and helmet-shape of the target. We performed the dosimetric comparison of automated non-coplanar volumetric-modulated arc therapy (HA) and intensity-modulated proton therapy (IMPT) in TSI for AS. A planning study was performed on 20 patients with AS. The clinical target volume (CTV) 1 and CTV2 were defined as gross tumor volume with margin and entire scalp, respectively. For HA, planning target volume (PTV) 1 and PTV2 were defined as CTV1 and CTV2 with 5-mm margins, respectively. For IMPT, robust optimization was utilized which accounted for a 5-mm setup and a 3.5% range uncertainty and dose was prescribed to CTV1 robust and CTV2 robust. The prescription doses were 70 Gy and 56 Gy in 35 fractions to PTV1 (CTV1 robust) and PTV2 (CTV2 robust), respectively, using the simultaneous integrated boost technique. The HA plan was performed using a 6-MV photon beam machine and a 1-cm thick virtual bolus. The HA plan included three half non-coplanar and one full coplanar arc fields. The HA plan used collimator angles of 15°, 60°, 15°, and 120° in the beam with couch rotations of 0°, 45°, 90°, and 315°, respectively. The IMPT plan was performed using a compact proton beam machine dedicated to pencil beam scanning. The IMPT plan used gantry angles of 70°, 150°, 70°, 150°, and 30° in the beam with couch rotations of 0°, 0°, 180°, 180°, and 270°, respectively. The IMPT plan was optimized assuming a relative biological effectiveness of 1.1. The dose distribution and dosimetric parameters for each plan were evaluated. All plans met the goals within the acceptable range in target volume coverage, conformity, and homogeneity. Table shows the dosimetric parameters of OARs in HA and IMPT plans. The doses receiving 0.1 cc of the volume for hippocampus, optic passway, eye, and lens in the IMPT plan were significantly lower than those in the HA plan. The IMPT plan showed a significant reduction in the percentage of brain volume receiving at least 5 Gy (V5 Gy) compared to the HA plan, while the HA plan showed a significant reduction of V10 Gy - V60 Gy and mean brain dose compared to the IMPT plan. The HA plan provided a shorter beam-on time (184 ± 9 s) than did the IMPT plan (446 ± 49 s). The HA and IMPT plans demonstrated different strengths with respect to OARs sparing.