Feasibility of Spot-Scanning Proton Arc Fractionated Radiosurgery to the Post-operative Cavity for Brain Metastases

Abstract

To determine dosimetric outcomes of Spot-Scanning Proton Arc (SPArc) fractionated radiosurgery to the post-operative cavity in brain metastases when compared to intensity modulated radiation therapy (IMRT) and intensity modulated proton therapy (IMPT) techniques. A previously treated post-operative brain metastasis cavity was re-planned using 3-field robust optimized IMPT and SPArc techniques. 5-field IMRT was planned according to institutional standards with a frameless setup. Robust optimization parameters were set to ±3.5% range and ±1mm setup uncertainties in x,y,z directions (21 worst-case-scenarios optimization). The treatment volume was prescribed to 25 Gy [RBE] in 5 fractions to 100% of volume for all plans. Multiple parameters were used to compare the plans including conformity index (CI) [total volume covered by 25Gy/target volume covered by 25Gy], gradient index (GI) [V50%RX/V100%RX], mean brain dose, V1Gy, V2Gy, V5Gy and V12Gy. The volume of the post-operative cavity was 73.12 cc. IMPT was superior to IMRT in all metrics except CI. SPArc had a similar CI as IMRT and was superior in all other metrics. When compared to IMPT, SPArc demonstrated an improved V12 and mean brain dose, consistent with a lower GI achieved with this technique, although did result in a higher low dose volume. A breakdown of the dosimetric parameters is located in Table 1. Proton therapy offered better dosimetric outcomes when compared to IMRT in treatment of post-operative cavity of brain metastases using fractionated radiosurgery. SPArc offered additional advantages of improved CI and GI. Further work is warranted to determine the clinical feasibility of SPArc.Abstract 3725; Table 1Comparison of various fractionated radiosurgery parameters for the three techniquesSPArcIMRT3-field IMPTConformity Index (ICRU)1.341.381.75Gradient Index2.093.092.73Mean brain dose (Gy [RBE])3.295.813.82V1Gy (cc)711.31032.6370.5V2Gy (cc)408.9830.4325.4V5Gy (cc)202.1446.9262.1V12Gy (cc)110.2196.4191.9SPArc = spot-scanning proton arc. IMRT = intensity modulated radiation therapy. IMPT = intensity modulated proton therapy. Open table in a new tab