Pencil Beam Scanning Proton GRID Therapy of Bulky Tumors: A Feasibility Study.

Abstract

PURPOSE/OBJECTIVE(S)\nTo evaluate a novel technique for spatially fractionated (GRID) therapy of bulky tumors using proton pencil beam scanning (PBS) and to compare dosimetry to a photon arc (VMAT) technique.\n\n\nMATERIALS/METHODS\nPBS and VMAT GRID plans were generated for five patients with bulky tumors. GRID targets were arranged along the long axis of gross tumor, spaced at 2 cm and 3 cm apart, and treated with a prescription of 15 Gy in a single fraction. PBS plans utilized two-beam multiple-field optimization with robustness evaluation. Dosimetric parameters including peak-to-edge ratio (PEDR), ratio of dose to 90% of the valley to dose to 10% of the peak (VPDR (D90/D10)), and volume of normal tissue receiving at least 5 Gy (V5) and 10 Gy (V10) were calculated.\n\n\nRESULTS\nGross tumor volume values ranged from 826-1468 cc. PEDR for PBS was higher than for VMAT at 7.2 ± 2.51 versus 2.4 ± 0.61 (2-cm spacing, P\u202f=\u202f0.02), and 8.4 ± 2.00 versus 2.8 ± 0.37 (3-cm spacing, P\u202f=\u202f0.01), respectively. VPDR (D90/D10) for PBS was lower than for VMAT at 0.034 ± 0.031 versus 0.41 ± 0.12 (2-cm spacing, P\u202f=\u202f0.004), and 0.008 ± 0.007 versus 0.33 ± 0.08 (3-cm spacing, P\u202f=\u202f0.002), respectively. Normal tissue V5 was lower for PBS than for VMAT at 343.2 cc ± 176.4 cc versus 917.6 cc ± 459.0 cc (2-cm spacing, P\u202f=\u202f0.03), and 250.3 cc ± 114.7 cc versus 520.7 cc ± 181.1 cc (3-cm spacing, P\u202f=\u202f0.02), respectively. Normal tissue mean dose was lower with PBS than with VMAT at 0.20 Gy ± 0.15 Gy versus 0.92 Gy ± 0.41 Gy (2-cm spacing, P = 0.03), and 0.17 Gy ± 0.11 Gy versus 0.82 Gy ± 0.33 Gy (3-cm spacing, P = 0.02), respectively.\n\n\nCONCLUSION\nPBS GRID for bulky tumors may result in improved dosimetry when compared to a VMAT approach. This PBS technique may be useful in bulky tumors that have a deep-seated location, complex shape, or adjacent critical organs.