Fractionated Lung IMPT Treatments

Treatment uncertainties in radiotherapy are either systematic or random. This study evaluates the sensitivity of fractionated intensity-modulated proton therapy (IMPT) lung treatments to systematic and random setup uncertainties. Treatments in which single-field homogeneity was restricted to within ±20% (IMPT 20% ) were compared to full IMPT (IMPT full ) for 10 patients with lung cancer. Four-dimensional Monte Carlo calculations were performed using patient computed tomography geometries with ±5 mm systematic or random setup uncertainties applied over a 35 × 2.5Gy(RBE) treatment course. Fifty fractionated courses were simulated for each patient using both IMPT delivery methods with random setup uncertainties applied each fraction and for 3 energy-dependent spot sizes (big spots, σ≈18-9 mm; intermediate spots, σ≈11-5 mm; and small spots, σ≈4-2 mm). These results were compared to Monte Carlo recalculations of the original treatment plan assuming zero setup uncertainty. Results are presented as the difference in equivalent uniform dose (ΔEUD), V 95 (ΔV 95 ), and target dose homogeneity (ΔD 1 -D 99 ). Over the whole patient cohort, the ΔEUD was 2.0 ± 0.5 (big spots), 1.9 ± 0.7 (intermediate spots), and 1.3 ± 0.4 (small spots) times more sensitive to ±5 mm systematic setup uncertainties in IMPT full compared to IMPT 20% . IMPT full is 1.9 ± 0.9 (big spots), 2.1 ± 1.1 (intermediate spots), and 1.5 ± 0.6 (small spots) times more sensitive to random setup uncertainties than IMPT 20% over a fractionated treatment course. The ΔV 95 is at least 1.4 times more sensitive to systematic and random setup uncertainties for IMPT full for all spot sizes considered. The ΔD 1 -D 99 values coincided within uncertainty limits for both IMPT delivery methods for the 3 spot sizes considered, with higher mean values always observed for IMPT full . The paired t-test indicated that variations observed between IMPT full and IMPT 20% were significantly different for the majority of scenarios. Significantly larger variations were observed in ΔEUD and ΔV 95 in IMPT full lung treatments in addition to higher mean ΔD 1 −D 99 . The steep intra-target dose gradients in IMPT full make it more susceptible to systematic and random setup uncertainties.