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Sensitivity of Fractionated Lung IMPT Treatments to Setup Uncertainties and Motion Effects

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S Dowdell

S Dowdell*, C Grassberger , H Paganetti , Massachusetts General Hospital & Harvard Medical School, Boston, MA


SU-E-T-292 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: Evaluate the sensitivity of intensity-modulated proton therapy (IMPT) lung treatments to systematic and random setup uncertainties combined with motion effects.

Methods: Treatment plans with single-field homogeneity restricted to ±20% (IMPT-20%) were compared to plans with no restriction (IMPT-full). 4D Monte Carlo simulations were performed for 10 lung patients using the patient CT geometry with either ±5mm systematic or random setup uncertainties applied over a 35 x 2.5Gy(RBE) fractionated treatment course. Intra-fraction, inter-field and inter-fraction motions were investigated. 50 fractionated treatments with systematic or random setup uncertainties applied to each fraction were generated for both IMPT delivery methods and three energy-dependent spot sizes (big spots - BS σ=18-9mm, intermediate spots - IS σ=11-5mm, small spots - SS σ=4-2mm). These results were compared to a Monte Carlo recalculation of the original treatment plan, with results presented as the difference in EUD (ΔEUD), V₉₅ (ΔV₉₅) and target homogeneity (ΔD₁-D₉₉) between the 4D simulations and the Monte Carlo calculation on the planning CT.

Results: The standard deviations in the ΔEUD were 1.95±0.47(BS), 1.85±0.66(IS) and 1.31±0.35(SS) times higher in IMPT-full compared to IMPT-20% when ±5mm systematic setup uncertainties were applied. The ΔV₉₅ variations were also 1.53±0.26(BS), 1.60±0.50(IS) and 1.38±0.38(SS) times higher for IMPT-full. For random setup uncertainties, the standard deviations of the ΔEUD from 50 simulated fractionated treatments were 1.94±0.90(BS), 2.13±1.08(IS) and 1.45±0.57(SS) times higher in IMPTfull compared to IMPT-20%. For all spot sizes considered, the ΔD₁-D₉₉ coincided within the uncertainty limits for the two IMPT delivery methods, with the mean value always higher for IMPT-full. Statistical analysis showed significant differences between the IMPT-full and IMPT-20% dose distributions for the majority of scenarios studied.

Conclusion: Lung IMPT-full treatments are more sensitive to both systematic and random setup uncertainties compared to IMPT-20%.

Funding Support, Disclosures, and Conflict of Interest: This work was supported by the NIH R01 CA111590.

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