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Four‐dimensional Monte Carlo simulations demonstrating how the extent of intensity‐modulation impacts motion effects in proton therapy lung treatments
Author(s) -
Dowdell Stephen,
Grassberger Clemens,
Paganetti Harald
Publication year - 2013
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.4829500
Subject(s) - proton therapy , monte carlo method , nuclear medicine , medicine , lung cancer , homogeneity (statistics) , intensity (physics) , physics , radiation therapy , radiology , mathematics , optics , statistics
Purpose: To compare motion effects in intensity modulated proton therapy (IMPT) lung treatments with different levels of intensity modulation.Methods: Spot scanning IMPT treatment plans were generated for ten lung cancer patients for 2.5Gy(RBE) and 12Gy(RBE) fractions and two distinct energy‐dependent spot sizes (σ ∼8–17 mm and ∼2–4 mm). IMPT plans were generated with the target homogeneity of each individual field restricted to <20% (IMPT 20% ). These plans were compared to full IMPT (IMPT full ), which had no restriction on the single field homogeneity. 4D Monte Carlo simulations were performed upon the patient 4DCT geometry, including deformable image registration and incorporating the detailed timing structure of the proton delivery system. Motion effects were quantified via comparison of the results of the 4D simulations (4D‐IMPT 20% , 4D‐IMPT full ) with those of a 3D Monte Carlo simulation (3D‐IMPT 20% , 3D‐IMPT full ) upon the planning CT using the equivalent uniform dose (EUD), V 95 and D 1 ‐D 99 . The effects in normal lung were quantified using mean lung dose (MLD) and V 90% .Results: For 2.5Gy(RBE), the mean EUD for the large spot size is 99.9% ± 2.8% for 4D‐IMPT 20% compared to 100.1% ± 2.9% for 4D‐IMPT full . The corresponding values are 88.6% ± 8.7% (4D‐IMPT 20% ) and 91.0% ± 9.3% (4D‐IMPT full ) for the smaller spot size. The EUD value is higher in 69.7% of the considered deliveries for 4D‐IMPT full . The V 95 is also higher in 74.7% of the plans for 4D‐IMPT full , implying that IMPT full plans experience less underdose compared to IMPT 20% . However, the target dose homogeneity is improved in the majority (67.8%) of plans for 4D‐IMPT 20% . The higher EUD and V 95 suggests that the degraded homogeneity in IMPT full is actually due to the introduction of hot spots in the target volume, perhaps resulting from the sharper in‐target dose gradients. The greatest variations between the IMPT 20% and IMPT full deliveries are observed for patients with the largest motion amplitudes. These patients would likely be treated using gating or another motion mitigation technique, which was not the focus of this study.Conclusions: For the treatment parameters considered in this study, the differences between IMPT full and IMPT 20% are only likely to be clinically significant for patients with large (>20 mm) motion amplitudes.