Four-dimensional Monte Carlo simulations demonstrating how the extent of intensity-modulation impacts motion effects in proton therapy lung treatments
- Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts 02114 (United States)
- Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts 02114 and Centre for Proton Therapy, Paul Scherrer Institut, 5232 Villigen-PSI (Switzerland)
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{sub 20%}). These plans were compared to full IMPT (IMPT{sub 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{sub 20%}, 4D-IMPT{sub full}) with those of a 3D Monte Carlo simulation (3D-IMPT{sub 20%}, 3D-IMPT{sub full}) upon the planning CT using the equivalent uniform dose (EUD), V{sub 95} and D{sub 1}-D{sub 99}. The effects in normal lung were quantified using mean lung dose (MLD) and V{sub 90%}.Results: For 2.5Gy(RBE), the mean EUD for the large spot size is 99.9%± 2.8% for 4D-IMPT{sub 20%} compared to 100.1%± 2.9% for 4D-IMPT{sub full}. The corresponding values are 88.6%± 8.7% (4D-IMPT{sub 20%}) and 91.0%± 9.3% (4D-IMPT{sub full}) for the smaller spot size. The EUD value is higher in 69.7% of the considered deliveries for 4D-IMPT{sub full}. The V{sub 95} is also higher in 74.7% of the plans for 4D-IMPT{sub full}, implying that IMPT{sub full} plans experience less underdose compared to IMPT{sub 20%}. However, the target dose homogeneity is improved in the majority (67.8%) of plans for 4D-IMPT{sub 20%}. The higher EUD and V{sub 95} suggests that the degraded homogeneity in IMPT{sub 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{sub 20%} and IMPT{sub 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{sub full} and IMPT{sub 20%} are only likely to be clinically significant for patients with large (>20 mm) motion amplitudes.
- OSTI ID:
- 22220301
- Journal Information:
- Medical Physics, Vol. 40, Issue 12; Other Information: (c) 2013 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA); ISSN 0094-2405
- Country of Publication:
- United States
- Language:
- English
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