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Comparison of Full Intensity Modulated and Single Field Uniform Dose Proton Beam Scanning Lung Treatments

S Dowdell

S Dowdell1*, C Grassberger1,2, H Paganetti1, (1) Department of Radiation Oncology, Massachusetts General Hospital & Harvard Medical School, Boston, MA, (2) Center for Proton Radiotherapy, Paul Scherrer Institute, Villigen, Switzerland

TU-A-108-8 Tuesday 8:00AM - 9:55AM Room: 108

Purpose: To determine differences in motion interplay effects between single-field uniform dose (SFUD) and intensity modulated proton therapy (IMPT) treatments for lung patients.

Methods: SFUD and IMPT treatment plans were generated for 10 lung cancer patients. The spot spacing was set to 0.7σ for all treatment plans and all plans used 2 fields delivered to the planning target volume (PTV). Treatment plans required that at least 95% of the PTV and 99% of the ICTV received the prescription dose. The ICTV was defined as the IGTV+8mm and covered the CTV in all 10 phases of the 4DCT. SFUD plans require each field homogeneously, within a given tolerance, cover the PTV, whilst IMPT has no such constraint.
Monte Carlo simulations were performed using the detailed timing structure of the proton fields and the patient 4DCT for 2.5Gy(RBE) and 12Gy(RBE) fractions using 2 different spot sizes (σ ~8-17mm and ~2-4mm). Simulations were performed on the different phases of the patient 4DCT. The 4D dose distribution was generated via deformable image registration, using the 50% phase as the reference. Degradation in the dose distributions due to motion and interplay effects was quantified via equivalent uniform dose (EUD), V₉₅ and dose homogeneity.

Results: All values are quoted as percentages of the dose per fraction. For 2.5Gy(RBE) fractions, the average EUD loss due to interplay is 3.7+/-3.6% and 4.4+/-3.3% for IMPT and SFUD respectively for the larger spot size. For the smaller spot size, the average EUD loss for IMPT and SFUD treatments is 12.2+/-10.5% and 15.1+/-12.2% respectively. The average EUD loss for 12Gy(RBE) fractions is within 1% of the 2.5Gy(RBE) values. No significant differences between the SFUD and IMPT plans were observed for V₉₅ and target dose homogeneity.

Conclusion: No significant difference in motion effects was observed between SFUD and IMPT treatments.

Funding Support, Disclosures, and Conflict of Interest: This project was supported by the National Cancer Institute Grant No. R01 CA111590

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