Purpose: Preclinical studies have measured an increase in the relative biological effectiveness (RBE) at the distal end of proton therapy beams, however, no clinical consequences have been reported. This study compares the radiation pneumonitis dose response in lungs that received a terminating proton beam with their contralateral lung. Methods: Thirty‐four patients treated with proton therapy for esophageal cancer who received restaging [18F]‐fluorodeoxyglucose positron emission tomography (FDG PET) were selected. Each received a proton field arrangement that included a lateral beam terminating in one lung. The pulmonary radiation inflammatory dose response was quantified for each lung separately as the slope of the FDG uptake versus dose using regression modeling. The dose‐response was compared within subject between the entrance and end‐range containing lungs using the related sample Wilcoxon signed rank test. Results: The spatial distribution of the regions that received between 1 and 10 Co‐60 Gy Equivalents (CGE) corresponded to the proton beam lateral and distal edges. The mean and range of SUV for the entire end range lung (0.65, 0.40 – 1.09) was lower than the entry lung (0.72, 0.40 – 1.17) (p=0.009). For the 1 to 10 CGE regions the dose response was higher in the end range lung (19.2 x 10‐ 2, range 0.8‐50 x 10‐2) versus the entry lung (10.2 x 10‐2, 0.03‐39 x 10‐2) (p < 0.001). Conclusions: This study found an enhanced radiation dose response in lungs that received radiation from the proton beam end‐range. This spatial variation in the dose response contradicts the prevailing isodose equals isoeffect assumption. This quantifiable enhancement represents a clinical manifestation of the increase in the RBE at the end range of proton beams.
ASJC Scopus subject areas
- Radiology Nuclear Medicine and imaging