The Role of Lung Lobes in Radiation Pneumonitis and Radiation-Induced Inflammation in the Lung: A Retrospective Study.

Objective: We examined the relative response to radiation of the upper lung lobes (UL) versus lower lung lobes (LL) of normal lung tissue using normalized [F]-fluorodeoxyglucose (FDG) uptake per radiation dose received per lung voxel in patients treated with either photons or protons and tested for correlation of the radiation response with clinical pneumonitis.

Methods: Seventy-five patients (photon (n = 51) or proton (n = 24)) treated for esophageal cancer from November 1, 2003 to May 15, 2011 who received restaging FDG-positron emission tomography (PET) imaging 1 to 3 months after chemoradiation were selected. UL and LL were contoured using the major fissure as the boundary, with the right middle lobe being included in the right UL structure. Pneumonitis toxicity was scored using the Common Terminology Criteria for Adverse Events, version 4.0 based on the consensus of 5 clinicians.

Results: LL had a higher mean dose (15.6 Gy vs. 10.4 Gy, p<0.001), higher mean standard uptake value (SUV) (0.78 vs. 0.56, p=0.001) and SUV in low dose regions (0.80 vs. 0.66 for 10 to 20 Gy, p=0.001), and lower mean dose response (0.015 vs. 0.019, p=0.003) compared to the UL. The mean dose ratio of UL vs. LL (p < 0.001), and SUV in the region of lung receiving 0-10 Gy (p=0.04), but not the dose response ratio of UL vs. LL (p=0.53) correlated with symptomatic pneumonitis.

Conclusion: Upper lung lobes had a greater pulmonary metabolic radiation response than lower lung lobes. Greater dose to UL relative to LL and higher SUV in the low dose region (10-20 Gy) on post-treatment PET correlated with symptomatic pneumonitis.