Quantitative evaluation of inflammatory response dynamics in the lung following proton and photon irradiation*

We analyzed lung density changes in lung cancer patients receiving stereotactic body radiation therapy with protons (SBPT) or photons (SBRT). Follow-up computer tomography (CT) scans were registered to pre-treatment scans using B-spline based deformable image registration. Dose response curves (DRC) were used to correlate the radiographic change in Hounsfield Units (HU) to the radiation dose and fitted using linear regression to provide a quantitative measure of normal lung response. CTs were also evaluated by a thoracic radiologist.
46 patients (23 SBPT/SBRT) were matched based on patient characteristics, quantitative assessment correlated highly with expert evaluation (p<10^-13,rho=0.6). The maximum response averages 3.8±3.0 (SBPT) versus 3.2±2.4 HU/Gy (SBRT). The lung response after SBRT significantly increases over the observation period (p=0.003), but not after SBPT, indicating an accelerated acute response after SBPT.
We demonstrate that quantitative normal lung response evaluation correlates well with established grading systems and differences in response dynamics after SBPT compared to SBRT, that warrant further investigation into their mechanisms of inflammation.