Impact of Treatment Modality on Post-Radiation Imaging in Post-Operative Head and Neck Cancers

Abstract

<h3>Purpose/Objective(s)</h3> Despite frequent dosimetric advantages to proton therapy over photon therapy in head and neck (H&N) cancers, there is concern for end of range biologic dose enhancement and subsequent increased mucosal toxicity with proton therapy. Herein, we evaluated differences in mucosal FDG avidity on post-treatment FDG PET-CTs between proton and photon therapy patients, as persistent activity is a surrogate for residual mucosal toxicity. <h3>Materials/Methods</h3> The cohort comprised ten proton and ten photon patients who received ≥40 Gy to a mucosal oropharyngeal site with curative intent after initial surgery and who remained loco-regional disease free ≥6 months post-treatment. All patients underwent follow-up PET-CT scans 3 months post-radiotherapy. Images centered over the treated primary were acquired 60 minutes after intravenous administration of 10 mCi of FDG. CT-based attenuation correction was applied to the data sets. The PET-CTs were fused to the planning CT using deformable registration using a commercial treatment planning system (TPS). Contours were generated in the deformed PET datasets using a 1.5 × Liver_SUV threshold, i.e., 50% above background; these uptake areas were later intersected with the mucosal CTV plus a 5 mm margin to create the region of interest (ROI). SUV maximum (SUV_MAX) values based on lean body mass were extracted for this ROI. <h3>Results</h3> The SUV_MAX values found in the post-treatment PET-CTs in the intersected ROI for the proton group ranged between 7.08 and 9.38, whereas in the photon group, SUV_MAX values ranged from 3.89 and 6.49. The likelihood of having SUV_MAX values above the 1.5 × Liver_SUV threshold was higher in the proton group (4 of 10 patients) compared to the photon group (2 of 10 patients). Clinically, an ulcer was visible in one proton patient at the area of FDG avidity. <h3>Conclusion</h3> Our results indicate that post-radiotherapy FDG avidity was more commonly observed in proton patients, which correlated with clinical mucosal toxicity.