ABSTRACT Background 18F-fluorodeoxyglucose-positron emission tomography/computed tomography (18FDG- PET/CT) is used in the clinical diagnosis and management of oncologic and inflammatory pathologies. It may also have utility in detecting tissue damage induced by radiotherapy (RT) used to treat various types of cancer. The aim of the present study was to use 18FDG-PET/CT to evaluate the effect of RT on the uptake of 18FDG by the cerebellum. Methods Thirty patients with head and neck cancer (HNC) were included in this retrospective study. The patients were treated with photon, proton, or combined photon/proton RT, in addition to chemotherapy. All patients received 18FDG- PET/CT imaging pre-treatment and 3 months post-treatment. The global mean standardized uptake value (SUVmean) of the cerebellum was determined for every patient by global assessment of 18FDG activity using OsiriX MD software. A two-tailed paired t-test was used to compare global SUVmean pre- and post-RT. Results The pre-treatment and post-treatment global SUVmean for the photon group were 5.26 and 5.51 (p: 0.42), respectively. As for the proton only group, the pre- and post-treatment global SUVmeans were 7.06 and 6.05, respectively. In the combined RT group, the pre- and post-treatment global SUVmeans were 6.14 and 6.19 respectively (p: 0.92). The differences between the pre- and post-treatment values failed to reach statistical significance for any of the treatment groups but it should be noted that there was a trend of increased 18FDG uptake in the cerebellum following photon therapy. This trend was not clear in the combined group. As for the proton group, p-value was not calculated as only two patients were included. Conclusion Although not statistically significant, the results showed an incremental increase in global SUVmean following treatment with photon RT likely reflecting the presence of mild radiation-induced inflammation in the cerebellum.