¹⁸F-fluorodeoxyglucose positron emission tomography to assess response after radiation therapy in anaplastic thyroid cancer

Aim: To assess the interest of ¹⁸F-fluorodeoxyglucose positron emission tomography (¹⁸F-FDG PET/CT) to evaluate the tumor response after radiotherapy (RT) in anaplastic thyroid cancer (ATC) patients. Methods and Materials: 92 patients were treated for ATC at our institution from 1987 to 2012, out of which 64 (70%) received an aggressive multimodal treatment and 28 (30%) a palliative treatment. In the multimodal treatment group, curative-intended surgery, chemotherapy, and RT were delivered in 35 (55%), 59 (92%), and 56 (88%) patients. The maximum standardized uptake value (SUVmax) was determined in tumor (T), nodes (N) and metastases (M) in each available ¹⁸F-FDG PET/CT. Results: The median follow-up was 3.2 years. The 1-year actuarial overall survival (OS) was 18% (median: 5.2 months) in the entire population and 27% (median: 7 months) in the multimodal treatment group. In the multivariate analysis, RT, surgery, and pre-RT chemotherapy independently predicted for OS, with HRs respectively of 0.1, 0.3, and 0.5. Quantification of FDG uptake with SUVmax was assessable in 26 (40%), 19 (30%), and 25 (39%) of ¹⁸F-FDG PET/CT performed initially (prior to any treatment), prior to RT, and after RT, respectively. Mean SUVmax significantly decreased in T (p < 0.001), but not in N (p = 0.1) and M (p = 0.3) during the assessment period, which might be related to the local effect of RT. Comparing pre- and post-RT ¹⁸F-FDG PET/CT, the T mean relative SUVmax decrease was lower (23 ± 54%) in the 4 patients that had a local relapse (LR) as compared with others in the 12 others patients (62 ± 33%; p = 0.3). A relative SUVmax decrease inferior to 20% significantly predicted for LR (p = 0.02). Conclusion: The prognosis of ATC patients remains dismal despite an aggressive multimodal treatment. Although our results were not significant, ¹⁸F-FDG PET/CT could potentially serve as a surrogate marker of treatment response in ATC.