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Objective  To analyze the pattern of failure in relation to pre-treatment [ 18 F] FDG-PET/CT uptake in head and neck squamous cell carcinoma (HNSCC) patients treated with definitive radio-chemotherapy (RT-CHT).Methods and materials  From 2012 to 2016, 87 HNSCC patients treated with definitive RT-CHT, with intensity modulated radiation therapy with simultaneous integrated boost, underwent pre-treatment [ 18 F] FDG-PET/CT (PET pre ), and MRI/CT for radiotherapy (RT) planning purposes. Patients with local recurrence, received [ 18 F] FDG-PET/CT, (PET rec ) at the time of the discovery of recurrence. In these patients, the metabolic target volume (MTV), MTV pre  and MTV rec  were segmented on PET images by means of an adaptive thresholding algorithm. The overlapping volume between MTV pre  and MTV rec  (MTV pre&rec ) was generated and the dose coverage of MTV rec  and MTV pre&rec  was checked on the planning CT using the D99 and D95 dose metrics. The recurrent volume was defined as: ‘‘In-Field (IF)’’, “Marginal recurrence” or ‘‘Out-of-Field (OF)’’ if D95 was respectively equal or higher than 95%, D95 was between 95 and 20% or the D95 was less than 20% of prescribed dose.    Results  We found 10/87 patients (11.5%) who had recurrence at primary site. Mean MTV pre  was 12.2 cc (4.6–28.9 cc), while the mean MTV rec  was 4.3 cc (1.1–12.7 cc). Two recurrences resulted 100% inside MTV pre , 4 recurrences were mostly inside (61–91%) and 4 recurrences were marginal to MTV pre  (1–33%). At dosimetric analysis, five recurrences (50%) were IF, 4 (40%) marginal and one (10%) OF. The mean D99 of the overlapping volumes MTV pre&rec  was 68.1 Gy (66.5–69.2 Gy), considering a prescription dose of 70 Gy to the planning target volume (PTV).    Conclusion  Our study shows that the recurrence may originate from the volume with the highest FDG-signal. Tumor relapse in the high-dose volume support the hypothesis that an intensification of the dose on these volumes could be further assessed to prevent local relapse.

Correlation of [18F] FDG-PET/CT with dosimetry data: recurrence pattern after radiotherapy for head and neck carcinoma