Background-aim: The aim of this retrospective study was to evaluate the role of 11C-Methionine PET/CT (MET-PET) in the restaging of adenoid cystic carcinoma (ACC) of head and neck, after primary treatments (surgery, radio-therapy or both). Methods: Four-teen patients (7 female, 7 male, mean age 57.2 years, range 23–79 years) underwent MET-PET, between May 2013 and March 2016, within 3.6 months from the last treatment (range 1–7; only one patient performed MET-PET after 14 months). Four out of four-teen (29%) patients were treated with Hadron or Proton Therapy or external radiotherapy (EBRT) only, three (21%) with surgery and seven (50%) with combined therapies (surgery + Radiotherapy (Hadron, Proton or EBRT)). All the findings were validated by means of imaging and/or clinical follow up (FU) for a mean time of 8.6 months (range 2–20 months). MET-PET were evaluated visually by two experienced nuclear medicine physicians. MET-PET results were classified as positive, negative or doubt. SUVmax was calcu- lated but not used to achieve the final diagnosis. Results: MET-PET was positive in nine (64%) of four-teen patients, negative in four (29%) and doubt in one case (7%). All the eight positive cases were confirmed to be true positives (TP) by clinical and/or radiological FU. MET-PET Positive Predictive Value (PPV) was 100% for the localization of local relapse and for the detection of lateral-cervical lymph-nodes metastasis. In six patients MET-PET was performed twice, the first before Hadron/Proton therapy, the second after the end of treatment. In four out-of six patients the second MET-PET has shown persistence of uptake of the radio tracer inside the PTV (radiotherapy planning target volume), with a SUVmax range between 4 and 8.8, compatible with minimum residual disease. In the other two cases MET-PET has demonstrated the appearance of an area of uptake outside the PTV: a lateral cervical lymph-node and a vertebra. These two last findings were diagnosed to be local and distant metastasis respectively. Four cases were negative on MET-PET, two of these were True Negative (TN) and two were False Negative (FN) (Negative Predictive Value 50%). The first FN case had local progression of disease after 1 year of FU. We consider this as typical long-term evolution that char- acterized this disease. The second FN patient showed faint uptake misinterpreted as post-actinic reaction but consistent with progres- sion of disease, as confirmed 3 months later by FU. MET-PET was doubt in one patient, were it was not possible to distinguish between post surgical changes and persistence of disease. Further investiga- tions are underway to clarify this aspect. Finally we would like to underline that, in our small population, we didn’t note any corre- lation between time elapsed from the end of primary treatment and the accuracy of MET-PET. Conclusions: MET-PET showed an excellent PPV (100%) and a low NPV (50%) to restage ACC. This is partially in contrast with what is generally observed in others head and neck tumors. Further studies are needed in order to assess the potential clinical impact of MET- PET to restage ACC.
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