Surgical treatment is the first therapeutic option in patients affected with medullary thyroid carcinoma (MTC). However, cure-rates are often low due to the high frequency of loco-regional metastases and recurrences. Therefore, post-operative hyper-calcitoninemia is a common feature in CMT. Despite these findings, traditional imaging techniques are often unable to localize tumour foci. In the last years the availability of new morpho-functional techniques might offer new chances for localization of occult MTC.
The aim of this study was to evaluate the ability of postoperative FDG-PET and OctreoScan to detect residual or recurrent MTC after surgery in comparison with conventional imaging techniques. Twenty-three patients had persistently elevated and progressively increasing calcitonin levels after standard surgical treatment for MTC. Conventional imaging techniques (including neck ultra-sonography and computed tomography of neck and chest in all patients, magnetic resonance and bone scintigraphy when appropriated) detected tumour foci in 7 of 23 patients (30%), identified as neck or mediastinal lymph node metastases in all but one with lung metastases. FDG-PET and OctreoScan were performed in 18 patients each detecting tumour foci in 8 (44%) and 5 (28%), respectively. Among the 16 patients undergone to both conventional imaging techniques, FDG-PET or OctreoScan, MTC foci could be identified with at least one of the imaging procedures in 8 patients (50%). New MTC foci, previously unrecognized by conventional radiologic techniques, were found at the FDG-PET in 3 patients who underwent surgery and consequent histological confirmation of MTC relapse. In all cases but one OctreoScan positivity was found in MTC foci previously revealed by the conventional imaging techniques.
In conclusion, in patients with occult loco-regional MTC which persists after surgery, FDG-PET shows the highest diagnostic performance. Its combined use with conventional imaging techniques makes possible to detect residual tumour in 50% of the patients with occult postoperative MTC.