Diagnostic capabilities of PET/CT with <sup>18</sup>F-DOPA in biochemical recurrence of medullary thyroid carcinoma: a retrospective study

Abstract

INTRODUCTION: Medullary thyroid carcinoma (MTС) is a rare neuroendocrine malignant neoplasm of the thyroid gland. In most cases, sporadic MTС is diagnosed at late stages due to the absence of specific symptoms. The main method of treatment of MTС is surgical — thyroidectomy and in most cases cervical lymphodissection. Current international guidelines suggest the use of basal calcitonin levels and cancer embryonic antigen (CEA) as markers of biochemical recurrence. In biochemical recurrence of the disease, it is advisable to search for tumor foci regardless of the level of cancer markers. Positron emission tomography combined with computed tomography (PET/CT) has the highest sensitivity and specificity for searching for local recurrence and distant metastases. Anatomical imaging methods (computed tomography (CT), magnetic resonance imaging (MRI)) have suboptimal sensitivity and specificity in detection of a recurrent tumor.OBJECTIVE: To explore the diagnostic capabilities of PET/CT with 18F-DOPA in patients with biochemical recurrence of medullary thyroid carcinoma, depending on the concentration of calcitonin in blood plasma.MATERIALS AND METHODS: To evaluate the diagnostic capabilities of PET/CT with 18F-DOPA in patients with biochemical recurrence of medullary thyroid carcinoma, 81 PET/CT studies were analyzed in patients after thyroidectomy. In most cases (76/81), at the time of the study, patients had elevated basal calcitonin levels (>10 pg/ml, including in 52/81 cases >150 pg/ml), which corresponded to a biochemical recurrence of the disease. In 6 cases, the calcitonin level was <10 pg/ml. PET/CT results were processed by visual image analysis, measurement of a semi-quantitative indicator of the maximal standardized level of radiopharmaceutical accumulation normalized to lean body mass (SUVlbm max) and the total volume of PET-positive tumor tissue (metabolic tumor volume).RESULTS: In 41 studies, foci of pathological accumulation of 18F-DOPA were identified, corresponding to recurrent tumor foci. DOPA-negative results in patients with elevated calcitonin levels were regarded as false negative. The highest sensitivity of the method was achieved at calcitonin levels >150 pg/ml. A noticeable positive correlation was found between the concentration of basal serum calcitonin, the number of pathological foci of 18F-DOPA hyperaccumulation and the total metabolic volume of tumor tissue. In most cases of biochemical recurrence, the PET method was superior to the CT method in detecting recurrent tumor foci.CONCLUSION: PET/CT with 18F-DOPA is the most informative method for molecular and structural imaging in patients with biochemical recurrence of medullary thyroid carcinoma. The results of the study directly correlate with the level of basal calcitonin in the blood.