Performing Calcitonin Immunocytochemistry on an Additional ThinPrep Slide in Fine-Needle Aspiration Diagnosis of Medullary Thyroid Carcinoma

Abstract

This study reviewed the institutional experience of performing calcitonin immunostain on an additional ThinPrep slide in fine-needle aspiration (FNA) diagnosis of medullary thyroid carcinoma (MTC). Thyroid FNA cases with MTC suspected or included in the differential diagnosis during cytologic evaluation and calcitonin immunostain performed were retrieved and reviewed. Calcitonin immunostain was performed in 132 cases with 41 positive, 81 negative, and 10 indeterminate results. All calcitonin-positive cases had a cytologic diagnosis of MTC while all calcitonin-negative cases were cytologically classified as non-MTCs except for two cases suspicious for MTC. In 10 cases with an indeterminate calcitonin result, diagnoses of non-MTC and suspicious for MTC were rendered in 6 and 4 cases, respectively. Histopathologic follow-up was available in 85 (64%) cases. All cytologically diagnosed MTC cases were confirmed on histopathology. In 3 MTC cases with an indeterminate calcitonin result, 1 case was misclassified cytologically as follicular neoplasm. The calculated sensitivity, specificity, positive predictive value, and negative predictive value of calcitonin immunostain were all 100% for diagnosing MTC. Our study demonstrates the feasibility of performing calcitonin immunostain on an additional ThinPrep slide. Calcitonin immunocytochemistry is a valuable adjunct test for FNA diagnosis and differential diagnosis of MTC.