Secretory activity in medullary thyroid carcinoma: a cytomorphological and immunocytochemical study

Medullary thyroid carcinoma (MTC) is a relatively rare thyroid malignancy of C-cell origin that secretes calcitonin. Although its varied cytomorphologic features are well described in literature, very little is mentioned about the morphologic manifestation of its secretory activity. This study, based on nine fine needle aspiration (FNA) samples from eight MTC patients, is an attempt to present the varied cytomorphologic features suggesting secretory activity in MTC as observed in Papanicolaou and MGG stained FNA smears and correlate them with the immunocytochemical (ICC) staining for calcitonin performed on FNA smears and the serum calcitonin values. The average number of cells in these nine samples was as follows: oval/triangular/plasmacytoid (56.7%), small round (23.6%), spindle-shaped (12.7%), and miscellaneous (7.1%). The cytomorphological features suggesting secretory activity, viz., fine cytoplasmic vacuoles, azurophillic granules, marginal vacuoles, and intracytoplasmic lumina (ICL) with secretions were present in eight, eight, five, and six samples, respectively. Material likely to be amyloid, based on morphological features, was present extracellularly in three samples and both intracellularly and extracellularly in six samples. Immunocytochemically, all the nine samples stained for calcitonin and all the three stained for chromogranin showed positive cytoplasmic reaction in the neoplstic cells. The background amyloid (in six samples), the coarse cytoplasmic granules (in two samples), and the contents of ICL (in one sample) were found to be positively stained for calcitonin. The intracytoplasmic secretory material appeared to be diffusing out of some cells both in the routine MGG stained smears and in the smears stained for calcitonin. Histopathology reports of seven samples in six patients confirmed the cytodiagnosis of MTC in all. Baseline serum calcitonin values in three cases and postoperative serum calcitonin levels during follow-up in three others were high. Thus, our study highlighted the morphological manifestations of secretory activity in MTC and the nature of secretory material as calcitonin, supported by immunocytochemical staining and serum calcitonin level.