International medullary thyroid carcinoma grading system: an Indian tertiary care centre experience

Medullary carcinoma (MTC) is a rare neuroendocrine thyroid neoplasm. The international medullary thyroid carcinoma grading scheme (IMTCGS), which has prognostic significance, has been introduced recently. The present study graded MTC cases using the IMTCGS and evaluated it in our study cohort.

All MTC thyroidectomy cases over 6 years were evaluated. Low-grade (LG) and high-grade (HG) were compared. Survival analysis included overall survival (OS), loco-regional free survival and distant metastasis free survival (DMFS).

Of 32 cases, 31.25% were HG and 68.75% LG. The mean age was 44.0 years and M:F ratio 1:1.146. HG patients were older and had tumour cells with high-grade nuclear features and prominent nucleoli and showed distant metastasis. Necrosis was found more in patients with high grade nuclear features. There was discordance between the high Ki67 (60%) and increased mitotic activity (20%). Univariate survival analysis revealed poor DMFRS and OS in the cohorts with high grade, Ki67 > 5% and coagulative necrosis. The multivariate cox regression analysis showed IMTCGS significantly associated with overall survival (HR 28.30, p = 0.009) and DMFS (HR 15.70, p = 0.02).

This is the first Indian study evaluating IMTCGS, a very simple and convenient grading system that can be readily used in any tertiary health care centre. IHC for Ki 67 should mandatorily be done irrespective of the low mitotic activity on the HPE and necrosis should be diligently searched in cases with high-grade nuclear morphology. HG MTC cohorts were associated with poor OS as well as DMFRS.