Endocrine Abstracts

Introduction: Collision tumors in the thyroid gland are uncommon entities constituting <1% of all thyroid tumors. These are defined as two distinct histological types of tumors arising concurrently within the thyroid, with the combination of Medullary Thyroid Cancer (MCT) and Papillary Thyroid Cancer (PTC) being the most common occurrence. PTC and MTC differ in management strategies and clinical courses, highlighting the importance of accurate diagnosis.

Patients and Methods: We conducted a retrospective analysis of patients diagnosed with MCT and PTC collision tumors in the thyroid, who were under follow-up from 2015 to 2021 at our tertiary care department. Among all cases of MTC and PTC during this period (177 PTC and 139 MTC, respectively), collision tumors represented 5.03% of MTC and 3.95% of PTC cases. We identified 7 cases (1 male, 6 females) with an average diagnosis age of 50.57 years (range 25-71 years).

Results: The left thyroid lobe was more frequently involved in both PTC (4/7, 57.1% of cases) and MTC (5/7, 71.4% of cases). 57.1% (4/7) of PTC, and 42.8% (3/7) of MTC were <1 cm, including two instances each of multifocal PTC and MTC. On average, MTC tumors, mean size 2.2 cm (0.8-7.7 cm), were more than twice the size of PTC tumors, mean size 0.95 cm (0.15-1.6 cm) - pT1 in three patients, pT2 in one patient, pT3 in three patiens, classified according to the TNM staging at time of surgery, however, all corresponding to pT1 TNM/AJCC 8th edition. The average follow-up period was 7.7 years. Genetic testing for germline RET proto-oncogene mutation was assesed in five patiens: two wild-type cases, two cases with RET polymorphisms linked to hereditary or sporadic MTC, and one case with a mutation in codon 634 (MEN 2 syndrome). Four patients underwent radioactive iodine (RAI) therapy (30-100 mCi); the other 3 patients with microcarcinoma (pT1a) did not receive RAI. Six patients maintained TSH levels between 0.5-2 mUI/l. By the last follow-up, all PTC patients showed excellent biochemical response (mean thyroglobulin levels 0.5 ng/ml, under 119.5 micrograms average dose of levothyroxine), and all patients had normalized calcitonin levels postoperatively except one (with the largest MTC tumor at 7.7 cm and two lymph node metastases, calcitonin levels were 12x upper normal limit). Histopathology analysis revealed focal chronic thyroiditis in two cases.

Conclusion: Collision tumors in the thyroid are not only rare but might also be underreported due to variability in sampling methods, particularly of the apparently unaffected lobe. The incidence rate in our series is similar with that reported in other case series. The aggressiveness of these co-occurring tumors appears similar to that of singular occurrences. However, recognizing this condition is critical for its therapeutic and prognostic significance.