SAT540 A Case of Simultaneous Medullary Thyroid Cancer (MTC) And Papillary Thyroid Cancer (PTC) In a Patient With Germline RET Mutation

Disclosure: N. Vaghasia: None. G. Jaiswal: None. Introduction: There are only handful of cases reported of concurrent MTC and PTC in a patient with germline RET proto-oncogene mutation. Case: A 41 year old female with no PMH presented to Endocrine clinic for evaluation of recently diagnosed RET mutation. Patient’s nephew passed away at 5 weeks of age from abnormal organogenesis of heart and brain. Genetic testing showed germline RET mutation. Subsequently her brother and his two children was tested positive for RET mutation. Patient underwent genetic testing which was positive for RET mutation. Patient was found to be heterozygous for p.V804M (c.2410G>A) mutation reported as moderate risk mutation in the RET gene. The result was consistent with an increased risk for RET-related cancers. No family history of MTC, pheochromocytoma or primary hyperparathyroidism was identified. Patient was asymptomatic on presentation and no previous symptoms concerning for any RET mutation related disease. Her physical examination was normal. Patient underwent testing for pheochromocytoma and hyperparathyroidism which was unremarkable. Patient’s calcitonin level was also normal. Patient had US thyroid done to evaluate for nodule and was found to have 1.62 x 0.97 x 1.50 cm mixed solid cystic, hypoechoic, irregular margins and punctate echogenic foci right sided nodule. No abnormal cervical lymph nodes. Patient underwent FNA of the suspicious nodule which was reported as benign based on Bethesda classification. Patient’s calcitonin level was undetectable <0.2 pg/ml (0.0-5.0). Patient underwent total thyroidectomy and on surgical pathology had three papillary microcarcinomas, classic type, 0.4 cm (right lobe), 0.3 cm (left lobe) and 0.1 cm (right lobe). Patient had medullary microcarcinoma 0.2 cm on left lobe. Background thyroid with C-cell hyperplasia. No lymphovascular invasion, no extrathyroidal extension and margins cleared. Discussion: Association of simultaneous MTC-PTC have been reported in few case reports in patient with germline RET mutation. There has been no risk factors or specific patient phenotype identified in previous cases to suggest any association between the two. One important feature in our case is the presence of highly suspicious thyroid nodule which on FNA was benign. This raise an important question on whether the co-existence of MTC and PTC in germline RET mutation decreases sensitivity of FNA. Also it is yet unknown if specific mutation like in our case p.V804M (c.2410G>A) is associated with higher incidence of multi-tumor pathology or not. Finally, our patient had normal calcitonin level despite having MTC which is unusual. Conclusion: Coexistence of MTC and PTC in germline RET mutation is an uncommon association and more information is required to understand common features, risk factors, phenotype and cancer aggressiveness in these patients. At this point it cannot be concluded that the association is a mere coincidence. Presentation Date: Saturday, June 17, 2023