LBODP099 A Young Patient With Undifferentiated And Advanced Medullary Thyroid Carcinoma With Negative RET - CASE REPORT

: A Young Patient With Undifferentiated And Advanced Medullary Thyroid Carcinoma With Negative RET – CASE REPORT INTRODUCTION: Medullary Thyroid Carcinoma (MTC) is a rare and aggressive form of thyroid cancer that arises from C cells, representing 5% of all thyroid malignancy. The biochemical characteristic of MTC is the elevation of serum calcitonin (CTN), which aids in the initial diagnosis and surveillance of this disease. Another important, but less specific marker in the follow-up is the carcinoembryogenic antigen (CEA). Although, it is observed in the Undifferentiated MTC that there is a disproportion between the expected values of CEA and CTN. CASE REPORT: A previously healthy 16 years old female patient was hospitalized for the investigation of cervical discomfort, involuntary weight loss and progressive back pain for 9 months. She had a BMI of 14.9kg/cm2, a palpable thyroid nodule measuring approximately 2. 0cm and painful cervical lymph node enlargement. Laboratory tests showed CTN 79.1pg/mL, CEA 4999.42ng/mL added with an independent PTH hypercalcemia. Thyroid US showed a solid nodule with lobulated contours, heterogeneous echotexture, predominantly hypoechogenic, with its longest axis parallel to the skin, measuring approximately 2.4×0.9×0.9 cm, located in the lower middle third of the right thyroid lobe (TI -RADS 4) added to multiple lymph nodes inthe anterior cervical area (in agreement with the neck CT). Regarding the staging Chest CT showed a metastasis suggestive nodulation; Cranial CT, bone scan and total spine MRI showed evidence of several infiltrative nodular formations with heterogeneous enhancement affecting the skullcap, sacrum, iliac bones and multiple vertebral bodies. Sacral bone biopsy was performed giving us the diagnose of a metastatic Medullary Thyroid Carcinoma (MTC). A FineNeedle Aspiration (FNA) of the thyroid nodule and cervical lymph node showed calcitonin levels of 31.1pg/mL added with positive immunohistochemistry for chromogranin,synaptophysin, CD56 and Calcitonin in a focal manner, confirming the hypothesis of Undifferentiated CMT. Genetic testing was performed by next-generation sequencing of the genes ATM, ATR, CDKN2A, EGLN1, FH, HRAS, KIF1B, KMT2D, MAX, MDH2, MERTK, MET, NF1, PIK3CA, RET, SDHA, SDHAF2, SDHB, SDHC, SDHD, TMEM127, TP53 (including promoter), VHL, which results were negative for all pathogenic variants, probably pathogenic or VUS therefore being considered a sporadic MTC. Patient was referred for cervical radiotherapy and started Vandetanib. CONCLUSION: Serum CTN is the main diagnostic and surveillance marker of MTC. CEA is another marker that must be taken into account. We report the case of a young patient diagnosed with metastatic RET negative MTC with disproportionate values of CTN and CEA, which suggests its undifferentiated feature, revealing the importance of joint assessment of these two markers. Presentation: No date and time listed