SUN-474 An Unusual Case of Poorly Differentiated Thyroid Carcinoma with an Excellent Prognosis

Background: Poorly differentiated thyroid carcinoma (PDTC) is a rare and aggressive subtype with morphological/behavioral features between differentiated thyroid carcinoma (DTC) and anaplastic thyroid carcinoma (ATC). Clinical case: A 43-year-old female presented with 3 cm right thyroid mass noted on US neck. FNA biopsy showed undifferentiated carcinoma, large cell type. Additional immune-stains were suggestive of ATC. Pre-surgery non-stimulated thyroglobulin (NSTG) was 311 (RR 0–55 ng/dl). Pathology post-total thyroidectomy with bilateral level VI lymph node dissection showed a 3.2 x 2.5 x 2.5 cm carcinoma with vascular and capsular invasion. Most of the mass consisted of very atypical pleomorphic cells, mitosis was difficult to find. The tumor did not show the widely invasive-destructive pattern commonly seen in ATC. An adjacent differentiated component showed predominantly follicular pattern and was described as dedifferentiated follicular carcinoma. All lymph nodes were negative for metastatic disease. Post-surgery NSTG was <0.2 (RR <0.1ng/ml as athyreotic), stimulated TG was 2.22 with negative TG antibodies. Four months later, she received 193.5 mCi radioactive iodine (RAI) therapy. The post-therapy scan showed no Iodine-131 avid uptake in neck or distant metastasis. Neck imaging and TG levels done periodically showed no structural or biochemical evidence of recurrence. Currently the patient is cancer-free for 14 years since diagnosis with no need for additional therapies. Discussion: PDTC accounts for 1–15% of all thyroid cancers. Although PDTC is rare, it is a clinically significant histological diagnosis as it represents the main cause of death from non-anaplastic follicular cell-derived thyroid carcinoma. The Turin proposal published in 2007 suggested three criteria for the diagnosis of PDTC which included the pattern of growth and high-grade features. PDTC presents more frequently with locally invasive extra-thyroidal disease, metastasis to regional lymph nodes and distant organs compared to DTC. Despite the capacity to have RAI uptake, there has been no evidence of significant improvement in survival due to tumor heterogeneity in differentiation. Recent data suggest that age more than 45 years, tumor size more than 4cm, extra-thyroidal extension, higher pathological T stage, positive margins, and distant metastasis predict worse prognosis. Conclusion: Our patient showed an excellent response to therapy in spite of having PDTC with positive margins. We hypothesize that this could be likely due to young age at the time of diagnosis, early detection of tumor while it was localized in the thyroid without distant metastasis as well as heterogeneity in the tumor with differentiated cells that are responsive to RAI. We conclude that with early detection, timely surgery, and adjuvant therapy, excellent prognosis can be achieved in patients with PDTC.