ODP446 Where's the Nodule?: Atypical Presentation of an Occult Metastatic Papillary Thryoid Carcinoma

Papillary thyroid Carcinoma (PTC) with apparent distant metastasis but lacking a primary thyroid cancer on pre-operative ultrasonographic examination, is referred by some as occult PTC. In most cases, primary carcinomas are identified after careful pathologic examination of the specimen but in some rare cases the primary tumors were not detected. Hereby, we describe the rare case of a patient with metastatic PTC without initial visualization on imaging of primary tumor location along with inconsistent pathology results between fine needle aspiration (FNA) biopsy and gross specimen pathology results. A 60-year-old female with past medical history of Diabetes Mellitus type 2 and Hypothyroidism presenting for follow-up visit after being found with neck nodules on head and neck CT scan. Thyroid and neck Ultrasound (US) were performed finding three right irregularly shaped hypoechoic lymph nodes (LN's) measuring > 1 cm, concerning for metastatic disease, but no thyroid nodules were noted. Patient denies family history of thyroid cancer, and no head and neck radiation exposure. Physical examination did not reveal goiter or palpable nodules. Laboratory results: TSH: 2.95 and Total T4: 9.32, both within normal range. Cervical LN's FNA biopsy was performed, yielding results consistent with PTC tall cell variant. Chest CT scan showed suspicious lesions for pulmonary micrometastasis. Total thyroidectomy was performed and gross pathology findings showed an infiltrating, follicular variant PTC with multifocal tiny primary tumors of the thyroid, with lesions measuring up to 0.2 cm, dimension which explains why nodules could not be identified on imaging studies. LN's FNA biopsy results and thyroid pathology did not correlate. Thyroid gross pathology was clearly PTC with follicular variant, infiltrating a small area away from greatest dimension tumor, with cells with apparent dedifferentiation to tall cell cytologic features. Due to high risk of recurrence, radioactive iodine therapy was recommended as adjunctive therapy for metastatic disease and eradication of residual neck disease to be started with levels of TSH>30 mU/ml. Postopertative Thyroglobulin level (Tg) was adequate < 0. 01 ng/ml but unreliable based on above normal range antithyroglobulin antibody > 1000 IU/ml, which increases risk of recurrence. Whole body scan also done showing bilateral lung metastasis and small neck metastatic disease. Repeated postoperative thyroid US also demonstrated a suspicious cystic lymph node in right central neck for which FNA with saline washout of aspirate for Tg measurement was ordered. Metastatic thyroid carcinoma without primary tumor after extensive thyroid sampling is a rare but existent phenomenon that can be encountered in daily practice. Genotyping or immunostaining mestastatic lesions could help direct the Pathologist to the source of the metastasis. Patients with a higher risk of recurrence are monitored more aggressively as it is believed that early detection of recurrent disease offers the best opportunity for effective treatment. Presentation: No date and time listed