Abstract #1406506: Hidden in Plain Sight: Incidental Diagnosis of Metastatic Papillary Thyroid Microcarcinoma without Radiologically-Apparent Thyroid Tumor

Abstract

Papillary thyroid cancer (PTC) is considered the most common form of thyroid cancer. Papillary thyroid microcarcinomas (PTMC) are defined as papillary carcinomas that are <1 cm and are often identified on surgical pathology but may be missed by ultrasound imaging. We present the case of a 24-year-old woman who initially presented with a right lateral neck lump. This had been noted for 3-4 years and she had been told this was likely a lipoma. Therefore, no workup was done until it began to grow during pregnancy. Following pregnancy, she underwent excision with findings of a level 5B lymph node 2.1 x 2.1 x 0.9 cm with pathology consisting of PTC, follicular variant. Neck ultrasound done in an imaging center and in our clinic did not reveal thyroid nodules or abnormal appearing lymph nodes. She was referred for total thyroidectomy with central neck and limited right neck dissection. Surgical pathology showed right superior lobe 0.5 cm PTMC, follicular variant with negative surgical margins. Seven out of twenty-four lymph nodes were positive in the right central compartment and in levels 3 and 4. The largest metastatic lesion measured 0.9 cm. Solid tumor profiling assay identified a NTRK3 fusion: ETV6(4)-NTRK3(14) mutation. She did well following surgery and had RAI ablation following thyroid hormone withdrawal. Follow up ultrasound 1 year later showed no signs of structural recurrence and thyroglobulin levels have remained undetectable. She is currently on 150 mcg of levothyroxine and doing well clinically. PTMC are identified with increased frequency due to improved ultrasound imaging and FNA techniques and may represent over 40% of PTC. PTMC are generally considered to carry a favorable prognosis. However, cervical lymph node metastasis is reported in 10-17% of patients with PTMC undergoing total thyroidectomy. There has been growing interest in NTRK fusion mutations in PTC. NTRK mutations have been previously described with higher frequency in pediatric populations but are increasingly recognized in adult cohorts. NTRK fusion mutations are identified in up to 13% of PTMC and some studies suggest higher risk of metastasis. This case is unusual in that nodal metastasis were present even without a sizable primary thyroid tumor. Although more studies are needed to elucidate the clinical and prognostic significance of NTRK fusion mutations in PTMC, their response to TRK inhibitors may represent a future pathway for treatment.