ODP494 Mediastinal Ectopic Thyroid Papillary Carcinoma with NTRK3 Fusion, Presenting with Thoracic Spine Metastasis
Authors: Khthir Rodhan 
Published: November 2022

Abstract

Background
Ectopic thyroid tissue may be encountered anywhere from the base of the skull to the lower neck. It is rarely seen in the mediastinum. Thyroid cancer in ectopic thyroid tissue is a very rare entity. We report a patient with papillary thyroid cancer arising from upper mediastinal ectopic thyroid tissue. The patient presented with thoracic spine metastasis with cord compression. Molecular testing showed SQSTM1-NTRK3chromosomal rearrangement.

Case Presentation
The patient is a 67 -year-old female, presented with upper back pain. She reported no weakness or sensory changes. MRI showed suspected metastatic disease to the second and third thoracic vertebrae (T2 and T3). Therewasepidural extension of tumor with cord compression and severe stenosis at T2. She underwent laminectomy and decompression surgeryat T1-T3 level. Final pathology report showed Metastatic thyroid carcinoma with papillary features. She underwent external beam radiation to the affected spines. CT chest abdomen and pelvis showed 3. 0×2.8×2.3 cm soft tissue mass in the left superior mediastinum extending into the supraclavicular region. FDG-PET scan showedHypermetabolic foci in the upper mediastinum with SUV max of 25. Fine needle Aspiration (FNA) of the upper mediastinal mass was consistent with papillary thyroid cancer. Molecular testing from the FNA sample using Thyroseq V3 showed SQSTM1-NTRK3 chromosomal rearrangement. Total thyroidectomy was performed; however, the upper mediastinal mass was not resected due to proximity to vascular and neuronal structures. Pathology of the resected thyroid was benign. Recombinant TSH stimulatedI-123 scan showed intense uptake in the neck area, most likely represents remnant thyroid tissue. No clear uptake in the mediastinal mass or spine metastasis. She underwent removal of the upper mediastinal mass with lymph node dissection at a referral center. Pathology showed soft tissue involved by papillary thyroid carcinoma with focal tall cell features, forming a 4×2.5×2.5 cm mass. Surgery was followed by ablation with 100 mci of radioactive iodine (I-131). Thyroglobulin level decreased from a baseline of 15272 ng/ml to 1857 ng/ml, post-surgery, external beam radiation and radioactive iodine ablation.

Conclusion
This case highlights the possibility of developing thyroid cancer in ectopic thyroid tissue and the unique challenges in the diagnosis and management of such rare disease. Our patient has very aggressive disease presentation that required multimodality treatment including thoracic spine decompression, total thyroidectomy, mediastinal ectopic thyroid cancer resection, external beam radiation to the affectedspine area, and high dose radioactive iodine treatment. The use of molecular diagnostics is very important to improve the outcome in such patient. The tumor has SQSTM1-NTRK3 chromosomal rearrangement. This rearrangement causesconstitutive activation of the NTRK3 in the Ras-Raf-MEK-ERK Pathway. This fusioncan be targeted by medications like Larotrectinib and Endtrectinib.
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