MON-336 Diagnostic Utility of I 123 Hybrid SPECT/CT in the Characterization of Retropharyngeal Masses

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Abstract Disclosure: M.I. Khan: None. S.I. Sherman: None. J.E. Langer: None. L.A. Loevner: None. C. Schmalbach: None. N. Vijayvargia: None. D. Weinberg: None. S. Rubin: None. J.W. Lazor: None. R. Viterbo: None. A. Kutikov: None. R. Satriale: None. K. Chernukha: None. N. Dioufa: None. A. Talwar: None. J.Q. Yu: None. G. Weinstein: None. Background: Differentiated thyroid cancer (DTC) rarely (0.43-5%) metastasize to retropharyngeal lymph nodes (RPLN, Nodes of Rouviere) in the retropharyngeal space (RPS). We describe the case of a patient with low grade uterine leiomyosarcoma and remote history of DTC who was identified to have increasing suppressed Thyroglobulin (Tg) and sub centimeter right level IV lymph nodes on neck ultrasound. Further evaluation with contrast CT neck identified right RPLN. I123 SPECT/CT was suggestive of DTC due to uptake of I123 in right RPLN. Suppressed Tg was undetectable six weeks after transoral robotic surgery (TORS) of right RPLN. Clinical case A 73 year old female with history of mixed papillary and follicular carcinoma of thyroid underwent total thyroidectomy and right lateral neck dissection in 1982. Patient was treated with radioactive iodine (RAI) in 1982 and 2005. During surveillance, suppressed Tg increased from 0.7 ng/ml in April 2018 to 1.6 ng/mL in May 2024. Serum TgAb levels were undetectable <0.9 IU/mL. Neck ultrasound identified two (0.5 cm and 0.7 cm) right level IV neck lymph nodes. Contrast CT neck and 18F-FDG PET CT identified hyperenhancing, PET avid (SUV 16.8) right RPLN 2.5x1.7x2 cm mass with multiple coarse dystrophic calcifications. NM I123 Dx WBS showed intense uptake in left cystic parotid mass. No clear focal lesion was identified with neck uptake value of 2.3%. I123 SPECT/ CT showed uptake in the right RPLN higher than adjacent soft tissue. Ultrasound guided fine needle aspiration biopsy of left parotid 1.2 cm cystic lesion was indeterminate. CT guided fine needle aspiration biopsy of the right RPLN was positive for papillary thyroid cancer. Patient underwent TORS of right RPLN with pathology consistent with 2.7 cm papillary thyroid cancer involving the soft tissue with focal osseous metaplasia and psamommatous calcifications. Tumor was present at peripheral tissue edge. Suppressed Tg decreased to <0.1 ng/ml six weeks after TORS. Three months after TORS of right RPLN, no iodine avid lesion was identified on NM I123 Dx WBS with neck uptake value of 1.4%. Discussion: RPS neck masses are a diagnostic challenge. RPS is bordered by visceral fascia of pharynx anteriorly, prevertebral fascia posteriorly, carotid sheath laterally, the skull base superiorly, and the hyoid bone inferiorly. Management options for metastatic RPLN in PTC include surgical resection, RAI, and EBRT. NM I123 Dx WBS identifies iodine avid tissue in retropharyngeal space. In case of presence of non-thyroid iodine avid tissue obscuring uptake of I123 in thyroid cancer metastases; I123 SPECT/CT may identify iodine avid masses in RPS, guiding prompt diagnosis and management. Presentation: Monday, July 14, 2025

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