SUN-362 - A Thyroid Cancer without A Thyroid Tumor: BRAF V600E PTC Emerging from A Cystic Level II Node

Benign thyroid tissue inclusions within cervical lymph nodes (“intranodal thyroid tissue”) are an underrecognized phenomenon that can mimic metastatic thyroid cancer, and rare cases of carcinoma arising in ectopic thyroid tissue have been reported. Cystic cervical nodal disease further complicates diagnosis because fine-needle aspiration (FNA) cytology may be nondiagnostic; thyroglobulin (Tg) in FNA washout can improve detection, particularly in cystic nodes.

A 73-year-old man with primary hyperparathyroidism underwent bilateral inferior parathyroidectomy with left thyroid lobectomy for an intrathyroidal parathyroid; pathology was benign. Later that month, MRI performed for right-hand numbness incidentally revealed a 1.7×0.8 cm cystic right level II neck lesion, and CT further characterized it as a new low-density cystic mass. Thyroid ultrasound demonstrated two sub-centimeter, predominantly cystic right-lobe nodules below ACR TI-RADS biopsy thresholds. Ultrasound-guided FNA of the cystic neck mass was nondiagnostic, but Tg washout was markedly elevated (35,875.2 ng/mL).

He subsequently underwent completion right thyroidectomy with central and right lateral neck dissection (levels II–IV). The right thyroid lobe showed follicular nodular disease with calcification and no carcinoma. One of 20 lymph nodes was essentially completely replaced by a 1.1 cm cystic papillary thyroid cancer (PTC), with no extranodal extension and BRAF V600E positivity. Early postoperative ultrasound showed no structural disease. A diagnostic radioiodine whole-body scan showed focal uptake anterior to the prior left thyroidectomy bed/thyroglossal duct region, consistent with residual thyroid tissue. The multidisciplinary tumor board recommended surveillance without adjuvant radioactive iodine treatment. On follow-up through late 2025, serial neck ultrasounds remained negative, and TG was < 0.2 ng/mL with negative TG antibodies, consistent with an excellent response.

This case highlights an incidentally detected solitary cystic level II PTC in which cytology was nondiagnostic, but FNA-Tg washout was decisive. In the absence of an identifiable intrathyroidal primary across 2-staged thyroid resections, the presentation is compatible with malignant transformation of ectopic thyroid tissue within a cervical lymph node. Recognition of intranodal thyroid inclusions and judicious use of FNA-Tg washout can expedite diagnosis and help tailor therapy; in single, isolated cervical nodal disease without extranodal extension and no primary tumor identified in the thyroid gland, surveillance without radioactive iodine treatment may be appropriate.

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