MON-406 - Collision of Malignancies: Synchronous Lung Adenocarcinoma and Metastatic Papillary Thyroid Carcinoma in A Single Pulmonary Mass Thirteen Years after Initial Diagnosis

Introduction:
Synchronous multiple cancers are uncommon and may complicate diagnosis and management. Differentiated thyroid carcinoma is typically indolent; while distant metastases might involve the lungs, late structural recurrence is unusual. We describe a patient with papillary thyroid carcinoma (PTC) treated with thyroidectomy and radioiodine iodine (RAI) who presented after a decade with synchronous primary lung adenocarcinoma and metastatic PTC within the same pulmonary mass.

Case presentation:
A 61-year-old woman with follicular variant PTC treated with total thyroidectomy and two courses of RAI in 2011 showed no residual iodine-avid disease on post-therapy whole-body scintigraphy (WBS). Serum thyroglobulin (Tg) was undetectable in 2012-2013 but became detectable at 0.6ng/mL in 2014, rising to 2.0-3.0 ng/mL in 2015 with negative WBS and persistently negative anti-Tg antibodies. She was lost to follow-up until 2024, when a 5.1-cm cavitary left lower lobe lung lesion was incidentally detected. Bronchoscopy suggested lung adenocarcinoma, and lobectomy confirmed lung adenocarcinoma with a separate nodule with PTC. Comprehensive genomic profiling (TEMPUS) demonstrated clonal independence, supporting 2 synchronous malignancies: lung adenocarcinoma and metastatic PTC. Serum Tg was 109.2 ng/mL. She received adjuvant chemotherapy for lung adenocarcinoma, then therapeutic RAI after thyrogen-stimulated WBS demonstrated multiple bilateral pulmonary nodules. She remains under surveillance with stable disease.

Discussion:
This case highlights the importance of maintaining PTC in the differential diagnosis of pulmonary nodules in patients with prior PTC, even years after apparent stability. Rising Tg levels with negative WBS may indicate indolent metastatic disease, revealing limitations of scintigraphy in detecting slow-growing or low-iodine-avid PTC. Molecular diagnostics were essential to confirm clonal independence and avoid misclassification as primary lung metastases. As precision oncology advances, genomic profiling may play an expanding role in the diagnosis and management of complex oncologic presentations.

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