Small-Sized Papillary Thyroid Carcinoma Mimicking a Mediastinal Lymphoma in an Adolescent With Hashimoto's...

Papillary thyroid carcinoma (PTC) is an uncommon neoplasm in pediatric and transitional age groups, characterized by a biologically more aggressive local behavior than in adults. Although its coexistence with Hashimoto's thyroiditis (HT) is widely documented, the impact of this chronic inflammation on massive lymphatic dissemination remains a subject of debate. We present the case of a 17-year-old male who presented with cervical lymphadenopathy and computed tomography imaging suggestive of primary superior mediastinal involvement (prevascular and pretracheal). Initial fine-needle aspiration biopsy (FNAB) of the right cervical lymph node revealed atypical follicular cells with nuclear grooves, consistent with metastatic PTC. The patient underwent total thyroidectomy combined with modified bilateral radical neck dissection and central compartment neck dissection. Definitive histopathological examination confirmed a multifocal PTC (classic type and follicular variant) measuring 1.1 cm in its largest diameter, with lymphatic invasion on a background of HT (pathological staging pT1b pN1b pMx). Compartmental lymph node analysis revealed a massive metastatic burden, with 13 of 39 lymph nodes exhibiting bilateral involvement, highlighted by a 4.0 cm macrometastasis in the right middle jugular chain. Following surgery, adjuvant radioactive iodine (I-131) ablation therapy was administered after thyroid hormone withdrawal, and subsequent post-ablation follow-up laboratory testing confirmed severe endogenous hypothyroidism with a markedly elevated thyroid-stimulating hormone (TSH) level. This clinical case highlights the striking clinicopathological discrepancy that can exist between the small size of a primary thyroid tumor and the potential for disproportionate lymphovascular dissemination in the adolescent population. When encountering bulky mediastinal and cervical masses that mimic lymphomas, PTC must be considered a critical differential diagnosis, justifying comprehensive preoperative mapping and an aggressive compartmental surgical approach.