Breast Cancer Metastasis to the Thyroid Presenting as a Collision Tumor With Papillary Thyroid Microcarcinoma and Oncocytic Adenoma: A Case Report

Thyroid metastases from breast cancer are uncommon, and their coexistence with primary thyroid neoplasms as collision tumors is even rarer, posing important diagnostic and therapeutic challenges.We report the case of a 57-year-old woman with hormone receptor-positive, HER2-negative breast cancer who developed disease recurrence 13 years after initial treatment, presenting with a cervical mass, persistent dry cough, and dysphagia.Evaluation revealed an intermediate-suspicion thyroid nodule according to the American Thyroid Association (ATA) classification, with successive fine-needle aspiration biopsies categorized as Bethesda III and IV.Right hemithyroidectomy demonstrated poorly differentiated metastatic breast carcinoma, confirmed by immunohistochemistry, coexisting within the same thyroid specimen with a 1.8-mm papillary thyroid microcarcinoma (PTMC) and a 7-mm oncocytic adenoma, constituting an exceptionally rare thyroid collision tumor.Staging studies subsequently revealed systemic metastatic disease, and treatment with ribociclib plus fulvestrant was initiated according to the phase III MONALEESA-3 regimen.This case highlights the possibility of very late recurrence of hormone receptor-positive breast cancer presenting as thyroid metastasis, illustrates the diagnostic complexity of thyroid collision tumors, and emphasizes the importance of considering metastatic disease in the differential diagnosis of thyroid nodules in patients with a history of breast cancer.Although current evidence does not support routine thyroid screening, this case raises the question of whether targeted thyroid evaluation in selected patients at high risk of late recurrence warrants further prospective investigation.