Triple-negative breast cancer with occult metastasis in parathyroid and ectopic thyroid adenomas: complete remission after chemotherapy

What happened, in summary

This 67-year-old Tunisian woman had triple-negative breast carcinoma of no special type and underwent radical mastectomy for invasive breast cancer staged pT3N1. No metastasis had been identified at the original breast cancer diagnosis, and adjuvant chemotherapy was planned. Before chemotherapy, however, blood testing found isolated hypercalcemia, with corrected calcium of 2.85 mmol/L, and very high parathyroid hormone of 1943 pg/mL. Ultrasound and a 99mTc-sestamibi scan pointed to an ectopic parathyroid adenoma near the right lower parathyroid region and upper mediastinum. Imaging also found a 25 × 22 mm jugulo-carotid mass, and the right thyroid lobe was multinodular. She underwent exploratory cervicotomy with right lobo-isthmectomy, right inferior jugulo-carotid dissection, and right paratracheal dissection. A 30 mm smooth mass was completely removed. Frozen section confirmed parathyroid adenoma, but final pathology found something more complex: the parathyroid adenoma contained multifocal deposits of large atypical cells, and the jugulo-carotid nodule was an ectopic benign thyroid adenoma also infiltrated by atypical cells. This meant the breast cancer had spread in an unusual and clinically silent way, discovered only because the hypercalcemia workup led to neck and mediastinal surgery. Immunohistochemistry supported metastatic breast carcinoma within these adenomas. The atypical cells were CK7-positive and negative for chromogranin, synaptophysin, ER, HER2, CK20, and GATA3, separating them from the parathyroid and thyroid tissue around them. After this occult metastatic spread was identified, she received systemic chemotherapy. As of December 2025, she remained in complete remission after 1 year of follow-up. Her course highlights that metastatic breast cancer can occasionally be found inside otherwise benign endocrine adenomas.