Incidental Detection of Aggressive HER2-Positive Breast Cancer on ^99mTc-Sestamibi Parathyroid Scintigraphy

A 71-y-old woman with primary hyperparathyroidism underwent ^99mTc-sestamibi SPECT/CT for preoperative localization. The study showed a left inferior parathyroid adenoma and, on extended field-of-view review, an intensely ^99mTc-sestamibi-avid 1.5-cm lesion in the left breast. Diagnostic mammography and ultrasonography classified the lesion as Breast Imaging Reporting and Data System category 5, prompting a core biopsy that revealed Nottingham grade 3 invasive ductal carcinoma of no special type (human epidermal growth factor receptor 2 [HER2] overexpression [3+]), with a Ki-67 index of 80%. After sentinel lymph node mapping, the patient underwent wide local excision and sentinel lymph node biopsy. Final pathology demonstrated a 1.9-cm grade 3 carcinoma with associated ductal carcinoma in situ and a 1-mm nodal micrometastasis identified in a sentinel lymph node. She completed adjuvant chemotherapy, radiotherapy, and HER2-targeted therapy, with no recurrence at 2-y follow-up. This case emphasizes deliberate review of the complete hybrid SPECT/CT field of view during parathyroid imaging.