Hydronephrotic ectopic pelvic kidney simulates sacral metastasis from breast cancer.

A 57-year-old woman with a history of a right modified radical mastectomy after neoadjuvant chemotherapy for stage IIIB infiltrating ductal breast carcinoma presented for follow-up bone scan. Initial inspection was suspicious for sacral metastasis, but on closer review the absence of renal activity in the left flank was noted. Correlation with CT scan confirmed a hydronephrotic ectopic kidney overlying the sacrum.