What happened, in summary

This 46-year-old woman had been managed for right invasive ductal breast carcinoma over the previous 2 years. Her breast cancer treatment included surgery, adjuvant chemotherapy, radiotherapy, and hormonal therapy. During staging investigations for the breast cancer, abdominal CT unexpectedly found a left renal mass. She had no back pain, no visible blood in the urine, and an unremarkable clinical examination, although urinalysis showed microscopic hematuria. CT described a well-defined, oval, multiseptated cystic mass in the mid-left kidney, measuring 50 × 69 mm and classified as Bosniak III. MRI upgraded the lesion to Bosniak IV, increasing concern for malignancy. The mass touched the superior segmental artery and renal vein but did not show vascular invasion. Because of its suspicious features and proximity to major vessels, partial nephrectomy was considered technically difficult, so she underwent laparoscopic total nephrectomy. Gross pathology showed a brownish multilobulated cystic mass measuring 7 × 5.5 × 4.6 cm, with multiple cystic spaces filled with clear fluid. Microscopy showed a multicystic tumor lined mainly by cuboidal epithelium, with stromal septa of variable thickness and no tumor aggregates in the fibrous septa. Immunohistochemistry showed PAX8 positivity in epithelial cells, progesterone receptor positivity in stromal cells, and estrogen receptor negativity. The final diagnosis was adult cystic nephroma, not breast cancer metastasis. She was discharged on postoperative day 4. As of December 2025, 1-year follow-up showed no symptoms and no radiologic signs of local recurrence or metastasis. The kidney finding was therefore managed as a separate renal tumor rather than a metastatic breast cancer event.