Renal Endometriosis: An Unusual Presentation of a Common Disease

Endometriosis is endometrial glands and stroma outside the uterine cavity. It occurs commonly in the pelvic organs, especially the ovary. The incidence is 8% to 10% in women of reproductive age. Urinary tract endometriosis represents only 1.2% of all cases and usually occurs in the bladder and ureter. Renal endometriosis accounts for less than 1% od urinary tract involvement and often presents with flank pain, hematuria, or renal mass and is commonly misdiagnosed on imaging as renal cell carcinoma. We report a case of a 45-year-old female with no significant history who presented with hematuria and flank pain. MRI showed a left upper pole multicystic renal lesion, 3.1 × 3 × 2 cm, with intervening parenchymal enhancement. The patient underwent a laparoscopic left partial nephrectomy. Macroscopic examination revealed a tan, circumscribed, partially cystic lesion confined to the kidney. Microscopy showed columnar glandular epithelium and stroma composed of bland spindle cells with scant cytoplasm and smooth muscle metaplasia. CD10 and desmin highlighted endometrial stroma and smooth muscle, respectively, consistent with endometriosis. Renal endometriosis is extremely rare. The pathogenesis is unknown, but several theories exist, including embryonic derived from Wolffian and Mullerian duct remnants, migratory from retrograde menstruation, lymphovascular metastasis, direct extension, or suboptimal immune response resulting in ectopic endometrial implantation. When presenting as a mass, renal endometriosis is often misdiagnosed as RCC on imaging. Due to accuracy of current imaging techniques, a biopsy or FNA is not typical. However, in some patients with renal endometriosis, detailed clinical history can reveal previous pelvic endometriosis and cyclic flank pain associated with menses. There have been reports of such patients who received an FNA prior to surgery and avoided nephrectomy. Therefore, in the appropriate clinical setting, FNA/biopsy may allow for the better management of patients by preventing the surgical overtreatment of renal endometriosis.