A Rare Mimicker: Renal Endometriosis and Its Diagnostic Pitfalls—Review of Reported Cases

Background/Objectives: Primary renal endometriosis with extrapelvic locations of endometriosis lesions is deemed extremely rare. Therefore, the aim of this review was to assess the current literature on renal endometriosis, encompassing a summary of recorded living-patient cases, imaging and pathological features, chosen therapeutic approaches, and patient outcomes. Methods: We performed a narrative review with a systematic identification and presentation of published cases of histopathologically proven kidney endometriosis. A literature search was carried out across PubMed, Scopus, and Web of Science for relevant studies, i.e., case reports and series that met predefined inclusion and exclusion criteria specified by the modified PECOS framework. Results: A total of 19 publications reporting a total of 20 reported cases were included in the in-depth analysis. The noted condition affected women of reproductive age, with a mean age of 37.4, with the most common presenting symptoms being lumbar or flank pain, gross hematuria, abdominal pain, and tenderness. Most were single-mass lesions detected on the right kidney, with a mean largest diameter of 6.7 cm. Most were CT-characterized as either hyperattenuating lesions, septate lesions with foci of calcifications and soft tissue components, or areas of necrosis or cystic change. As CT presented features that would not allow the exclusion of malignant kidney tumors, most patients were treated surgically with nephrectomy. Diagnosis was confirmed by histopathology of recorded endometrial glands and stroma. Conclusions: The diagnosis of these lesions is exceptionally challenging because of their nonspecific symptoms, the rare association with the menstrual cycle, the rarity of dysmenorrhea, and the occasional coexistence of ovarian endometriosis, with no distinct endometriosis-specific clinical and imaging pattern emerging.