Several case studies have identified the diagnostic difficulty posed by renal endometriosis, particularly in its differentiation from a neoplasm. Conventional imaging modalities are limited in discriminating between endometriosis and a cystic malignancy (7). The majority of previously reported cases were indeed cystic or demonstrated a cystic component. Although a cystic component was suspected radiologically in our case, this was not found at gross inspection, with the inflammation and associated edema possibly accounting for this radiological impression.
Clinically, endometriotic lesions with a prominent smooth muscle component may give rise to a significant mass effect, which can mimic a malignancy on imaging studies. Several reports have identified that endometriosis has the potential to mimic a neoplasm, as the disease presents with multiple components in varying quantities, such as smooth muscle (8). Large amounts of fibromuscular tissue are more associated with deeply infiltrating endometriosis (9). The fact that renal endometriosis is exceedingly rare, accounting for 0.1-1% of all cases (3), coupled with the knowledge that lesions with a smooth muscle predominance are atypical and difficult to separate from a neoplastic process, contributed to the diagnostic dilemma in our case.
Pre-operative distinction from an angiomyolipoma on core biopsy can be difficult in lesions that have a prominent smooth muscle component, with endometrial glands in endometriosis and entrapped renal tubules in an angiomyolipoma both demonstrating positivity for PAX8. In this situation, CD10 and ER/PR immunohistochemistry, confirming endometrial stroma, is necessary. In addition, smooth muscle predominant angiomyolipomas may be negative for HMB45, especially in a core biopsy.
The right kidney was involved in our case, and in previous case reports where laterally is indicated, the majority of the lesions were also right-sided (3, 5, 7, 10). Additionally, the concurrence or history of other foci of endometriosis outside of the kidney is mentioned in only two of the thirteen previous cases. The curious right-sided predilection and the frequent absence of endometriosis elsewhere suggests that renal endometriosis may develop from a residual Mullerian remnant in or adjacent to the kidney, rather than through retrograde menstruation or a metaplastic process. The proximity of the paramesonephric duct to the mesonephros during embryogenesis may account for this phenomenon. During embryonic development, the urogenital ridges eventually differentiate into the kidneys, ureters, reproductive ducts and gonads (11). The same primitive coelomic epithelium also invaginates near the anterior aspect of the mesonephros (foetal kidney) and expands caudally to form the Mullerian ducts, which are the origin of the female reproductive tract (11). The intermediate mesoderm eventually develops into the kidneys and parts of the reproductive system.
Another possible hypothesis for the occurrence of endometriosis in this case is ectopic implantation of endometrial tissue, known as Mullerianosis. The embryonic rest theory proposes that cells of Mullerian origin within the peritoneal cavity may be induced to form endometrial tissue, which can result in endometriosis at various locations along the migration pathway of the Mullerian system (12). In one study investigating this theory, 4 out of 36 foetuses examined at post-mortem demonstrated the presence of misplaced endometrium in five different ectopic sites (13). This suggests that one cause of endometriosis may be the dislocation of primitive endometrial tissue outside of the uterine cavity during gestational development, as opposed to retrograde menstruation.
Such ectopic misplacement or fusion of tissue at embryogenesis is recognised in other areas of human biology. For example, adrenal cortical rests in the testes and testicular adnexa occur frequently, not just in exclusive cases of congenital adrenal hyperplasia (14). In addition, splenogonadal fusion is a recognised anomaly, which is characterised by congenital fusion between the spleen and testicular tissue, often presenting as a testicular mass. This specific congenital malformation is believed to occur due to the close proximity of the developing gonad and spleen during gestational development, resulting in an abnormal connection between these structures during early embryological development, which facilitates their fusion. Following this fusion, once gonadal descent begins, the attached splenic tissue subsequently follows the gonadal path (15). Misplacement of endometrial tissue or indeed fusion of the developing kidney and Mullerian ducts may account for the rare presence of renal endometriosis in our case.
In conclusion, this is a rare case of renal endometriosis, which clinically and radiologically mimicked a neoplastic process due to the prominent smooth muscle component and associated inflammation and edema. Histopathological correlation with multidisciplinary team input and collaboration was required to accurately diagnose this entity and ensure that an appropriate management plan was implemented. The aetiology of renal endometriosis remains unclear with several theories hypothesised (2); however, given the lack of a history of endometriosis and the absence of other foci of disease in this patient, together with the smooth muscle predominant phenotype, this may represent a case of reno-mullerian fusion or endometrial displacement during gestational development.