Renal endometriosis mimicking a malignancy– a rare case of Reno-Mullerian fusion?

Diarmuid O’Connor, Kevin G. Byrnes, Kilian Walsh, Gerard J. O’Sullivan, Teresa McHale
In: Research Square · 2021 · doi:10.21203/rs.3.rs-425619/v1 · W3171668368
preprint OA: green CC0 ⤵ 1 in-corpus citation
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⚙ AI-generated summary by claude@2026-06, 2026-06-08 ⓘ

This case study reports a rare instance of renal endometriosis with a significant smooth muscle component mimicking malignancy, potentially arising from reno-mullerian fusion.

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⚙ AI-generated deep summary by claude@2026-06, 2026-06-08 ⓘ

This preprint reports a rare, asymptomatic 49-year-old perimenopausal woman who had an incidental 4.2 cm right renal upper-pole mass on imaging, initially raising concern for malignancy; after multidisciplinary review, she underwent open radical nephrectomy because of adherence to adjacent structures. Histology unexpectedly showed endometriosis and endosalpingiosis with a significant smooth muscle component, histiocytic inflammation, and lymphoid aggregates, with immunohistochemistry supporting an endometrial/stromal origin and no atypia or malignancy. The authors note the key limitation that pre-operative imaging and even core biopsy can be diagnostically challenging when renal lesions have smooth-muscle predominance and overlap with entities such as angiomyolipoma or mixed epithelial/stromal tumors. Relevance to endometriosis: this paper is centrally about endometriosis — it documents renal endometriosis that mimicked a renal neoplasm and discusses possible embryologic mechanisms (reno–Mullerian fusion/endometrial displacement) underlying this condition.

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Abstract

Abstract Endometriosis is a common gynaecological condition characterised by ectopic endometrial tissue growth beyond the uterine cavity. Urinary tract endometriosis represents only 1.2% of all cases, with renal endometriosis accounting for less than 1% of urinary tract involvement. An asymptomatic, 49-year-old, perimenopausal Irish female was found to have an incidental mass at the upper pole of the right kidney on imaging studies. Histological analysis was recommended to outrule a renal malignancy and an open radical nephrectomy was performed following multidisciplinary input. Histological analysis surprisingly revealed the presence of endometriosis and endosalpingiosis, accompanied by a significant smooth muscle component, which mimicked a renal neoplasm. The aetiology of renal endometriosis remains unclear. 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. Keywords Endometriosis, smooth muscle, reno-mullerian fusion, malignancy

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endometriosis

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