Beyond the uterus: diagnostic puzzle unravelled as benign metastasising leiomyomatosis with retroperitoneal and pulmonary involvement.

Benign metastasising leiomyomatosis (BML) is a rare entity characterised by the spread of benign smooth muscle cells, most commonly from uterine leiomyomas, to distant sites-most often the lungs. We present a case of a woman in her 40s who presented with abnormal uterine bleeding. Initial imaging revealed multiple uterine fibroids, a retroperitoneal mass mimicking gastrointestinal stromal tumour. HRCT chest showed thin-walled cystic lung nodules creating a suspicion of lymphangioleiomyomatosis (LAM). Following total abdominal hysterectomy with bilateral salpingo-oophorectomy and excision of the retroperitoneal lesion, histopathology and immunohistochemistry (IHC) confirmed a spindle cell tumour probably leiomyoma. Although the lung lesions were initially suggestive of LAM, the IHC study of the retroperitoneal lesion showed negative HMB-45 staining, and the genetic study also ruled out tuberous sclerosis complex mutations. Hence, we finally came to the diagnosis of BML. This case highlights the diagnostic challenges posed by cystic lung lesions in BML and underscores the need for a multidisciplinary approach incorporating histology, IHC and genetic testing to differentiate BML from mimickers like LAM or low-grade leiomyosarcoma.