Retroperitoneal Desmoid Tumour Masquerading as Malignancy: A Case Report

Desmoid tumour is a rare, benign soft tissue tumour. It is noted to be clinically aggressive but pathologically benign, showing a high local recurrence rate; however, it cannot metastasize. Although it is benign, it can impact functionality. The majority of these cases remain sporadic, and the aetiology is unknown. The retroperitoneum is a rare site, and there are few reports of desmoidtype fibromatosis occurring in this region. It is difficult to distinguish from other tumours and to identify the tumour’s origin. Here, the authors present a case of a 36-year-old female who presented with abdominal pain and a mass in her abdomen. Imaging studies revealed a large retroperitoneal mass extending from the L5 vertebral body up to the inferior endplate of the L1 vertebral body. Exploratory laparotomy was performed, and the retroperitoneal mass was resected. Grossly, it was a capsulated dark brown to grey-coloured mass measuring 15×12×10 cm. The mass was sent for histopathology {Haematoxylin and Eosin (H&E)} study and reported as a retroperitoneal desmoid tumour, which was further confirmed on Immunohistochemistry (IHC) by showing immunopositivity for Desmin, Muscle-Specific Actin (MSA), and Beta-catenin. The patient is doing well with no further complaints to date. No disease relapse has occurred. Follow-up has been done every three months till the writing of the present report.