# Fibromatosis of the accessory breast mimicking a malignant tumor: a rare case report and literature review

## TL;DR

A rare case of fibromatosis in the accessory breast was reported, initially mistaken for a malignant tumor but confirmed benign through histopathology.

## Contribution

First reported case of fibromatosis occurring in the accessory breast, highlighting diagnostic challenges and management considerations.

## Key findings

- The tumor was initially suspected to be malignant but was confirmed benign via histopathology and immunohistochemistry.
- No recurrence was observed at the 2-year follow-up, indicating a favorable prognosis.
- The case underscores the need for histopathological evaluation to avoid misdiagnosis.

## Abstract

Fibromatosis is a rare neoplasm characterized by fibroblastic and myofibroblastic proliferation. Fibromatosis that occurred in the accessory breast has never been reported in the literature worldwide. We present a case of extra-abdominal fibromatosis of the accessory breast occurring in a 36-year-old Chinese female patient. The tumor was hard with unclear boundaries and adhered to surrounding tissues (6 × 4 × 3 cm). Despite that malignancy was initially suspected based on clinical, ultrasound, and magnetic resonance imaging (MRI) results, histopathology and immunohistochemistry showed an unexpected outcome. At the 2-year postoperative follow-up, there was no recurrence; the prognosis was explained to the patient. This case emphasizes the importance of clinical suspicion and histopathological evaluation and the need to raise awareness to promote early diagnosis and appropriate management of fibromatosis. We also present a literature review of varied presentations and treatment options for extra-abdominal fibromatosis.