Cellular fibroma in the Douglas cavity, mimicking a malignant neoplasia: fibroma, fibrosarcoma or mitotically active cellular fibroma?

Ovarian fibroma is a benign stromal tumour composed of spindle/ovoid fibroblastic cells producing collagen. Approximally 10% of fibromas are densely cellular with small amount of collagen. In these cases, if mild nuclear atypia is present, they are best addressed as cellular fibroma. However cellular fibroma may show a greater mitotic activity and therefore they should be referred as mitotically active cellular fibromas. Mostly benign, it is necessary to differentiate them from malignant tumours such as fibrosarcomas. We report a case of an unusual presentation of mitotically active cellular fibroma, detected in the Douglas cavity of a young woman, with normal appearing ovaries and uterus, mimicking a malignant neoplasia clinically and on imaging. In fact abdominal mass may be associated with acute pain, resulting in clinical emergency, really difficult to distinguish from a frank malignancy, before surgical procedure. We described the clinical, radiological and pathological characteristics of our case and we make a comparison of what previously described in literature. The differential diagnosis among those entities is based on the microscopic features such as atypia and the number of mitoses. However, according to their dimensions, it may be necessary to generously sample these tumours and sometimes, to perform a panel of immunohistochemical markers, in order to make a correct diagnosis, establish the best treatment and the right follow-up. In fact, the prognosis is not certain, due to the possible recurrence, especially if not completely excised.