US and MRI appearance of a collagenous fibroma (desmoplastic fibroblastoma) of the shoulder

Collagenous fibroma (desmoplastic fibroblastoma) is a recently described rare benign tumor. Since this tumor was characterized by Evans in 1995, fewer than 100 cases have been reported in English literature with the largest series published by Miettinen and Fetsch. Preoperative radiological diagnosis is important to avoid over-treatment and unnecessary extensive procedures, but is difficult because diagnostic imaging findings for collagenous fibroma have not been established. Here we report the US and MRI appearance of a lesion localized deep under the deltoid muscle in a 34-year-old woman with a palpable painless mass. Histological findings were consistent with a collagenous fibroma. As far as we know, this is the first description of US appearance of a collagenous fibroma in English literature.