Teaching Case

A 53-year-old man presented with a 3 month history of pain at the right base of the neck and right sopraclavicular mass. At admission, an US examination showed a large mass with a low echogenicity and smooth, sharply defined margins; because of this, a muscular haematoma was suspected.

MRI was performed using axial T1- and T2-weighted images. T1-weighted images revealed an oval soft tissue mass in the right supraclavicular fossa with well-defined margins with anterior subcutaneous fat tissue and irregular and ill-defined margins with paraspinal muscles, from which it probably had originated. This soft tissue mass was homogeneously isointense on T1-weighted images relative to skeletal muscle, and heterogeneously hyperintense on T2-weighted images. Strong enhancement was seen after contrast administration. Before surgical excision, a CT examination was also performed and provided an optimal vascular map of the supraclavicular region. Other similar lesions were not found in the patient.
The patient was treated surgically, and histological analysis showed bland sheets of spindle cells in dense collagen stroma, as typical features of fibromatosis.

Fibromatosis refers to a family of soft tissue lesions characterised by proliferation of benign fibrous tissue, which is composed of fusiform cells surrounded and separated by abundant collagen. Although histologically benign, this lesion tends to gradually infiltrate adjacent soft tissues.
The signal intensity is reflective of the tumour histology. Areas of high signal in T2-weighted sequences reflect abundant tumour cellularity. Regions of low signal intensity on the T2-weighted images reflect the presence of fibrosis and hypocellularity. The main differential diagnosis of aggressive fibromatosis on imaging is malignant soft-tissue sarcoma. Aggressive fibromatosis typically shows an infiltrative growth pattern, crosses fascial boundaries, and does not show central necrosis, even in the largest lesions. Additional MRI findings of aggressive fibromatosis are bands of low signal intensity across all sequences even if low signal is not seen in all cases of aggressive fibromatosis, and prominent low signal intensity, particularly on T2-weighted images, may be seen in pigmented villous nodular synovitis, giant cell tumors of the tendon sheath, calcified masses, and malignancies such as fibrosarcoma or malignant fibrous histiocytoma. Although MRI can help to find the correct diagnosis of aggressive fibromatosis, tissue confirmation is required for a final diagnosis in all cases.