[A hard subcutaneous swelling: not always as malignant as it seems].

Background: Nodular fasciitis is a rare, benign soft tissue tumor in young adults on the arms or trunk.

Case Description: A 24-year-old woman with previous pretibial melanoma in situ presented with a painless, rapidly growing subcutaneous hard swelling on the right forearm since two weeks. Ultrasound showed a subcutaneous, irregularly shaped, vascularized mass, suspicious for malignancy. Three days later, the swelling had subsided spontaneously. A new ultrasound showed a smaller, oval, sharply demarcated lesion with an inflammatory aspect. Histopathological-immunohistochemical examination of a biopsy showed a spindle cell lesion consisting of (myo)fibroblasts with some erythrocyte extravasation, consistent with nodular fasciitis. Three months later the nodule was resolved spontaneously without treatment.

Conclusion: Nodular fasciitis should be included in the differential diagnosis in case of hard, fast-growing, subcutaneous bumps in young adults. By ultrasound, it is difficult to differentiate from a malignant soft tissue tumor. A biopsy is necessary to confirm the diagnosis with certainty.