A 39-year-old female patient with familiar neurofibromatosis developed a sarcoma in the hollow of the knee that appeared clinically and sonographically as a Baker cyst. Following an operation to remove the tumour and subsequent local radiation treatment multiple metastases appeared distant to the primary tumour from which the patient died. On the observation of a Baker cyst in a patient with neurofibromatosis the possibility of malignant degeneration must be kept in mind and an operation carried out immediately.
Adult Diagnosis, Differential