Background: Nodular fasciitis (NF) is currently considered a self-limited\nclonal neoplastic process. It shares the rearrangement of USP6-gene with\naneurysmal bone cysts and myositis ossificans. The presented case is of interest\nas this is a rare site of presentation of NF; so far only few single cases of\nintraarticular NF have been reported with documented USP6-gene rearrangement.\nIntraarticular neoplasias of the knee joint are rare; the most frequent\nbeing tenosynovial giant cell tumor (TSGCT). Given a nationwide annual incidence\nrate of 14 for the lower extremity and about 75% affecting the knee\njoint about 10 new cases involving the knee joint can be expected per 1 million\npersons/year. All other types of benign neoplasms are comparably rare\nwhile malignant intraarticular processes are extremely rare with most of them\nreported as single case studies. Aim: We report our case to emphasize the\nimportance of preoperative diagnostics including the option of biopsy. Intraarticular\nmalignant processes are extremely rare and frequently are operated\non accidently with negative consequences for the patient. Tactics and techniques\nto treat benign processes depend on the correct pathologic diagnosis.\nCase presentation: The 38 year old man noticed slowly increasing swelling of\nhis left knee joint after wakeboarding. Because of continuing discomfort 2\nmonths later MRI diagnostic revealed, apart from retropatellar cartilage lesions,\na popliteal mass compatible with a Baker cyst. The lesion of interest\n(later diagnosed as NF) was neither recognized by the radiologist nor the\ntreating clinician. During the following 8 months the patient felt increasing\nswelling of the knee joint. The repeat MRI documented the crescent intraarticular\nsolid synovial mass in the medial patellofemoral recess without signs\nof hemosiderin impregnation. A percutaneous sonographically guided 16G\nneedle biopsy was performed. Histologically, bland myofibroblastic proliferation\nsuggestive of nodular fasciitis (NF) was found. The next generation sequencing\n(NGS) demonstrated the presence of MYH9-USP6 gene fusion,\nconfirming the diagnosis of NF. The lesion was excised under arthroscopic\ncontrol. At 1 year follow-up the patient is asymptomatic. Conclusion: The\ncase is of interest because of its rare pathology. The decision how to treat was\nbased on pathologic biopsy diagnostics including the USP6-gene rearrangement.\nIn view of similar presentation of the rare malignancies we also want to\nstress the importance of definitive diagnostics which generally are possible\nonly through biopsy.
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