Intraarticular Nodular Fasciitis in the Knee Joint with USP 6-Gene Rearrangement —A Case Report with Special Attention to Diagnostics of Intraarticular Lesions

Background: Nodular fasciitis (NF) is currently considered a self-limited clonal neoplastic process. It shares the rearrangement of USP6-gene with aneurysmal bone cysts and myositis ossificans. The presented case is of interest as this is a rare site of presentation of NF; so far only few single cases of intraarticular NF have been reported with documented USP6-gene rearrangement. Intraarticular neoplasias of the knee joint are rare; the most frequent being tenosynovial giant cell tumor (TSGCT). Given a nationwide annual incidence rate of 14 for the lower extremity and about 75% affecting the knee joint about 10 new cases involving the knee joint can be expected per 1 million persons/year. All other types of benign neoplasms are comparably rare while malignant intraarticular processes are extremely rare with most of them reported as single case studies. Aim: We report our case to emphasize the importance of preoperative diagnostics including the option of biopsy. Intraarticular malignant processes are extremely rare and frequently are operated on accidently with negative consequences for the patient. Tactics and tech-niques to treat benign processes depend on the correct pathologic diagnosis. Case presentation: The 38 year old man noticed slowly increasing swelling of hemosiderin impregnation. A percutaneous sonographically guided 16G needle biopsy was performed. Histologically, bland myofibroblastic proliferation suggestive of nodular fasciitis (NF) was found. The next generation se-quencing (NGS) demonstrated the presence of MYH9-USP6 gene fusion, confirming the diagnosis of NF. The lesion was excised under arthroscopic control. At 1 year follow-up the patient is asymptomatic. Conclusion: The case is of interest because of its rare pathology. The decision how to treat was based on pathologic biopsy diagnostics including the USP6-gene rearrangement. In view of similar presentation of the rare malignancies we also want to stress the importance of definitive diagnostics which generally are possible only through biopsy.

ment. Intraarticular neoplasias of the knee joint are rare; the most frequent being tenosynovial giant cell tumor (TSGCT). Given a nationwide annual incidence rate of 14 for the lower extremity and about 75% affecting the knee joint about 10 new cases involving the knee joint can be expected per 1 million persons/year. All other types of benign neoplasms are comparably rare while malignant intraarticular processes are extremely rare with most of them reported as single case studies. Aim: We report our case to emphasize the importance of preoperative diagnostics including the option of biopsy. Intraarticular malignant processes are extremely rare and frequently are operated on accidently with negative consequences for the patient. Tactics and techniques to treat benign processes depend on the correct pathologic diagnosis. Case presentation: The 38 year old man noticed slowly increasing swelling of his left knee joint after wakeboarding. Because of continuing discomfort 2 months later MRI diagnostic revealed, apart from retropatellar cartilage lesions, a popliteal mass compatible with a Baker cyst. The lesion of interest (later diagnosed as NF) was neither recognized by the radiologist nor the treating clinician. During the following 8 months the patient felt increasing swelling of the knee joint. The repeat MRI documented the crescent intraarticular solid synovial mass in the medial patellofemoral recess without signs

Introduction
Nodular fasciitis (NF) is currently considered a self-limited clonal neoplastic process. It shares the rearrangement of USP6-gene with aneurysmal bone cysts and myositis ossificans. The presented case is of interest as this is a rare site of presentation of NF; so far only few single cases of intraarticular NF have been reported.
Intraarticular neoplasias of the knee joint are rare; the most frequent being tenosynovial giant cell tumor (TSGCT). Given a nationwide annual incidence rate of 14 for the lower extremity [1] and about 75% affecting the knee joint [2] about 10 new cases involving the knee joint can be expected per 1 million person/year. All other types of benign neoplasias are comparably rare while malignant intraarticular processes are extremely rare and mostly are reported as single case studies. An overview of intraarticular tumors of synovial joints recently was given by Jang et al. [3]; a list of diagnoses to be considered with reference to most recent reports is given in Tables 1-3 to assist in the differential diagnostic evaluation of images.
We report our case to emphasize the impact of preoperative biopsy in spite of the fact that intraarticular malignant processes are extremely rare as the treatment approach for benign processes also depends on the pathologic diagnosis.
Based on accessible location and histological typing, arthroscopically assisted resection was considered suitable for complete tumor removal. Arthroscopic tumor resection was performed, with uneventful functional recovery (Figure 4). The resected specimen showed macroscopically typical features of NF ( Figure  5). Control-MRI 7 months later (12/2019) showed, apart from postoperative variations, no signs of tumor recurrence (Figure 6).
At 1 year follow-up the patient is asymptomatic.

Discussion
The intra-articular mass in the presented case was not appreciated/recognised as pathologic formation in the first MRI performed elsewhere. The enlarging mass documented in the repeat MRI 8 months later did not exhibit a pattern clearly attributable to a specific diagnosis. Considering the possibility of a malignant process (e.g. synovial sarcoma) it was decided to perform a sonographically guided core biopsy leading to the diagnosis of NF confirmed by the presence of the MYH9-USP6 rearrangement in the NGS study. The presence of the USP6 gene rearrangement appears to be a factor involved in the development of neoplastic processes with the tendency to spontaneous regression, such as NF [6] [7] [8]. Both myositis ossificans and aneurysmal bone cyst belong to the spectrum of lesions with MYH9-USP6 gene fusion and are considered as a model of "don't touch lesions" [9]. Further observation to expect possible spontaneous regression was discussed with the patient; however with documented enlargement and increasing discomfort he opted for active treatment. Arthroscopically a marginal en-bloc resection could be performed. The MRI 7 months postoperative showed no evidence of tumor persistence and the patient is asymptomatic at 1 year follow-up. NF commonly occurs in sucutaneous tissue, skeletal muscle, vessels [2], but is rarely seen intraarticular [10]. MYH9-USP6 gene fusion identified in intraarticular NF of the knee was first described by Miyama et al. [10] in two cases; a third case of intraarticular NF of the knee with MYH9-USP6 gene fusion was reported by Igrec et al. [11]. Even though intraarticular NF diagnosed without proven USP6-gene-rearrangement may behave and be treated similar as NF in other locations we would caution to pool and mix data as this may prevent to estimate possible differences according to the site of affection.
In the reported cases of intraarticular NF the lesions were usually operated without prior biopsy, often assumed to be TSGCT.
However, given the impact of inappropriate initial treatment of sarcomas of any site cautious diagnostic assessment of any articular process is mandatory.
Imaging of TSGCT, the most frequent intra-articular neoplasia, is fairly typical and when hemosiderin deposits are present almost pathognomonic. However most other benign processes exhibit features also observed in TSGCT.
Synovial processes can be considered as already having contaminated the entire joint as is the case of extraarticular neoplasm having invaded the joint. Treatment modalities for benign processes may differ according to the definitive pathologic diagnosis ranging from observation (sometimes indicated in TSGCT, maybe also for NF) over arthroscopic procedures (indicated for most benign neoplasms) to open resection as favored e.g. for synovial hemangioma [16] [17].
An overview regarding diagnosis and management of neoplasms involving synovial joints recently was given by Jang et al. [3] including osseous processes. Chebib et al. [15] report on 15 cases of intraarticular sarcoma; synovial sarcoma (n = 6) and extraskeletal myxoid chondrosarcoma (n = 3) being the most frequent.
Imaging of intraarticular masses are classified by Adams et al. [13] according to absence or presence of calcification and bone erosions in radiography, and MRI features subdivided in four categories as solitary focal mass, multifocal solid masses, diffuse solid mass and diffuse synovitis (Table 1).
Sheldon et al. [12] classify synovial processes according to their nature as non- This case confirms the experience of the three reported cases of NF with USP6-gene rearrangement and adds informations referring to diagnosis and management of rare intraarticular neoplasm.

Conclusions
The diagnosis of intra-articular NF can be confirmed by the presence of MYH9-USP6 gene fusion and should be considered in the evaluation of intra-articular lesion.
Though the most frequent synovial process is TSGCT treatment strategies should be developed upon all possible other diagnoses including sarcoma, metastases, tumor-like lesions as well as infection or malformation.
Using a check-list may assist in the evaluation.
In the work-up of intra-articular synovial processes of the knee we follow a check-list as given in Tables 1-3 with reference to respective reports and the WHO classification of tumours of soft tissue and bone [2].

Patient Consent
The patient was informed that data from his case would be submitted for publication, and an informed consent was obtained.

Conflicts of Interest
The authors declare no conflict of interest regarding the publication of this paper. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this article.