Purpose or Case Report: The prevalence, imaging features, and etiologies of pediatric intra-articular soft tissue masses are not well described in the literature. Therefore, intra-articular masses can present diagnostic dilemmas for pediatric radiologists. The purpose of this study was to further evaluate pediatric intra-articular soft tissue masses. Our hypothesis was that intra-articular masses have a limited differential diagnosis and are mostly benign.
Methods & Materials: In this IRB-approved retrospective study, we reviewed all knee MRIs performed at our institution between 2001 – 2019 (n = 3915). Our inclusion criteria were knee MRIs with an intra-articular soft tissue mass. Our exclusion criteria were cases with no masses or intra-articular lesions with clear radiologic diagnoses (e.g. ganglion cyst, venous malformation). Multiple radiologic characteristics were evaluated (size, focality, joint involvement, T1/T2/post-contrast features, radiographic correlate, calcification, and bone destruction). Radiologic diagnoses and pathologically confirmed diagnoses were collected when available. The authors re-reviewed the MRI studies that met the inclusion/exclusion criteria.
Results: 26/3915 (<1%) cases were identified that satisfied the inclusion/exclusion criteria of the study. 20/26 underwent biopsy. Patient age ranged from 7 months to 19 years with a mean of 11 years. Histologic diagnoses included pigmented villonodular synovitis (PVNS) (7/20, 35%), venous malformations (2/20, 10%), inflammatory arthritis (2/20, 10%), synovial sarcoma (1/20, 5%), kaposiform hemangioendothelioma (1/20, 5%), giant cell tumor of tendon sheath (1/20, 5%), PTEN hamartoma (1/20, 5%), nodular fasciitis (1/20, 5%), tuberculous osteomyelitis (1/20, 5%), synovial osteochondromatosis (1/20, 5%), benign myofibroblastic lesion (1/20, 5%), and benign synovial tissue (1/20, 5%). Of the 6 remaining cases without biopsies, suspected radiologic diagnoses included PVNS (3/6, 50%), juvenile idiopathic arthritis (2/6, 33%), and hemarthrosis (1/6, 17%). 19/20 (95%) of the lesions were benign. 1/20 (5%) were malignant (synovial sarcoma). The most frequent diagnosis was PVNS. 5/10 (50%) of the PVNS cases were focal and 5/10 (50%) were multifocal. All PVNS cases (10/10, 100%) had hypo-intense signal on T2.
Conclusions: Intra-articular soft tissue masses of the knee in children are rare, with a prevalence of <1% in our study. Nonetheless, it is important to be aware that the majority (>95%) are likely to be benign with PVNS being the most likely diagnosis.