Diagnostic Challenge at the Knee: A Large (15 Cm) Rapidly Growing Calcified Mass with Features of Desmoplastic Fibroblastoma

Background: Desmoplastic fibroblastoma is a rare, benign fibrous tumor typically characterized by slow growth and absence of calcification. However, atypical presentations can pose significant diagnostic challenges. Here, we present a case of a large prepatellar mass with unusual features that complicated the preoperative and differential diagnosis. Case Presentation: A 45-year-old male presented in June 2023 with a progressively enlarging mass in the left extremity over one year prior to presentation, following direct trauma to the area. While desmoplastic fibroblastomas are typically slow- growing, the rapid enlargement of this mass raised suspicion for more aggressive entities such as sarcomas. Clinical examination revealed a large (13 × 15 cm), firm, well-defined, and non-pulsatile mass in the prepatellar region. Plain radiography revealed calcifications within the soft tissue mass-a feature rarely associated with desmoplastic fibroblastoma and more commonly seen in other lesions like synovial sarcomas or calcifying aponeurotic fibromas. The mass was surgically excised and appeared well -circumscribed and non-infiltrative to the surrounding tissues. Histopathological examination revealed dense fibrocollagenous tissue with perivascular lymphocytic infiltration, a finding that overlaps with other fibrous tumors and necessitated clinicopathological correlation to confirm the diagnosis. Conclusion: This case highlights the diagnostic dilemma posed by large fibrous lesions when clinicoradiological and histopathological features overlap. It underscores the critical importance of integrating all available clinical, imaging, and histological data; particularly when confirmatory immunohistochemistry is unavailable. Furthermore, it expands the known spectrum of desmoplastic fibroblastoma to include presentations with rapid growth and calcification, serving as a reminder for clinicians to consider this rare entity even in the presence of atypical features.