Dilemmas in Diagnosing Popliteal Swellings: Nodular Fasciitis Masquerading as Baker's Cyst

This is a case of a popliteal swelling which was clinically misdiagnosed as a simple popliteal cyst, which turned out to be a benign fibroblastic tumor featuring nodular fasciitis (NF). A swelling in the popliteal fossa by and large is considered to be a Baker's cyst. However, an array of other lesions which can be encountered such as fibroma featuring NF, meniscal cysts, lipoma, aneurysms, bursitis of the biceps femoris tendon, and schwannoma should also be borne in mind since clinical and radiological evaluation may not always be conclusive. This case report presents fibroma featuring NF masquerading as Baker's cyst, which has not been described in any literature that the authors could find hitherto, making it a unique case.

A 20-year-old male man presented to the outpatient department with a solitary palpable mass on the posterior aspect of his right knee for the past 3 years associated with difficulty in squatting for the past 3 months. The mass gradually was well defined and increased gradually in size to the current size of 10*9 cm, irregular soft to firm in consistency, spherical, non-tender, and non-reducible on the posterior aspect of the popliteal fossa. Local ultrasonography showed evidence of heterogeneous hypoechoic lesion with signs of inflammation; underlying bony cortices appeared to be normal. A clinical diagnosis of Baker's cyst was made and an excisional biopsy was performed. Microscopically, histologic sections showed fibroblasts arranged in loose fascicular patterns intermixed with small amount of collagen and myxoid stroma.

Popliteal masses need not always be simple Baker's cyst, and careful evaluation of the mass in the popliteal fossa is always mandatory. Radiologic investigations may not always be conclusive and biopsy is the gold standard for diagnosis.