Perforating Baker's cyst mimicking cutaneous malignancy

A 51-year-old woman was referred to the Department of Dermatology with two skin tumors localized below the right popliteal fossa. They initially appeared as two erythematous patches, that within 4 months grew into tumors. Along with the growth of the tumors, the patient suffered from increasing pain of the knee and calf and was recurrently treated for suspected, ipsilateral erysipelas of the lower leg. Her medical history included knee osteoarthrosis, fibromyalgia, obesity, and a smaller Baker's cyst diagnosed 4 years previously. Physical examination showed two, non-tender skin tumors and a larger, subcutaneous firm mass in the popliteal fossa. The largest tumor, measuring 5 × 3.5 cm, was ulcerated, oozing, and cystic on palpation (Figure 1a). Differential diagnoses included skin carcinoma, bacterial abscess, and perforation of the Baker's cyst to the skin. Histology based on a biopsy from the largest tumor showed fibrosis and an inflammatory infiltrate dominated by histocytes, as in a foreign body reaction, supporting the diagnosis of a ruptured Baker's cyst. Magnetic resonance imaging revealed a 12 × 7 cm fibrotic Baker's cyst with a possible fistula to the skin (Figure 1b). Shortly thereafter, she was admitted with Staphylococcus aureus-mediated septic arthritis. Acute arthroscopic surgery confirmed communication from the knee joint to the skin through a large, infected, fistular Baker's cyst. Following antibiotic treatment and surgery including synovectomy and excision of the cyst the woman ultimately underwent total knee arthroplasty. Baker's cysts (or popliteal cysts) are common and usually associated with other intra-articular pathologies.1 Larger cyst can be palpated and seen as subcutaneous masses in the popliteal fossa and rare complications include rupture, infection, and pseudothrombosis.2, 3 To our knowledge, this is the first reported case of a Baker's cyst fistulating to the skin and presenting as ulcerating skin tumors. Anna Sophie Quaade: Investigation, resources, visualization, writing – original draft, writing – review and editing. Claus Zachariae: Resources, supervision, writing – review and editing. Mattis Enggaard Jørgensen: Resources, visualization, writing – review and editing. Anne Birgitte Simonsen: Conceptualization, resources, investigation, supervision, writing – review and editing. The authors have no conflicts of interest to declare. The patient in this manuscript has given written informed consent to publication of the case details. Ethical approval: non-applicable. Information supporting the findings in this manuscript is available from the corresponding author upon reasonable request.