Unusual presentation of popliteal soft tissue sarcoma: not every swelling in the knee is a Baker's cyst
An 80-year-old female presented to the general practitioner with 3-month-old history of gradual onset swelling in the back of her right knee, painful right leg and swelling in her right ankle. There was no history of trauma, constitutional symptoms, weight loss or loss of appetite. She lived alone and was independently mobile.\nShe suffered from gout, hypertension and asthma but otherwise quite fit for her age.\nOn examination, she had a nontender 5 × 5 cm fluctuant and nonpulsatile lump in popliteal fossa with no overlying skin changes.\nAn ultrasound scan (Fig. ) revealed a large mass lesion with heterogeneous appearances in the popliteal fossa extending between the two heads of the gastronemii and large blood vessels in the central region of the mass. It measured ∼13 × 5.5 × 4.5 cm in dimension and a magnetic resonance imaging (MRI) scan was suggested to rule out mass arising from the popliteal vessels being enveloped by the mass.\nThe MRI scan of the right knee (Figs and ) with gadolinium-enhancement showed a large mass lesion in the posterior aspect of the right knee extending into the popliteal fossa and into the upper third of the right leg having appearance of bursal pathology. Post gadolinium images showed evidence of circumferential rim enhancement but no internal enhancement indicating the presence of a fluid filled lesion. The mass (Figs and ) measured ∼8.39 × 6.14 × 12.72 cm and was seen between semimembranosus and biceps femoris muscles. There was no evidence of any infiltration of either the sciatic nerve or the popliteal vessels. A likely diagnosis of complicated synovial cyst with a differential diagnosis of haematoma was reported and a provisional diagnosis of Baker's cyst was established.\nShe was admitted a month later in the medical ward with right leg swelling to rule out deep vein thrombosis. On admission, she had calf tenderness, right lower leg pitting oedema, mild erythema around the foot and ankle and faint peripheral pulses. Venogram ruled out deep vein thrombosis and X-ray of the knee revealed extensive ill-defined calcification within the soft tissue mass in the popliteal fossa extending posteriorly to the proximal tibia but no other bony abnormality was found.\nTwo months later she was admitted under vascular surgeons with critical ischaemia of the right leg. Doppler examination revealed swollen calf, reduced sensation in the foot and absent pulses in posterior tibial and dorsalis pedis arteries.\nAn urgent incisional biopsy of the popliteal mass was performed, which revealed soft tissue sarcoma (STS). A CT scan confirmed soft tissue mass consistent with STS in the popliteal fossa with metatstasis to the lungs and stomach.\nPatient was offered above-knee amputation which she declined and subsequently developed gangrene of her foot. This patient had a rapid worsening of her symptoms over a few months but unfortunately her diagnosis was delayed till she developed neurovascular compromise and already had secondary lesions implying a very poor prognosis and she chose not to have any intervention.