Working Diagnosis:
Stage 1 grade IB fibrosarcoma

Treatment:
The patient underwent a radical resection of the popliteal fibrosarcoma with skin grafting. A chest computed tomography scan (CT) was done before the resection which showed a 4.4 mm noncalcified pulmonary nodule. It was too small to biopsy so a repeat CT scan was done and showed no increase in size or metastatic pathology, so no radiation therapy or adjuvant chemotherapy was indicated.

Outcome:
The patient had no other abnormal growths and noted no unintentional weight loss, however, did not adhere to regular yearly follow-ups.

Author's Comments:
There are a variety of masses that can originate from the posterior compartment of the knee such as popliteal cysts, varicose popliteal veins or popliteal artery aneurysms. Diagnosing these requires a systematic approach to determine the compartment and structures involved (1).
Popliteal cysts are the most frequent synovial cysts of the knee (1). Popliteal cysts can be described as an enlargement of the gastrocnemius-semimembranosus bursa behind and communicating with the knee joint. They most commonly develop in the setting of knee trauma, osteoarthritis, or inflammatory arthritis (2). Differential diagnoses for a popliteal cyst include seroma, abscess, hematomas, or neoplasms. They are frequently asymptomatic but can present with posterior knee pain and swelling (3). However, these cysts have also been known to extend medially, laterally, and superiorly and have the propensity to occasionally rupture(1). When ruptured, the popliteal (Baker) cyst demonstrates the "crescent sign,"- an ecchymotic area below the medial malleolus. Ruptured cysts may mimic deep venous thrombosis (pseudothrombophlebitis) and may be associated with ecchymosis from the popliteal fossa to the ankle and can be treated with rest, elevation, and non-steroidal anti-inflammatory medications (4). Symptomatic patients often experience relief with joint aspiration and intra-articular glucocorticoid injection while, asymptomatic popliteal cysts, require no treatment (5). In rare instances, cysts may cause compression of adjacent structures, leading to compression syndrome, posterior tibial nerve entrapment, or popliteal artery occlusion (6).
A large part of the knee joint is covered by a synovial membrane. Most tumors originating from the joint will be composed of synovium. These are as rare as soft tissue sarcomas which make up 1% of all malignant tumors (1). Given the density and characteristics of the mass, they are often mistaken for osteochondroma or myositis ossificans (conversion of stem cells into osteoid tissue) rather than a baker's cyst. Soft tissue sarcomas arise from mesenchymal tissue other than bone and are commonly seen in the extremities (7). When found in the popliteal fossa, soft tissue sarcomas are considered extra compartmental and have poorer prognosis depending on the type due to poor surgical margins, but can still be treated with a high rate of limb salvage (8). The treatment approach is usually a surgical resection, which can be challenging due to the proximity to neurovascular structures (9).
Many malignant masses that present as cysts in the popliteal region may constitute various differential diagnoses besides Baker cysts (2). These masses warrant a systematic approach in diagnosis that examines factors such as affected compartment and anatomical structures, as well as characteristics of the lesions (1). As such, clinical characteristics and specified imaging such as MRI can help differentiate between the pathologies by determining the internal composition of the mass. Popliteal cysts typically appear as an ellipsoid mass with a uniform low signal intensity on T1-weighted MRI and high signal intensity on T2-weighted MRI (8). Popliteal fibrosarcomas rely on a combination imaging (preferably ultrasound and MRI) to arrive at a conclusive diagnosis (10).

Editor's Comments:
This case demonstrates the different imaging findings with soft tissue sarcomas and the characteristics on plain x-rays and MRI. After promptly recognizing the physical exam and imaging findings were not consistent with a common popliteal cyst, the treatment team appropriately referred this patient to orthopedics oncology. Although rare, this case illustrates the importance of considering soft tissue sarcomas and other malignancies when diagnosing soft tissue masses in the popliteal space.

References:
Larbi, A., et al. “Imaging of tumors and tumor-like lesions of the knee.” Diagnostic and Interventional Imaging, vol. 97, no. 7–8, 2016, pp. 767–777, https://doi.org/10.1016/j.diii.2016.06.004.
Demange, M. K. (2011). Baker's cyst. Revista Brasileira de Ortopedia (English Edition), 46(6), 630-633. https://doi.org/10.1016/s2255-4971(15)30317-7
Telischak NA, Wu JS, Eisenberg RL. Cysts and cystic appearing lesions of the knee: A pictorial essay. Indian J Radiol Imaging 2014;24:182-91.
Han, Hyun Ho, et al. “A Cystic Mass in the Popliteal Fossa and Its Differential Diagnosis.” Archives of Plastic Surgery, U.S. National Library of Medicine, July 2015, www.ncbi.nlm.nih.gov/pmc/articles/PMC4513059/
Handy, J R. “Popliteal cysts in adults: a review.” Seminars in arthritis and rheumatism vol. 31,2 (2001): 108-18. doi:10.1053/sarh.2001.27659
Sanchez, Jorge E et al. “Compression syndromes of the popliteal neurovascular bundle due to Baker cyst.” Journal of vascular surgery vol. 54,6 (2011): 1821-9. doi:10.1016/j.jvs.2011.07.079
Shah, A., et al. “A diagnostic approach to popliteal fossa masses.” Clinical Radiology, vol. 72, no. 4, 2017, pp. 323–337, https://doi.org/10.1016/j.crad.2016.11.010.
Rudiger, Hannes A., et al. “Soft Tissue Sarcomas of the Popliteal Fossa: Outcome and Risk Factors.” European Journal of Surgical Oncology, vol. 33, no. 4, Elsevier BV, May 2007, pp. 512–17, https://doi.org/10.1016/j.ejso.2006.11.009.
Pritsch, Tamir, et al. "Popliteal Sarcomas: Presentation, Prognosis, and Limb Salvage." Clinical Orthopaedics & Related Research 455.(2007): 225-233. Journals@Ovid Full Text. Web. 01 January. 2024. .
Vibhakar, A. M., Cassels, J. A., Botchu, R., Rennie, W. J., & Shah, A. (2021). Imaging update on soft tissue sarcoma. Journal of Clinical Orthopaedics and Trauma, 22, 101568. https://doi.org/10.1016/j.jcot.2021.101568

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