To review, our patient was a 60-year-old woman with a large soft tissue mass that involved both the anterior and posterior compartments of the left knee, with extensive extra-articular extension and osseous erosions present.
Her differential diagnosis included both benign and malignant neoplastic disease, massive ganglion cyst, inflammatory arthritis, and crystalline arthritis. We considered inflammatory and crystalline arthritis less likely due to the chronicity of symptoms. Although a benign neoplasm was in the differential diagnosis, we considered the most likely diagnosis to be a primary sarcoma, given the MRI characteristics and apparent aggressiveness of the mass. Inflammatory markers were ordered to rule out inflammatory and autoimmune disease, the results of which were negative. Ultimately, tissue sample for pathologic analysis was needed for diagnosis.
After seeing the patient in clinic, she was referred for an ultrasound-guided core needle biopsy. Pathologic findings showed this was a low-grade myxoid lesion; however, not enough tissue was available for final diagnosis. Core needle biopsies have been shown to have acceptable — although definitely lower — diagnostic accuracy compared with open biopsy.
The patient returned to the clinic for a discussion of next steps. We advocated for open biopsy. Staging studies were also ordered at this time, including a CT scan of her chest, abdomen, and pelvis, and a bone scan, given the high index of suspicion for sarcoma. The staging study findings were negative.
Open biopsy yielded diagnostic tissue. The final pathologic diagnosis was juxta-articular myxoma. This was a surprising diagnosis due to the high-grade features seen on MRI, including bone erosion, infiltrative growth pattern, massive mass size, and heterogeneous MRI spin echo sequencing with central contrast voids. Although myxomas can have some heterogeneity with contrast enhancement, they are typically well circumscribed and homogenous on both T1- and T2-weighted images.
Although many soft tissue tumors can arise around the knee, juxta-articular myxomas are rare. They can have many overlapping characteristics, radiographically and histologically, with various tumors, and all factors must be considered when making the diagnosis. Myxomas are a benign, hypocellular, spindle cell neoplasm with myxoid stroma. They lack features of malignancy, such as cellular atypia, frequent mitoses, necrosis, or hyperchromasia. Histologically distinguishing a myxoma from a low-grade myxofibrosarcoma, which will demonstrate features of malignancy, is arguably quite difficult.
Macroscopically, myxomas present as cystic formations of soft or friable consistency, are white to yellow in color, and are typically 2-6 cm. Myxomas are more common in men in their third to fifth decade but can occur in all age ranges. Although intramuscular myxomas and juxta-articular myxomas can have similar histologic features, they likely represent distinct entities. Juxta-articular myxomas lack the Gs alpha mutation that is common to sporadic myxomas and those associated with Mazabraud syndrome.
Juxta-articular myxomas most commonly occur around the knee but have been reported at other joints.[9,10,11] Other intra-articular pathologic findings are also typically associated with them, such as a meniscal tear or arthritis. Although benign and usually isolated to subcutaneous adipose tissue, juxta-articular myxomas rarely behave more aggressively clinically, with involvement of deeper structures (eg, tendon, bone, capsule, neurovascular structures). Destructive features, infiltrative growth, and massive size are rarely reported.[12,14]
In a series of 65 patients with juxta-articular myxoma, Meis and Enzinger reported the mean size to be 3.8 cm and the maximum size to be 12 cm. Although metastasis has not been reported, rapid growth can occur. Juxta-articular myxomas have a recurrence rate of approximately 30% after resection. To our knowledge, our patient's case is one of the largest juxta-articular myxomas reported in the literature. The massive size and destructive features seen on imaging were atypical for this tumor. A careful diagnostic workup ultimately led to the correct diagnosis.
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Cite this: Alexandra Scoles, Babe Westlake, Nicholas S. Tedesco. My Strangest Case: A Woman With Varicose Veins and Leg Pain - Medscape - Sep 02, 2020.