case_images: PMC7163288_01
case_images: Pre-operative magnetic resonance image of the right femur. The lesion was located beneath the vastus medialis muscle, and the vastus intermedius muscle showed fatty degeneration. Axial view of the right distal femur on (a) T1-weighted imaging, (b) T2-weighted imaging, and (c) gadolinium-enhanced, T1-weighted, fat-suppressed imaging. (d) Axial view of the right proximal femur and (e) sagittal view of the right femur on T1-weighted imaging show that fatty degeneration stretched the full length of the vastus intermedius muscle.
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case_images: T1-weighted images and slightly high signal intensity on T2-weighted images; the mass was located under the vastus medialis muscle and merged into the vastus intermedius muscle (Fig. 1a, b). The lesion showed intense, heterogeneous gadolinium enhancement on T1-weighted fat-suppressed images (Fig. 1c). A considerable part of the vastus intermedius muscle appeared to have turned into fat tissue with some fibrous elements (Fig. 1d, e).
case_images: Limited range of motion in the right knee was relieved by tumor resection. (a) Under general anesthesia, the affected knee had some flexion contracture preoperatively, with a range of motion of 0-90 . (b) Postoperatively, the knee contracture had improved, with a range of motion of 0-130 .
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case_images: On physical examination at our clinic, the affected leg was 35 mm longer than the opposite leg, and the range of motion of the right knee was limited (extension-flexion range was 0-80  on the right and 0-140  on the left) (Fig. 2a). Since the primary goal of the surgery was to improve the range of motion of the knee, an additional transverse cutting of the fascia of the vastus intermedius muscle successfully relieved the tension and improved the knee flexion to 120  (Fig. 2b).
case_images: Surgical margin, macroscopic findings, and tumor histology. (a) The lesion was marginally resected (red dotted line) along the distal intermediate vastus muscle and its fascia (yellow dotted line). (b) Grossly, the specimen was oval and as large as 3 cm at its greatest diameter. On the resected specimen, small white flecks were observed, suggesting calcification. (c) Histologically, the lesion had multiple calcifications surrounded by round cells arranged in short arrays. The stroma was largely hyalinized.
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case_images: We resected the distal vastus intermedius muscle and its fascia, but laterally only at the transient zone to the vastus medialis and lateralis muscles in order to preserve the extension function, running the risk of local recurrence (as agreed with the patient's parents) (Fig. 3).
case_images: Plain radiographs of both lower extremities. Plain radiography showed that limb inequality remained even 1 year after tumor resection.
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case_images: As of the 12th month after surgery, the knee extension-flexion range has been maintained at 0-110 , and the leg length discrepancy has not shown an apparent increase from 35 mm (Fig. 4).