Characteristic magnetic resonance imaging features of atypical spindle cell/pleomorphic lipomatous tumor

Atypical spindle cell/pleomorphic lipomatous tumor (ASPLT) is a newly classified benign lipomatous neoplasm characterized by diverse histological features, leading to broad differential diagnoses. This study evaluated the magnetic resonance imaging (MRI) features of ASPLT to identify markers for distinguishing them from other soft tissue tumors. Nineteen patients with histologically confirmed ASPLT across multiple institutions were retrospectively evaluated. MRI features, including tumor margins, capsules, septa, fat content, hyperintense areas on T2-weighted imaging (T2WI), edema, and contrast enhancement were assessed by two experienced radiologists. Interobserver agreement was evaluated using Cohen's kappa coefficient. Fourteen men and five women [median age: 70 years; interquartile range (IQR), 62.5-74.5 years] were included, with common tumor sites including the femoral (seven) and thoracic dorsal (three) regions. The median lesion size was 90.0 mm (IQR, 48.5-135.7 mm). MRI revealed well-defined margins in 42.1% of patients, capsules in 89.5%, septa in 100%, and varied fat content. T2WI high-signal areas appeared in 94.7% of tumors. Contrast enhancement occurred in 93.8% of tumors, with 93.8% and 75% showing enhancement in fat-suppressed T2WI high- and low-signal areas, respectively. The interobserver agreement for key features ranged between moderate and good, with κ values of 0.42 for enhancement area [95% confidence interval (CI): 0.13-0.71], 0.57 for capsule presence (95% CI: 0.15-0.98), 0.60 for hyperintense areas on T2WI (95% CI: 0.35-0.85), and 0.64-0.89 for other features. Although κ=0.42 was classified as moderate, the wide confidence interval indicated variability in agreement. This study analyzed ASPLT imaging features, highlighting trends in fat and nonfatty components. ASPLT typically shows low-fat content on T1WI, T2WI high-signal areas, and varying degrees of contrast enhancement. Differential diagnoses included spindle cell lipoma, atypical lipomatous tumor/well-differentiated liposarcoma, dedifferentiated liposarcoma, and myxoid liposarcoma. Although pathological diagnosis remains crucial, recognizing imaging differences may enhance early diagnosis and treatment.