Case report
A 45-year-old Japanese man was referred to us with a few day-history of a subcutaneous mass in the left shoulder area. The patient had arrhythmogenic right ventricular cardiomyopathy associated with arrhythmia after cardioverter-defibrillator implantation and dyslipidemia. There was no family history of mesenchymal tumors or a previous history of trauma to the left shoulder area.
A physical examination revealed a 7-cm dome-shaped subcutaneous mass on the left shoulder with poor mobility to the lower floor (Figure 1a ). Magnetic resonance imaging (MRI) showed a subcutaneous mass measuring approximately 6.0 × 5.0 × 2.5 cm without findings of invasion into the deep muscle layer (Figure 1b ). The tumor had a well-demarcated margin from the surrounding tissue and a heterogeneous structure with a septum. Fat-suppressed T1-weighted images revealed fat-suppressed areas, suggesting the presence of a fatty component (Figure 1c ). Based on these findings, we initially considered the tumor to be an atypical lipomatous tumor or liposarcoma. Therefore, we dissected the tumor with a narrow margin for a histopathological examination. Dissection from the surrounding tissues was relatively easy. The excised mass was covered with an elastic, soft, and thin capsule that was yellowish in color and contained hard, whitish nodules.
The histopathological examination showed the lobular proliferation of mature adipocytes, bundles of collagen fibers, and spindle-shaped cells (Figure 1d and 1e ). Adipocytes and spindle-shaped cells had neither atypical nuclei nor mitosis. Immunohistochemical staining revealed that spindle-shaped cells were positive for CD34 and negative for S100 protein, MDM2, α-smooth muscle actin (αSMA), cyclin-dependent kinase 4 (CDK4), signal transducer and activator of transcription 6 (STAT6), and desmin. Less than 5% of tumor cells were positive for mindbomb E3 ubiquitin protein ligase 1 (MIB1) (Figure 2 ). Based on these results, the patient was diagnosed with SCL. Twelve months after surgery, neither recurrence nor postoperative complications have been observed.