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A dedifferentiated liposarcoma with inflammatory myofibroblastic tumor‐like features

To the Editor: Although various degrees of inflammatory cell infiltration and fibrosis are commonly observed in liposarcoma, well-differentiated liposarcomas (atypical lipomatous tumors)—which show extensive inflammatory cell infiltration—have been classified as an inflammatory variant of well-differentiated liposarcoma. In 2010, Lucas et al. were the first to report cases of dedifferentiated liposarcoma (n1⁄4 6) with extensive lymphoplasmacytic infiltration and histology resembling that of inflammatory myofibroblastic tumor (IMT). They called these cases ‘dedifferentiated liposarcoma with IMT-like features’ and noted the risk of misdiagnosis as IMT, which is a much less aggressive neoplasm than dedifferentiated liposarcoma. Since then, no case of dedifferentiated liposarcoma with IMT-like features had been reported, but we recently encountered a case of dedifferentiated liposarcoma of the retroperitoneum which required a differential diagnosis from IMT, IgG4-related disease, and Castleman's disease. We eventually diagnosed the case as dedifferentiated liposarcoma with IMT-like features, and we report the case as follows. A 65-year-old Japanese man visited a nearby hospital for the examination of an abdominal mass lesion that had been revealed by an abdominal echo examination as part of his medical checkup. As a retroperitoneal tumor was confirmed, he was referred to our hospital for further examination and treatment. He had no remarkable medical history and no remarkable family history other than an appendectomy for acute appendicitis. Laboratory tests on admission showed no abnormality. Post-contrast abdominal CT demonstrated a well-circumscribed mass lesion measuring 11 8 cm in diameter at the retroperitoneum; the mass lesion was slightly enhanced in the delayed contrast phase (Fig. S1a). No fat component was found in the mass lesion by in-phase and opposed-phase MRI (Fig. S1b,c). Under the clinical diagnosis of retroperitoneal tumor without invasion of surrounding organs, we performed a laparoscopic resection of the retroperitoneal tumor. The postoperative course was uneventful, and no recurrence was found as of the time of this writing (2 years after surgery). The resected specimen showed an elastic, hard, welldefined and yellowish-white-colored tumor measuring 12.5 9.8 7.3 cm (Fig. S2). Histologically, the margin of the tumor was bordered by lymphoid tissues that partially formed lymphoid follicles (Fig. 1a). No atypia was observed at adipocytes surrounding the tumor (Fig. S3a). In the tumor, a significant deposition of collagen fibers with focal hyalinization and infiltration and aggregations of lymphocytes and plasma cells were extensively observed (Fig. 1b). Spindle-shaped cells with mild nuclear atypia were found between the collagenous fibers (Fig. 1c). No necrotic lesion was found in the tumor, and mitotic figures were not apparent. Small vessels with hyalinization which suggest the hyaline vascular type of Castleman's disease were also observed. No lipogenic lesion was found in the tumor by the pathological examination. As a differential diagnosis based on hematoxylin-eosin (HE) staining results, we considered IMT, the hyaline vascular type of Castleman's disease, IgG4-related diseases, and liposarcoma. In an immunohistochemical analysis, IgG4-positive plasma cells were scarcely found in the specimen; IgG4related disease was therefore ruled out. Although many aSMA-positive spindle-shaped cells were observed, we considered these cells reactive myofibroblasts because the spindle-shaped cells with nuclear atypia were negative for a-SMA and no anaplastic lymphoma kinase (ALK)-positive cells or pan-Trk positive cells were recognized. Therefore, the differential diagnosis of IMT was denied. Spindle-shaped cells with mild nuclear atypia diffusely expressed CDK4 (Fig. 1d), p16 (Fig. S3b), and MDM2 (Fig. S3c, performed at Kyushu University). Expressions of CDK4, p16, and MDM2 were not observed at the surrounding adipose tissue (Fig. S3d). The Ki-67 labeling index of the atypical cells was 14.8%. These immunohistochemical findings indicated the diagnosis of liposarcoma. The primary antibodies used are summarized in Table S1. Based on these HE and histochemical findings, we made the final diagnosis of dedifferentiated liposarcoma with IMT-like features. Typically, a dedifferentiated liposarcoma consists of a component of well-differentiated liposarcoma adjacent to a non-lipogenic sarcoma of dedifferentiated component. Usually, the dedifferentiated component shows high-grade morphology resembling undifferentiated pleomorphic sarcoma (which is a synonym for malignant fibrous histiocytoma: MFH). However, low-grade dedifferentiation characterized by the presence of fibroblastic spindle cells with mild nuclear atypia has been recognized. If the present case had contained high-grade morphology resembling an MFH, the diagnosis of dedifferentiated liposarcoma with an inflammatory MFH-like component would be considered.