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Hibernoma is an uncommon benign tumor arising from brown fat. These tumors are well-described in the literature, and occur throughout the body with the majority of cases arising in the thigh, shoulder, and upper back, with some in the retroperitoneum. Several morphologic variants have been described, including typical, spindle, lipoma-like, and myxoid. The overlap with a well-differentiated liposarcoma/atypical lipomatous tumor is well-known, and the two lesions can be differentiated by immunostaining and in-situ hybridization for MDM2. In this report, we describe a case of hibernoma with a dense lymphoplasmacytic infiltrate arising in the retroperitoneum, a morphologic variant not previously described in the literature.

Abstract Background Intramuscular / cellular myxomas and low-grade myxofibrosarcomas are two different tumor entities with a significant histological overlap, especially if dealing with small biopsies. Despite the morphological similarities, they differ considerably in their biological behaviour. Intramuscular / cellular myxoma rarely shows signs of recurrence and never metastasizes, in contrast to myxofibrosarcoma that tends to recur more aggressively and to metastasize haematologically. Therefore, it is of great importance to distinguish these lesions - evaluation of GNAS mutation status could be of tremendous help. Methods We reviewed 13 cases with intramuscular / cellular myxomas. The 13 cases included 5 men and 8 women, aged from 33 to 71 years (mean age 55.5 years). Immunohistochemistry was performed as well as next generation sequencing. Ten cases were located in the lower extremities and three cases were located in the upper extremities. Two lesions were initially misdiagnosed as a low-grade myxofibrosarcoma. Results Performing next generation sequencing 12 out of 13 specimens showed a GNAS mutation. Conclusions Our findings demonstrate that GNAS mutations are more common in intramuscular / cellular myxomas, than had been reported in literature in the past. Next generation sequencing for determining GNAS mutation status on small biopsies or diagnostically challenging cases facilitates the diagnosis of intramuscular / cellular myxoma and separates this tumor entity from its mimics.