Introduction: Hibernoma is a very rare benign tumor that arises from vestigial remnants of fetal brown adipose cells and usually manifests as a slowly growing, painless soft-tissue mass. It mainly occurs in adults, in the third and fourth decade of life, slightly more in women and is commonly seen in the subcutaneous regions of the back, neck, thighs and retroperitoneum. It was originally described in 1906 by Merkel, who named it "pseudolipoma". In 1914, Gery derived the name hibernoma from the tumor's histological similarity to brown fat in hibernating animals. A hibernoma may be confused with a lipoma clinically and cannot be completely distinguished from hypervascular lesions such as lipossarcoma.
Methods: A 36-year-old woman presented with pain and edema of the left leg. It was diagnosed with non-recent femoro- -popliteal venous thrombosis, was medicated with rivaroxaban and prescribed compression stocking.
Results: The edema subsided after 2 weeks but she still complained of pain in the thigh several weeks after first visit along with subtle localized soft enlargement in the upper thigh. It was requested a CT scan that showed a nodular image with 60X 47 mm medially to vastus intermedius and beneath the sartorius and rectus femoris muscles, which was suspected to be a lipossarcoma. In this context, a magnetic ressonance imaging was requested and showed contact with femoral vessels with no cleavage plan, suspected to be a mixoid lipossarcoma. The biopsy didn't show malignancy. She was operated with local excision of the mass and preservation of adjacent structures. Pathologic evaluation revealed a hibernoma with 11.5 cm, PS100 positive and MDM2 negative. The patient was evaluated at outpatient clinic 6 months after surgery and had no evidence of relapse.
Conclusion: The first clinical manifestation of this patient was a deep vein thrombosis and the diagnosis of the lipomatous tumor was delayed. Clinical awareness of less frequent causes of DVT is a key point to timely detection of this lesions that are rare and curable. The increased vascularity of this lesion raised suspicion of malignancy. Malignancy potential is perhaps the most difficult aspect to ascertain in this patient, being only completely disclosed after surgical excision. Optimal treatment is complete surgical resection. Local recurrence does not occur with complete excision. No reports of metastases of malignant transformation have been identified in the reviewed literature.