In my previous post here on Lablogatory, I discussed the diagnosis and comparison of two mediastinal fine needle aspiration (FNA) cases – thymoma and thymic carcinoma. I tooted my own horn of how I instantly recognized the tumors on Rapid On-Site Evaluation (ROSE), as the characteristics were exactly how I remembered them from my cytology knowledge bank formulated in grad school. Here’s a case that completely threw me off my game. I had never seen this type of tumor nor heard of it, at least not to my memory, but that’s the beauty of lab medicine—we’re continuously learning.
A 43 year old female with hypertension and no cancer history presented to a vascular surgery clinic for treatment of varicose veins, and an ultrasound was performed, noting a mass in the left inguinal region. The patient subsequently had an MRI, which demonstrated a predominantly fatty mass in that area with enhancement and probable necrosis within the lesion. The differential diagnosis determined by imaging was fat necrosis versus liposarcoma. With this risk of malignancy, the patient came to our institution for biopsy and further guidance. The ultrasound department visualized the left inguinal mass of mixed echogenicity, measuring 3 centimeters with a focal area of central necrosis.
After receiving two FNA passes of the patient’s left inguinal mass from the radiologist, I made mirror-image smears of the samples, air-drying one slide for Rapid On-Site Evaluation (ROSE), fixing the other in 95% Ethanol, and rinsing the needles in Hanks Balanced Salt Solution to later make a FFPE-Cell Block.
I remember my differentials – Lipomatous tumor of unknown etiology versus clear cell renal cell carcinoma versus adrenal cortical carcinoma. I knew it was a neoplasm of sorts and that we had adequate material for a diagnosis. But I could not make a definitive diagnosis, and it mind-boggled me. That’s when my cytopathology director reviewed the case with me, and I went straight to the cytology encyclopedias.
The FNA specimen was signed out as a “Benign-appearing adipose tissue neoplasm, consistent with hibernoma.
Hibernoma was also diagnosed on the concurrent core biopsy specimen by the surgical pathologist on service.
Hibernomas are rare brown fat tumors that typically develop where brown fat is normally distributed throughout the body, such as the upper back, thigh, and retroperitoneum.2 Brown fat, or brown adipose tissue is responsible for non-shivering, mitochondria-rich thermogenesis.3 From the cytology images, one can appreciate the small, eccentric nuclei and capillaries, featuring three cell types: mature adipocytes (think lipoma), lipoblast-like cells (think liposarcoma), and hibernoma cells, which appear to be highly, but uniformly vacuolated adipocytes with granular cytoplasm.
Two months after the initial biopsy, the patient underwent a radical resection of her left thigh hibernoma en bloc with a portion of the iliopsoas muscle and femoral nerve neurolysis. The intraoperative findings showed a 5.2 centimeter well-circumscribed mass directly beneath the femoral vessels, beginning at the common femoral artery and extending to the level of bifurcation of the superficial femoral artery and profunda. The mass was adherent to the posterior wall of the vessel, but fortunately did not involve the adventitial layer. The mass, however, was more adherent to the pectineus muscle and inseparable from the middle portion of the iliopsoas muscle. The mass was also adherent to the hip, and in order to clear the mass from that space, an arthrotomy was made.
The surgical pathologist signed out the case as follows:
– Hibernoma with focal myxoid changes, 5.3. cm. The inked margins showed no tumor.
In the middle of the hibernoma, there was a nodular myxoid lesion with spindle cells. Due to a question of liposarcoma, cytogenomic microarray analysis (CMA) was performed which was negative for genomic imbalances. Immunostain performed on a frozen section of tissue showed that the atypical cells were positive for Desmin, confirming that they are skeletal muscle.
If this case was diagnosed as a liposarcoma rather than hibernoma, one would see atypical lipoblasts with more prominent capillaries, like a well-differentiated liposarcoma. Depending on the type of liposarcoma, one might also identify a myxoid stroma or round cells.2
Hibernomas are a unique kind of tumor where the consensus on how to manage them remains split – some favor observation, while others suggest surgical intervention. From the literature, there are no reports to suggest metastasis or malignant degeneration/transformation, but many do favor a resection if feasible.1
- AlQattan, A. S., Al Abdrabalnabi, A. A., Al Duhileb, M. A., Ewies, T., Mashhour, M., & Abbas, A. (2020). A Diagnostic Dilemma of a Subcutaneous Hibernoma: Case Report. American Journal of Case Reports, 21, 1–5. https://doi.org/10.12659/ajcr.921447
- Cibas, E. S., & Ducatman, B. S. (2009). Cytology: Diagnostic Principles and Clinical Correlates, Expert Consult – Online and Print (3rd ed.). Saunders.
- Cypress, A., & Khan, C. (2010). The Role and Importance of Brown Adipose Tissue in Energy Homeostasis. Curr Opin Pediatr, 22(4), 478–484. https://doi.org/10.1097/MOP.0b013e32833a8d6e
Taryn Waraksa, MS, SCT(ASCP)CM, CT(IAC), has worked as a cytotechnologist at Fox Chase Cancer Center, in Philadelphia, Pennsylvania, since earning her master’s degree from Thomas Jefferson University in 2014. She is an ASCP board-certified Specialist in Cytotechnology with an additional certification by the International Academy of Cytology (IAC). She is also a 2020 ASCP 40 Under Forty Honoree.