Keywords
hibernoma - breast benign neoplasia
Palavras-chave
hibernoma - neoplasia mamária benigna
Introduction
Hibernomas are rare benign neoplasms composed of brown fat cells.[1] These tumors arise from remnants of fetal brown fat, which commonly tends to involve
by the first weeks after birth and be replaced by white adipocytes.[2] Histologically, this kind of adipose tissue is very similar to the one found in
hibernating animals, thus the term hibernoma.[1]
Although uncommon, it has been described that hibernomas occur more often in areas
where residual brown fat is found, such as the interscapular region, axilla and the
groin.[3] Mammary hibernomas are extremely rare, with only a few cases described in the literature.[2]
[3]
[4]
[5]
[6]
Case Report
A 42 years-old female patient presented with a 6 months history of a lump in the upper
outer quadrant of her right breast. The lesion was painless, with no association to
either edema or papillary discharge. She had her menarche when she was 12, her obstetric
history was G1P1A0 and she had been using oral contraception for 6 years. There was
no familiar history of breast cancer.
The initial clinical assessment showed a 10.0 × 10.0 cm lobulated, well-defined soft
mobile mass, with no evidence of axillary or supra/infraclavicular fossae lymphadenopathy.
A previous mammography revealed a regular, partially defined nodule in the upper outer
quadrant of her right breast. Additionally, an ultrasound scan of the lesion confirmed
a hypoechoic solid nodule of ∼ 10.2 × 5.0 cm, which occupied most of the upper outer
and the upper inner quadrants of the right breast.
The histopathological evaluation of this mass by core biopsy showed fibrous-adipose
tissue associated with areas of steatonecrosis, without any evidence of neoplasia
in those samples. Therefore, the patient was submitted to partial mastectomy.
The surgical specimen was constituted by a yellowish round mass of 12.0 × 8.0 × 4.0
cm, with well-defined boundaries and elastic consistency, showing compact surface
after being sectioned. The resection was marginal. Microscopically, the tumor was
composed predominantly by lobules of large round to polygonal cells with abundant
multivacuolated cytoplasm, well-defined membrane and central nuclei with fine chromatin
and prominent nucleoli, which is consistent with a brown fat tumor, admixed with regular
white adipocytes and small blood vessels ([Fig. 1]). In addition, the surgical margins were microscopically negative. The diagnosis
of mammary hibernoma was confirmed by immunohistochemistry, which revealed positivity
for S100 protein; the tumor cells were negative for CD31, CD34, CD68 and topoisomerase
([Fig. 2]).
Fig. 1 High power microscopic view (H&E stain) demonstrating large polygonal cells with
abundant multivacuolated, eosinophilic, cytoplasm.
Fig. 2 Microscopic view demonstrating immunohistochemical positivity for S100 (A) and negativity
for CD34 (B).
Discussion
Hibernomas are uncommon benign tumors histologically composed of a specialized form
of adipose tissue known as brown fat.[2] Although the brown adipocytes are gradually replaced by white fat after birth, foci
of remnant brown fat may persist in adults; thus, hibernomas might theoretically arise
in any area where these foci are found, although a de novo brown fat differentiation
is reported as possible.[2]
[5] These tumors are mainly being reported in the thigh, shoulder, back, neck, chest,
arm, abdominal cavity and retroperitoneum, accounting for only 1.6% of benign lipomatous
neoplasms, with a slight predominance in adult men and a higher incidence between
the third and fourth decades of life.[5]
[7]
Among the mammary benign tumors, hibernomas are one of the rarest with only five cases
previously described in the literature.[2]
[3]
[4]
[5]
[6] Most cases present with an asymptomatic growing mass, although this tumor might
cause symptoms due to adjacent structures compression, or it can be even diagnosed
as an incidental finding at a radiological routine examination.
Grossly, hibernomas are well-circumscribed, lobulated and sometimes partly encapsulated.
As sectioned, the cut surface has a yellowish brown hue with a rubbery texture.[6] On histological examination these tumor exhibit a lobular pattern with pale and
eosinophilic multivacuolated fat cells with small, central, or eccentric nuclei admixed
with capillaries with a varying degree of differentiation.[5]
[6] Nuclear atypia and mitotic figures are exceptionally rare.[7] The histopathological diagnosis is based upon morphological features. However, although
not necessary, immunohistochemistry may be used to confirm adipocyte differentiation.[5]
Corroborating with our findings, immunohistochemical studies of hibernomas show global
positivity for S100 protein in both eosinophilic and pale cells with variably intensity
and negativity of CD34.[5]
[6] Although CD31 was previously reported to might be positive in normal and neoplastic
brown fat cells, in our case it was negative.[5]
Differential diagnosis include other lipomatous neoplasms, such as lipoblastoma and
well-differentiated liposarcoma, adult rhabdomyoma and fat necrosis.[6] The likelihood of diagnostic confusion with other tumors with a complete surgical
excision is, however, minimal.
The present case was resected with marginal excision. The largest series published
to date about hibernomas (170 cases derived from the files of the Armed Forces Institute
of Pathology – AFIP) revealed no recurrence or aggressive behavior, even though many
of these tumor were incompletely excised with a mean follow-up period of 7.7 years.[7]
In conclusion, hibernomas are benign tumors closely related to brown adipose tissue,
which is rarely present in the human breast. Nevertheless, differential diagnosis
must be done to rule out malignant neoplasias.
