Mammary hamartoma has a reported incidence of 0.1% to 0.7%.2,6,9
The true incidence is probably higher, as pointed out by Daya et al
and substantiated by our own experience that with the increasing use of breast imaging more hamartomas are likely to be identified.
Hamartoma has the typical mammographic appearance of lucent lesions containing fat, varying radio dense fibrous and adenomatous elements, a sharp margin, and sometimes a thin capsule. Lobulated densities are dispersed within the encapsulated fat, described as a “slice of salami”. The ultrasound shows sharp definition and displacement of surrounding structures. It contains sonolucent fat and echogenic fibrous components with a heterogeneous internal echo pattern.11
The MRI shows the presence of internal fat density in addition to the smooth well defined hypointense rim and internal heterogeneous enhancement, which are characteristic of breast hamartoma.
The pathology of hamartoma remains poorly defined; the original definition1
was used for a clinically discrete nodule composed of a variable amount of epithelial elements in a fibrofatty stroma. Early attempts to subclassify hamartomas according to the histological parameters resulted in a confusing state of affairs. A three category classification of the “fibrous”, “fatty”, and “fibro fatty” hamartoma has been put forward by McGuire and Cohn,12
and Jones et al
suggested a four category classification of “encapsulated fibrocystic changes”, “fibroadenoma with fibrous stroma”, “fibroadenoma- like”, and “circumscribed adenolipoma”.5
Neither of these descriptive classification systems has been widely adopted. The current criteria used by practising pathologists have not been described in a detailed manner.13
Fechner has described the difference in lobular distribution and the presence of fat in hamartomas as the differentiating features against the more common fibroadenomas.10
In the literature, several good reviews have been published encompassing moderate numbers of cases.2,3,5,6,14
In most of these series, efforts were devoted to the identification of definitive histological criteria. Different authors had used different criteria, but overlap exists.
The presence of lobules within a fibrotic stroma, which surrounds and extends to between individual lobules and obliterates the usual interlobular specialised loose stroma, is most characteristic. Most authors2,3,5,6
have alluded to this interlobular fibrosis. The same is seen in our series of 25 cases, in which most showed such a feature. However, this feature is not unique to hamartomas. In sclerosing lobular hyperplasia (fibroadenomatoid mastopathy), which is a well defined, rounded mass with enlarged lobules showing increased numbers of intralobular glands, the presence of interlobular stromal sclerosis mimics hamartoma. Its frequent association with fibroadenoma, and the absence of fat in the stroma, may help to distinguish sclerosing lobular hyperplasia from hamartoma.
Adipose tissue within the stroma is also commonly reported in hamartomas. In most series,2,3,5,6
adipose tissue is present in more than 90% of the cases, although the volume of adipose tissue generally accounts for 10–20% of the lesion volume. Charpin et al
reported cases consisting of up to 70% fat.6
We report similar findings, with fat being present in all cases, and ranging from 5% to 90%, with a mean volume of 31% of the hamartoma.
The presence of pseudo-angiomatous stroma within the hamartoma has been reviewed and described in detail by Fisher et al
Subsequent experience by other authors3,15
found this to be a constant observation, even though the incidence varies from a high 71%2
to a low 16–20%.3,15
In our series, pseudo-angiomatous stroma was found in 32% of the hamartomas.
“The correct identification of hamartoma is important because there are the problems of recurrence and coincidental epithelial malignancy”
Epithelial changes were also seen in our series. Simple epithelial hyperplasia without atypia occurred in 40%, whereas cystic changes occurred in 32%, apocrine metaplasia was present in 16%, overlapping with cystic changes, and adenosis occurred in 4%. Cystic changes with or without concomitant apocrine metaplasia was also reported as a common feature of mammary hamartoma,2,3,6
even though in another series15
cystic changes were present in 20% of the cases reported. In our series, 32% showed significant cyst formation.
Other rare features that have been described in the literature, but occurring in a smaller proportion of cases include microcalcification,3
myoid (smooth muscle) differentiation,3,5,6,16
and stromal giant cells.6
To add to the list is one case with focal ossification within the stroma of the hamartoma seen in our series.
Take home messages
- Hamartomas do not possess specific diagnostic histological features, and diagnosis is therefore difficult
- The presence of fibrous tissue within the lobules, or fibrous tissue and fat in the stroma, with or without pseudo-angiomatous changes, should alert the pathologist to the possibility of a hamartoma
- The role of fine needle aspiration cytology and needle core biopsy in making the diagnosis is limited, and requires clinical and radiological correlation to avoid underdiagnosis
- Although hamartomas are benign, coincidental malignancy can occur, and the issue of potential recurrences has not been resolved
For all the features that have been evaluated, none is unique to hamartoma. The presence of fibrosis or hyaline fibrosis within the stroma can be seen in a variety of breast lesions, including fibroadenoma, in which it constitutes the major differential diagnosis, as well as sclerosing lobular hyperplasia. Within the spectrum of fibrocystic changes, sclerosis, stromal fibrosis, apocrine metaplasia, and cystic changes of the ducts are common; pseudo-angiomatous changes in the stroma have also been reported in many types of breast lesions.17
The correct identification of hamartoma is important because there are the problems of recurrence and coincidental epithelial malignancy. In the series of Daya et al
two of 25 cases recurred after local excision, after an interval of seven and 18 months, and in our series, one lesion recurred after 10 months. This is different from the experience of other authors, who reported no recurrences.2,5,6,14,15
More experience is clearly needed. Furthermore, there are occasional case reports8,18–20
of coincidental in situ or invasive ductal or lobular carcinoma occurring in hamartomas. Although such cases are rare, it is imperative for the pathologist to be diligent in sampling hamartomas for suspicious areas.
The use of FNAC in diagnosing breast lesions has now been well established and proved to be accurate. The role of FNAC in diagnosing hamartoma remains limited. In the literature, only one report has commented on its lack of usefulness in this respect.14
In another cytology report of a single case of FNAC of a breast hamartoma,21
a diagnostic label could not be assigned to the FNAC specimen, although the hamartoma was confirmed by the histology of the excision specimen. Similar findings occurred in our series, in which none of the hamartomas was diagnosed correctly at FNAC. The two main contributing factors are the scanty materials obtained during the aspiration and the lack of specific cytological or architectural features in hamartoma. For the rare cases in which the FNACs were of moderate to high cellularity, differentiation from fibroadenoma was not possible. Even core biopsy is of limited value, especially when the imaging findings or the clinical impressions were not provided.
Diagnosing hamartoma of the breast is difficult, especially in biopsy or FNAC. The pathologist who sees fibrous tissue within the lobules, or fibrous tissue and fat in the stroma with or without pseudo-angiomatous changes, should be alerted to the possibility of a hamartoma. Correlation with the imaging findings and clinical impression may avoid the embarrassing situation of diagnosing “no significant pathology” in a palpable, radiologically distinct lesion. The radiologist who performs FNAC or needle core biopsy should remember that FNAC can rarely yield sufficient sample for diagnosis, and that both FNAC and needle biopsy are unlikely to provide enough information for the pathologist. Good communication of imaging findings is essential. The surgeon should also realise that although hamartomas are benign, coincidental malignancy may occur, and the issue of potential recurrences has not been resolved.