Demonstration of polyclonal origin of giant fibroadenoma of the breast.

We have shown that fibroadenoma of the breast is polyclonal and that phyllodes tumour is monoclonal in origin. It is not known whether a giant fibroadenoma which is histologically identical to the more usual type of fibroadenoma but grows to be a huge mass, like a phyllodes tumour, is polyclonal or monoclona. Clonal analysis was conducted on the DNA samples extracted from the paraffin sections of a giant fibroadenoma resected from a 21-year-old woman. The method used was based on trinucleotide repeat polymorphism of the X-chromosome-linked androgen receptor gene and on random inactivation of the gene by methylation. Clonal analysis showed that the giant fibroadenoma and the adjacent normal breast tissue are polyclonal in origin. Although the term giant fibroadenoma has often been used interchangeably with the term benign phyllodes tumour, because of their similarity in clinical appearance, our present results demonstrate that a giant fibroadenoma is a polyclonal fibroadenoma that has attained an immense size and is different from the monoclonal phyllodes tumour.