What is the most probable diagnosis?

Diagnosis

Phyllodes Tumour

Discussion:

The tumours vary in size from a few centimeters to massive lesions involving the entire breast. The larger lesions often have bulbous protrusions (phyllodes is Greek for "leaflike") due to the presence of nodules of proliferating stroma covered by epithelium. In some tumours these protrusions extend into a cystic space. This growth pattern can also occasionally be seen in larger fibroadenomas and is not an indication of malignancy. Phyllodes tumours are distinguished from the more common fibroadenomas on the basis of cellularity, mitotic rate, nuclear pleomorphism, stromal overgrowth, and infiltrative borders. Low-grade lesions resemble fibroadenomas but are more cellular and contain mitotic figures. High-grade lesions may be difficult to distinguish from other soft tissue sarcomas and may have foci of mesenchymal differentiation (eg. Rhabodomyosarcoma or liposarcoma). The frequency of chromosomal changes increases with grade and the majority of high-grade lesions are reported to have amplification of EGFR. Recurrent phyllodes tumours are often of a higher grade than the presenting lesion.