Phyllodes tumors (from Greek: phullon), are a rare type of biphasic fibroepithelial mass that form from the periductal stromal and epithelial cells of the breast. They account for less than 1% of all breast neoplasms. They were previously termed cystosarcoma phyllodes, coined by Johannes Muller in 1838, before being renamed to phyllodes tumor by the World Health Organization in 2003. Phullon, which means "leaf" in Greek, describes the unique papillary projections characteristic of phyllodes tumors on histology. Diagnosis is made via a core-needle biopsy and treatment is typically surgical resection with wide margins (>1 cm), due to their propensity to recur. Signs and Symptoms Phyllodes tumors typically present as a firm, mobile, and palpable mass that is painless in nature. On physical examination, the mass can demonstrate a smooth or nodular texture depending on its size. In addition, larger masses can stretch the overlying breast tissue leading to nipple retraction, chest wall fixation, and in advanced cases, ulceration from pressure necrosis. Phyllodes tumors can grow to a variety of sizes, ranging from 0.8 to 40 cm, with the average tumor growing to between 4 to 8 cm. Their growth can be slow, rapid, or demonstrate alternating growth patterns. A notable feature however is that their growth rarely regresses, a distinguishing feature from fibroadenomas which tend to change in size based on a female's menstrual cycle. Causes Experts are unsure of the exact etiology of phyllodes tumors. Genetically, phyllodes tumors have shown to have a higher incidence with conditions such as Li-Fraumeni syndrome and BRCA1 / BRCA2 mutations. In addition, males with a history of gynecomastia have shown an increased incidence of developing a phyllodes tumor. Mechanism The pathogenesis behind the development of a phyllodes tumor is not well-defined. Some theories suggest a genetic cause while other literature supports the involvement of hormone and growth factor receptors, cell signal transduction pathways, and cell cycle markers. Certain receptors include estrogen / progesterone, glucocorticoid, and HER2. The most well-studied cell signal transduction pathway includes the Wnt pathway, which is a highly-conserved genetic pathway between species. In the Wnt pathway, gene transcription via B-catenin is a highly-regulated process by proteins including, but not limited to, c-myc, c-jun, Fra, and cyclin D1. Mutation of any these proteins can lead to the un-regulated, rapid growth characteristic of phyllodes tumors. In addition, vascular factors promoting angiogenesis have been shown to further promote growth of these breast masses.