Can phyllodes tumors have a spindle cell component?

Phyllodes Tumors Are Spindle Cell Neoplasms

Yes, phyllodes tumors are fundamentally spindle cell lesions—they are composed of a benign epithelial component and a cellular spindle cell stroma that forms their characteristic leaf-like architecture. 1, 2

Histologic Composition

• Phyllodes tumors are biphasic breast neoplasms containing both epithelial and stromal components, with the stromal component being the defining cellular spindle cell element 2, 3
• The neoplastic spindle stromal cells create the classic leaf-like architecture with subepithelial stromal condensation that characterizes these tumors 3
• The stromal spindle cells differentiate phyllodes tumors from fibroadenomas, which arise from the same intralobular fibrous tissue origin but have different stromal characteristics 4

Stromal Cell Characteristics

• The spindle cell stroma in phyllodes tumors is more proliferative than in fibroadenomas, showing higher PCNA and Ki-67 positivity 4
• Malignant phyllodes tumors are classified among high-grade spindle cell lesions of the breast, alongside spindle cell carcinomas and matrix-producing metaplastic carcinomas 3
• The stromal spindle cells are typically positive for CD34 (72.5% of cases), CD117, and bcl-2 3, 5

Important Diagnostic Pitfall: Keratin Expression

• Contrary to common understanding, stromal spindle cells in phyllodes tumors can express keratins in 11.9-28.4% of cases, though this staining is focal and patchy (affecting only 1-5% of stromal cells) 5
• This keratin positivity increases with tumor grade and is significantly associated with malignant features including cellularity, necrosis, and cystic change 5
• Do NOT rely solely on keratin immunostaining to exclude phyllodes tumor on core biopsy—this can lead to misdiagnosis as metaplastic carcinoma 5
• In contrast, spindle cell components of metaplastic carcinomas show wider keratin distribution (up to 90% of cells) and express multiple keratin markers 5

Differential Diagnosis Considerations

• Spindle cell-predominant phyllodes tumors may be misdiagnosed as sarcomatoid carcinoma or smooth muscle tumors 6
• The diagnostic challenge is particularly significant in core biopsies where the characteristic leaf-like architecture may not be sampled 6, 5
• In any rapidly growing or large (>2 cm) breast mass, perform excisional biopsy rather than relying on core needle biopsy to definitively exclude phyllodes tumor 1