• Primary stromal tumors of breast, benign or malignant, with a non-neoplastic epithelial component; FNA shows a dimorphic population of non-neoplastic epithelium and neoplastic stroma;

• Large, cellular stromal fragments with well-defined borders (leaf-like projection);

• High stromal cellularity, varying from bland monomorphism to high pleomorphism;

• Presence of single intact spindle-shaped stromal cells, which are long, wavy with abundant pale cytoplasm and plum nuclei;

• Naked oval nuclei also seen, and may show highly atypical;

• Features indicating malignancy: marked stromal overgrowth; stromal cells in diffuse sheets, poorly cohesive clusters and numerous single cells; high mitotic activity; prominent atypia; and intranuclear cytoplasmic invaginations.

• Differential Diagnosis of Fibroadenoma and Phyllodes tumors

  	Fibroadenoma	Phyllodes Tumors
  Clinical	More common, round, younger age	Rare, larger, irregular, lobulated, rapid growth
  Epithelium	Abundant	Less abundant, non-neoplastic, similar to fibroadenoma
  Stromal	Maybe prominent,	High cellularity, leaf-like projections
  Stromal cells	Spindle, smaller	Long, wavy, benign or malignant
  Squamous metaplasia	Rare	Common
  Apocrine metaplasia	Common	Rare
  Naked oval nuclei	Present	Present
  Single intact cells	Epithelial cells	Spindle stromal cells

Reference

• The art and science of cytopathology. Richard M Demay, MD. 1996