Phyllodes Tumor Classification
CAP-based benign / borderline / malignant classification with full reasoning shown.
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Educational and decision-support tool only. This application does not replace microscopic examination, multidisciplinary assessment, institutional protocols, or professional pathologic diagnosis. Classification should be based on the complete histomorphologic assessment of the tumor.
Reference
About phyllodes classification
Phyllodes tumor classification is based on an integrated morphologic assessment of stromal cellularity, stromal atypia, stromal overgrowth, mitotic rate, tumor border, and malignant heterologous elements — not an additive numeric score like the Nottingham grading system used for invasive breast carcinoma. This tool applies a rules-based interpretation of the CAP criteria and explains its reasoning, rather than simply tallying points.
CAP histologic features
| Feature | Benign | Borderline | Malignant |
|---|---|---|---|
| Stromal cellularity | Mild | Moderate | Marked |
| Stromal atypia | None/Mild | Mild/Moderate | Marked |
| Stromal overgrowth | Absent | Absent/very focal | Present |
| Mitotic rate | ≤4/10 HPF (<2.5/mm²) | 5–9/10 HPF (2.5–5/mm²) | ≥10/10 HPF (≥5/mm²) |
| Tumor border | Circumscribed | Usually circumscribed/focal infiltrative | Infiltrative |
| Heterologous elements | Absent | Absent | May be present (establishes malignant regardless of other features) |
Classification rules
Malignant requires all five major histologic features to be present simultaneously (or a qualifying malignant heterologous element). Benign requires the mild/absent pattern across all features. Borderline falls between, without meeting full malignant criteria.
Override rule: presence of a malignant heterologous mesenchymal component can establish malignant phyllodes tumor even when all other malignant histologic criteria are not present.
Exception: well-differentiated liposarcoma / atypical lipomatous tumor alone should NOT automatically classify the tumor as malignant.
Note: the distinction between benign and borderline phyllodes tumor is not always well-defined, and there is no universal agreement on which histologic features should receive greater emphasis. When entered findings don't fit a clean pattern, this tool flags them as discordant and recommends professional correlation rather than forcing a classification.
Stromal overgrowth and field size
Stromal overgrowth is present when at least one low-power microscopic field contains stroma only, without epithelial elements. Low-power field = 4× objective with 10× eyepiece, or 22.9 mm².
HPF size varies between microscopes. When using mitoses/mm², the high-power field area should be determined for the specific microscope.
Source
College of American Pathologists. Protocol for the Examination of Resection Specimens From Patients with Phyllodes Tumor of the Breast. Version 1.1.0.1. September 2022. Applies to resection specimens including excision, lumpectomy, quadrantectomy, and partial/total mastectomy.