A paper in Histopathology, published on 8 October by an international group of breast pathologists, addresses a gap in phyllodes tumour grading. Stromal mitotic activity is one of the main criteria separating benign, borderline and malignant tumours, yet how to count mitoses has not been clearly specified, so pathologists variously average across fields or count the most active area.
The group recommends the hotspot method: find the most mitotically active stromal area and count there, using a protocol that accounts for the field diameter of the individual microscope so that counts per area are comparable between laboratories. The count is then combined with the other grading criteria — stromal cellularity, atypia, overgrowth and the tumour border. The paper also discusses digital counting tools.
This is expert opinion rather than an outcome study, but it standardises a step on which the grade, and therefore the extent of surgery and follow-up, depends. Laboratories should check that their microscopes' field diameters are known and that the method is stated in reports.
- Count stromal mitoses in the most active area (hotspot), not as an average across the tumour.
- Know your microscope's field diameter and convert counts to a standard area.
- Combine the count with cellularity, atypia, stromal overgrowth and border — no single feature grades the tumour.
- State the counting method in the report so surgeons and other pathologists can compare.
- Consider digital counting where available to improve reproducibility.
Why it matters
Different counting methods can move a tumour between benign, borderline and malignant, changing surgery and follow-up.
Don't overread it
This is an expert recommendation, not an outcome study showing that hotspot counting predicts recurrence better.
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