Stromal mitotic activity is a key criterion for grading phyllodes tumours as benign, borderline or malignant, yet the method for counting it has never been clearly set out. An international group of breast pathologists reviewed whether counts should come from the most mitotically active area or an average across several areas.
They recommend the hotspot method: find the area of highest stromal mitotic activity and count there, adjusting for the field diameter of the microscope in use so that counts per area are comparable between laboratories. Mitotic count should be combined with the other grading criteria rather than used alone. The paper also addresses digital tools for counting.
This is an expert recommendation without new outcome data, but it addresses a common source of disagreement between pathologists on borderline and malignant tumours, which drives surgical margins and follow-up.
For laboratories, adopting a written, standard method and stating it in the report is a straightforward change.
- Count stromal mitoses in the most mitotically active area of a phyllodes tumour.
- Calibrate counts to your microscope's field diameter so results are per standard area.
- Combine the mitotic count with stromal cellularity, atypia, overgrowth and border in grading.
- State the counting method and area in the report.
- Consider digital counting tools where validated in your laboratory.
Why it matters
Inconsistent counting methods are a likely reason two pathologists grade the same phyllodes tumour differently.
Don't overread it
This is an expert recommendation; it does not yet show improved prediction of recurrence.
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