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What is a Bloom Richardson score?

Published in Breast Cancer Grading 2 mins read

The Bloom Richardson (BR) score, also known as the Nottingham Histologic Grade, is a crucial grading system used to assess the aggressiveness of invasive breast cancers, specifically "invasive no-special-type" (formerly known as invasive ductal carcinoma). It provides a numerical score that helps oncologists and patients understand the tumor's differentiation and potential behavior.


Understanding the Bloom Richardson Score

The Bloom Richardson score is a pathology-based grading system that evaluates three key morphologic features of breast cancer cells observed under a microscope. Each feature is assigned a score, and the sum of these scores determines the overall grade of the tumor. The total BR score can range from 3 to 9, indicating varying degrees of tumor differentiation and aggressiveness.

The Three Morphologic Features

As specified in Note 4, the BR score is based on an assessment of:

  1. Degree of Tubule Formation / Histologic Grade: This feature assesses how well the tumor cells form glandular structures (tubules), similar to normal breast tissue.
  2. Mitotic Activity: This refers to the rate of cell division within the tumor, indicating how quickly the cancer cells are proliferating.
  3. Nuclear Pleomorphism / Nuclear Grade of Tumor Cells: This evaluates the size, shape, and uniformity of the tumor cell nuclei, reflecting how abnormal they appear compared to normal cells.

Each of these features is individually scored on a scale from 1 to 3, with 1 being the least aggressive (well-differentiated) and 3 being the most aggressive (poorly differentiated).

How the Bloom Richardson Score is Calculated

The individual scores for tubule formation, nuclear pleomorphism, and mitotic activity are summed to arrive at a total Bloom Richardson score. This total score then corresponds to one of three grades:

Feature Evaluated Score 1 (Low Aggressiveness) Score 2 (Moderate Aggressiveness) Score 3 (High Aggressiveness)
Tubule Formation >75% of tumor forming tubules 10-75% of tumor forming tubules <10% of tumor forming tubules
Nuclear Pleomorphism Small, uniform nuclei Moderate variation in size and shape Marked variation, large, irregular nuclei
Mitotic Activity Low mitotic count (e.g., 0-5 mitoses/10 HPF*) Moderate mitotic count (e.g., 6-10 mitoses/10 HPF*) High mitotic count (e.g., >10 mitoses/10 HPF*)

*HPF: High Power Field, a common unit for microscopic counting.

Interpreting the Final Score

The sum of the three individual scores (ranging from 3 to 9) determines the overall histologic grade of the breast cancer:

  • Grade 1 (Score 3-5): Well-differentiated tumor, indicating slow growth and less aggressive behavior.
  • Grade 2 (Score 6-7): Moderately differentiated tumor, representing intermediate growth and behavior.
  • Grade 3 (Score 8-9): Poorly differentiated tumor, suggesting fast growth and more aggressive behavior.

This grading system is a critical tool for pathologists and oncologists in determining prognosis and guiding treatment decisions for patients with breast cancer.