Introduction Modified Scarff Bloom and Richardson score (or Nottingham histologic grading) has become widely accepted as a powerful indicator of prognosis in breast cancer. It combines nuclear grade, tubular formation, and mitotic rate. Each element is given a score of 1 to 3 (1 being the best and 3 the worst) and the score of all three components are added together to give the ‘grade’.

The majority of studies that analyze the reliability of this grade, compare it to that found on the surgical specimen. Few studies have examined its interobserver variability in core biopsies.

Objective To evaluate the interobserver variability of Nottingham histologic grade in scoring breast cancer in core biopsies among 2 general pathologists.

Methods This is a retrospective study of 65 cases of invasive ductal carcinoma that were independently evaluated by two pathologists and graded according to the Nottingham histologic system. A detailed histopathological assessment was carried out and analyzed statistically using the Kappa agreement score.

Results The mean size of biopsies was 15 mm. There was a substantial agreement among the 2 pathologists in scoring tubular formation, pleomorphism, and final grading (Kappa=0.7, 0.65 and 0.8 respectively). A fair agreement was noted in scoring mitosis (Kappa=0.35).

Conclusion The interobserver variability of Nottingham grading in scoring breast cancer in core biopsies remains good. The relatively weak agreement in scoring mitosis is secondary to the small size of the micro-biopsies, not covering the 2 mm2 fields necessary to grade this parameter. This often leads to an extrapolation of the number of mitoses.