Breast screening unit size and performance on self-assessment (PERFORMS)

The UK Breast Screening Programme (UKBSP) is comprised of approximately 100 individual Breast Screening Units (BSUs) that vary in size (measured by number of women screened). Previous research using UKBSP real life data (Blanks, Bennett, Wallis and Moss, 2002), attributed differences in performance, related to BSU size, to smaller units' slightly lowered cancer detection rates and positive predictive value scores (compared to large/medium units).

All BSUs on the UKBSP annually take part in the PERFORMS scheme as way of self-assessing their film-reading skills. We looked at the performance of all film-readers who had completed the last PERFORMS round (SA08) by BSU size in order to explore any group differences mediated by unit capacity.

Each BSU's size was approximated by ranking each unit by number of readers who had completed the last PERFORMS round. Subsequently, these BSUs were allocated into three main groups approximating their unit's size: small = 1 to 4 readers, 30% (n = 157); medium = 5 to 7 readers, 34.5% (n = 181); and large = 8 or more readers), 35% (n = 186). Several performance measures were compared, including 'percentage correct recall' and 'malignancies detected' (measures of sensitivity), 'percentage correct return to screen' (a measure of specificity), and negative and positive predictive value scores.

Analysis of variance (one-way) did not produce any significant findings (P = not significant) for any of the measures, indicating equivocal performance. Descriptive statistics showed smaller units scored less than 1% below medium/large BSUs for malignancies detected, correct recall and negative predictive value only.

Unlike real-life screening, smaller units perform at a similar level to all others on self-assessment.