A COMPARISON OF THE EFFECTIVENESS OF 28 KV (GRID) VERSUS 25 KV (NO GRID) MAMMOGRAPHIC TECHNIQUES FOR BREAST SCREENING

The purpose of this study was to compare the clinical effectiveness of breast screening using a mammographic technique of 25 kV without a gr id, with one of 28 kV with a grid. Effectiveness is judged by cancer d etection, interval cancer rates, sensitivity and specificity calculati ons and tumour characteristics. The doses on standard physics tests gi ven by the three machines when these exposure factors are selected wer e compared to see whether there is ally scientific basis for recommend ations on which is the more effective technique. The experiment was un dertaken in the prevalence round of a screening programme set up in th e UK in 1987. The main comparison is on 25078 women randomized to one or other technique after March 1989. Comparison can also be made with the preceding 8482 women, who were examined by the 25 kV method, but n ot randomized and in whom there were a variety of other differences. I n the randomized group there was no statistically significant differen ce in cancer detection. A minor difference in overall numbers without statistical significance was seen in favour of the 28 kV grid techniqu e, but is offset by a greater interval cancer rate in this group. Smal l cancer detection was equal in the two groups. By contrast, the first 8482 women showed significantly worse screening performance, both in lower overall and small cancer detection rate, and in increased number of interval cancers, for which the explanation is likely to be comple x. The dose measurements show that the use of a higher tube potential with the grid mitigates the dose increase that may have been expected. The choice between these two techniques is therefore neither automati cally made by greater cancer detection nor made on grounds of dose. Th ere was a minor dose penalty in using the 38 kV technique with grid.