The Breast (1995) 4,203~204 0 1995 Pearson Professional Ltd I Short report I Entry into the tamoxifen prevention trial depends on women’s estimates of the population risk of breast cancer K. Thirlaway, L. Fallowfield, G. Evans* and A. Howell* CRC Communication and Counselling Research Centre, Department of Oncology, University College London Medical School, London, *Breast Family History Clinic, Nightingale Breast Screening Centre, University Hospital of South Manchester, Manchester, UK INTRODUCTION Women who have an increased risk of developing breast cancer are currently being offered the opportunity to join a prevention trial. Women aged 45-65 years with a greater than two-fold increased risk are randomized to re- ceive either tamoxifen or placebo for 5 years. Younger women with an even higher risk may also be randomized. The rationale for the trial is based on the observation that women with breast cancer who take tamoxifen after sur- gery have fewer new tumours in the contralateral breast compared with controls. The factors influencing women’s decisions about participating in a prevention trial have not been thoroughly investigated although, in groups with a high genetic risk of other diseases, an individual’s perception of the general population risk appears to be important.’ Risk perceptions of women are often inaccu- rate, for example only 33% of 200 women attending a breast cancer family history clinic correctly understood the general population risk and only 41% correctly understood their own risk despite careful counselling.2 Knowledge about the general population risk of devel- oping breast cancer could be a reference against which women can evaluate their own risk and this may then influence behaviour. We investigated the relationship between understanding of breast cancer risk and the decision about whether or not to join the tamoxifen prevention trial. PATIENTS AND RESULTS Women concerned about their own breast cancer risk were referred by their GPs or surgeons to the Manchester Address correspondence to: K. Thirlaway, CRC Communication and Counselling Research Centre, Department of Oncology, University College London Medical School, 3rd Floor Bland Sutton Institute, 48 Riding House Street, London WlP ?PL, UK breast cancer family history clinic.2 Following consulta- tions with a geneticist or an oncologist about breast cancer risks, eligible women were invited to enter the prevention trial. The trial began in November 1993 and during the subsequent S-month period 149 women were offered the trial and a parallel psychosocial study in which women complete a range of psychological assess- ments at 6 monthly intervals for 5 years. As part of their initial assessment, after consultation with an oncologist, women were asked to estimate whether the approximate chances of a woman in the UK getting breast cancer during her lifetime were less than 1 in 100, 1 in 55 or 1 in 12. Analysis of these data using the Mantel-Haenszel test for linear association showed that acceptance of par- ticipation in the prevention trial was related to an indi- vidual’s assessment of risk in the general population. Seventy-five women declined to enter the trial and of these, 63 (84%) correctly understood the general popula- tion risk to be 1 in 12 whereas significantly fewer, 51 (66%) of the 74 women who agreed to enter the trial, knew the correct answer (P < 0.008) (Fig.). COMMENT These results suggest that women who underestimate breast cancer risk in the general population are more likely to accept an intervention that might reduce their own breast cancer risk. Misinterpretation of both general population and personal risk is common in several stud- ies of women at high genetic risk2” as well as women over 50 years in the national mammographic screening programme. It appears that a significant proportion of women may be making decisions about participation in prevention programmes based on inaccurate perceptions of breast cancer risk. These findings have important implications for the counselling and information given to high risk women. 203