Describing the building process of the halls.md Breast Cancer Risk Calculator.
Documenting the methods used by the halls.md Risk Calculator is essential, so the results can be trusted. The basis for risk calculation methods is described in references 1 through 5. See the references, below.
The internal workings of my Gail model 1 calculator start by calculating the "summary relative risk" using the answers from questions 1 through 6.
Then you have the option of adding several extra relative risk factors (from questions 7 through 12) which were not part of model 1. These extra factors are:
- a) mammographic density (ref 6),
- b) age-specific tamoxifen effect (ref 7),
- c) dose-specific alcohol effect (ref 9),
- d) age-specific correction factors to adjust for Black race (using data from ref 5),
- e) presence of LCIS on a biopsy (ref 10), and
- f) oral contraceptives effects (ref 11)
The relative risks of these extra factors are multiplied with the summary relative risk.
Then, the absolute risks at 10-year, 20-year, 30-year and lifetime(90) are calculated using the graphical interpolation method described in reference 1. Polynomial equations are used in place of the curves graphed in reference 1.
However, model 1 is known to underestimate absolute risks in older women particularly, so age-specific correction factors are applied (from ref 5). Then, a correction is applied for not participating in screening mammography (ref 8). I have not yet implemented a correction factor for one problem: The model 1 calculator can significantly overestimate the chance of cancer in women with many risk factors. This is a known problem with model 1 and it can be made worse by including extra risk factors.
The internal workings of my NSABP model 2 calculator also start by calculating the summary relative risk, using the basic Gail model method. However, my model 2 calculator uses slightly different values for relative risks than those published in reference 5 for model 2. The Appendix (below) lists the relative risk figures used by my calculator, in comparison to the published values from reference 5. To explain the difference, if you try to read the text of reference 5, you’ll see that the mathematics described are very complicated, and unfortunately, not all the mathematical terms used are fully defined in reference 5. I suspect that the relative risk values used in my calculator, have merged the effects of mathematical terms "h*I(t)" and "F(t)", whereas these terms are presumably used in discrete mathematical operations in the NCI’s downloadable "Breast Cancer Risk Assessment Tool", (which implements model 2). Model 2 does not include the risk of developing Ductal Carcinoma In-Situ (DCIS). Therefore, the NSABP model 2 results will be lower than Gail model 1 results. In my risk calculator, there is an option, that will add in the risk of in-situ cancers (such as DCIS) into the model 2 estimate.
Because this calculator includes extra risk modifiers and correction factors that
were not originally part of model 1 or model 2, it is not possible to demonstrate that the results are accurate from published population-based studies. Therefore, please consider these results to be an estimate.
Steven B. Halls, MD, FRCPC.
Click here to read the Appendix, with more details about modifications to the standard Gail Model.
Back to the Breast Cancer Risk Calculator.
- Benichou J, Gail MH, Mulvihill JJ, Graphs to estimate an individualized risk of breast cancer. J Clin Oncol 1996; 14:103-110.
- Gail MH, Brinton LA, Byar DP, Corle DK, Green SB, Chairer C, Mulvihill JJ, Projecting individualized probabilities of developing breast cancer for white females who are being examined annually. J Natl Cancer Inst 1989; 81:1879-1886.
- Spiegelman D, Colditz GA, Hunter D, et al.: Validation of the Gail et al. model for predicting individual breast cancer risk. J Natl Cancer Inst 1994; 86:600-607.
- Bondy ML, Lustbader ED, Halabi S, et al.: Validation of a breast cancer risk assessment model in women with a positive family history. J Natl Cancer Inst 1994; 86: 620-625.
- Costantino JP, Gail MH, Pee D, Anderson S, Redmond CK, Benichou J, Wieand S, Validation studies for models projecting the risk of invasive and total breast cancer incidence. J Natl Cancer Inst 1999; 91:1541-1548.
- Byrne C, Schairer C, Wolfe J, Parekh N, Salane M, Brinton LA, Hoover R, Haile R, Mammographic features and breast cancer risk: Effects with time, age and menopause status. J Natl Cancer Inst 1995; 87:1622;1629.
- Fisher B, Costantino JP,Wickerham DL at al, Tamoxifen for prevention of breast cancer: Report of the national surgical adjuvant breast and bowel project P-1 study. J Natl Cancer Inst 1998; 90:1371-1388.
- Spiegelman D, Colditz GA, Hunter D, Hertzmark E. Validation of the Gail et al. model for predicting individual breast cancer risk. J Natl Cancer Inst 1994; 86:600-607.
- Smith-Warner SA, Spiegelman D, S Yaun, et al. Alcohol and breast cancer in women: A pooled analysis of cohort studies. JAMA 1998; 279:535-540.
- Bodian CA, Perzin KH, Lattes R, Lobular Neoplasia. Long term risk of breast cancer and relation to other factors. Cancer 1996:78:1024-1034.
- Collaborative Group on Hormonal Factors in Breast Cancer. Breast cancer and hormonal contraceptives: collaborative reanalysis of individual data on 53 297 women with breast cancer and 100 239 women without breast cancer from 54 epidemiological studies. Lancet 1996; 347:1713-1327.