© 2005 Oxford University Press
CORRESPONDENCE |
RESPONSE: Re: Role of Detection Method in Predicting Breast Cancer Survival: Analysis of Randomized Screening Trials
Affiliation of authors: Department of Biostatistics and Applied Mathematics, M. D. Anderson Cancer Center, University of Texas, Houston, TX
Correspondence to: Donald A. Berry, PhD, Department of Biostatistics and Applied Mathematics, Unit 447, The University of Texas M. D. Anderson Cancer Center, 1515 Holcombe Blvd., Houston, TX 77030 (e-mail: dberry{at}mdanderson.org).
We thank Paci et al. (1) for their correspondence. They suggest that we should have performed a survival analysis by intention to treat with or without adjustment for tumor characteristics. Intention-to-treat analyses are critical in assessing treatment (or screening) benefit in randomized trials. This assessment was not our focus. Rather, in Shen et al. (2), we were interested in comparing the cancer-specific survival distributions by actual method of detection. There were no "nonattenders" in the Canadian National Breast Cancer Screening Studies (CNBSS) trials because all women in the screening groups had at least one mammogram; subsequent cancers were either interval cancers or detected at a later screening examination. There were 81 cancers diagnosed among the 9931 nonattenders in the screening group in the Health Insurance Plan (HIP) trial (3). Such tumors were not diagnosed by screening examinations, and they were neither interval nor incident cancers. They were properly included in the screening group for any assessment of the benefits of screening. However, an analysis that was based on an intention to treat that compared the survival distributions between breast cancers in the screening group including nonattenders and those in the control group was not relevant for addressing our question, whether or not there was an adjustment for tumor characteristics. It is unclear what kind of intention-to-treat analysis Paci et al. would consider for these cases.
For the HIP trial, we summarized the tumor characteristics of nonattenders and control subjects separately in our Table 1 (2). Also, as indicated in Shen et al., we compared the cancer-specific survival distribution of the 81 nonattenders with that of the 301 control subjects and found no statistically significant difference. The calculations of Paci et al. in Table 1 in their study are actually quite consistent with this observation. Their 95% confidence intervals for the hazard ratio of disease-specific survival between never-responders (i.e., the same group that we called nonattenders in Shen et al.) and the noninvited women (i.e., control group) include 1.00 with (model 4) or without (model 2) adjustment for tumor characteristics. Even though the estimated hazard ratio is somewhat higher for never-responders, cancer-specific survival of "never-responders" in the invited group was not statistically significantly different from that of patients in the noninvited group. Regarding Table 1 of Paci et al., the "all invited" results are irrelevant for the question we addressed in our study (2). We were interested in how a women's cancer was actually detected and not how it might have been detected.
Paci et al. suggest that we should have analyzed more recent screening trials. We would like to have analyzed all eight of the randomized screening trials, but we had access to data from only these three trials. Actually, the two CNBSS trials were among the more recent of the randomized trials.
REFERENCES
(1) Paci E, Ponti A, Crocetti E, Zappa M, Segnan N. Re: Role of detection method in predicting breast cancer survival: analysis of randomized screening trials. J Natl Cancer Inst 2005;97:1853–4.
(2) Shen Y, Yang Y, Inoue LY, Munsell MF, Miller AB, Berry DA. Role of detection method in predicting breast cancer survival: analysis of randomized screening trials. J Natl Cancer Inst 2005;97:1195–203.
(3) Shapiro S, Venet W, Strax P, Venet L. Periodic screening for breast cancer. The Health Insurance Plan Project and its sequela. Baltimore (MD): Johns Hopkins University Press; 1988.
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J Natl Cancer Inst 2005 97: 1853-1854.
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