© The Author 2007. Published by Oxford University Press.
CORRESPONDENCE |
Re: Dietary Fat Reduction and Breast Cancer Outcome: Interim Efficacy Results From the Women's Intervention Nutrition Study
Affiliations of authors: Department of Family and Preventive Medicine, Cancer Prevention and Control Program, Moores UCSD Cancer Center, University of California, San Diego, La Jolla, CA (JPP, LN, KM); Cancer Prevention and Population Sciences, Roswell Park Cancer Institute, Buffalo, NY (JM)
Correspondence to: John P. Pierce, PhD, Cancer Prevention and Control Program, Moores UCSD Cancer Center, University of California, San Diego, 3855 Health Sciences Dr, #0901, La Jolla, California 92093-0901 (e-mail: jppierce{at}ucsd.edu).
Chleblowski et al. (1) recently reported an interim analysis of the Women's Intervention Nutrition Study trial and concluded that a reduction in dietary fat intake may influence relapse-free survival following breast cancer. This effect was observed mainly in the subgroup of patients with previously diagnosed hormone receptor–negative tumors. The accompanying Editorial (2) noted that the study's conclusion is contrary to much of the literature and raised the possibility that the analysis may have been confounded by either the higher frequency of mastectomy or greater weight loss in the intervention group.
However, a simpler explanation for the observed result could be differences between study arms in the completeness of the interim efficacy data. The authors note that 84% of the intervention group and 89% of the control subjects were successfully contacted in the 12 months before the analysis closeout date. This 5% lower outcome ascertainment rate in the intervention group was nearly double the absolute difference in study outcomes reported between the two groups. The authors did not comment on whether this differential follow-up varied by body mass index, surgery status, or hormone receptor status of the initial tumor. It is not clear that the intent-to-treat approach used by the authors would account for this potentially critical loss to follow-up.
That this suggestive finding is contrary to findings of several observational cohort studies but in line with the large Women's Health Initiative randomized trial of incident breast cancer, as noted by Thiebaut et al. (2), emphasizes the importance of controlled randomized trials to furthering the knowledge base. A more definitive result would have been achieved in this trial with better evidence that the effect of the dietary intervention had been maintained. The authors used a completers-only analysis when presenting the level of change achieved in the trial in table 3. An impressive 40% between-group difference in energy from fat was reported at the 12-month study assessment. However, differential assessment rates make it difficult for the reader to ascertain how well this difference was maintained. At the 3-year assessment, information was obtained from 67% of the intervention group and 74% of control subjects. At the 5-year assessment, information was reported for only 39% of the intervention group compared with 44% of the control group. If even modest proportions of nonresponders were no longer following the intervention, the indicated intervention effect for both dietary pattern and weight would need to be reduced substantially.
REFERENCES
(1) Chleblowski RT, Blackburn GL, Thomson CA, Nixon DW, Shapiro A, Hoy MK, et al. Dietary fat reduction and breast cancer outcome: interim efficacy results from the Women's Intervention Nutrition Study. J Natl Cancer Inst (2006) 98:1767–76.
(2) Thiebaut ACM, Schatzkin A, Ballard-Barbash R, Kipnis V. Dietary fat and breast cancer: contributions from a survival trial. J Natl Cancer Inst (2006) 98:1753–5.
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J Natl Cancer Inst 2006 98: 1767-1776.
J Natl Cancer Inst 2007 99: 900-901.
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