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BMJ No 7119 Volume 315 Letters Saturday 22 November 1997
New method for expressing survival in cancerEditor,Whilst I would agree with Vaidya et al(1) that the impact of a predicted 10-year survival at the time of diagnosis of breast cancer could be very different in a 30-year old as compared with a 70-year old woman, I do not find that their proposed change in the methodology for plotting survival curves resolves the issue of determining the true survival rates in breast cancer. In a standard Kaplan-Meier survival plot, there are vertical error bars that represent the confidence intervals for the true population survival curve, for which the survival of the sample under study is used as an estimate. However Vaidya's methodology would necessitate including additional horizontal error bars, representing the uncertainty that arises from using the average life expectancy of a historical population as an estimate for the anticipated survival of the contemporaneous sample of women with breast cancer, had they not developed breast cancer. Furthermore it remains unclear as to exactly how one would calculate those error bars. An alternative methodology for determining the “cured fraction” of a group of patients is that proposed by Gamel, using a parametric analysis of the Kaplan-Meier curve based on a log-normal function.(2)(3) It has the advantage of not requiring any knowledge about the life expectancy of the underlying population from which the patients with breast cancer have been drawn, and has suggested that there is at least one adjuvant chemotherapy regimen that can increase the cure rate as opposed to simply retarding relapse.(4) There are several possible statistical definitions of cure in breast cancer, but the one most pertinent to an individual patient is personal cure, wherein she eventually dies of another cause without having ever suffered from the disease again.(5) Unfortunately an individual patient cannot in her lifetime ever be guaranteed such a cure, and adjusting the way survival curves are plotted does not alter this.
David Cameron Senior registrar
Directorate of Oncology,
Reference
1 Vaidya J S, Mittra I. Fraction of normal remaining life span:
a new method for expressing survival in cancer. BMJ 1997;
314:1682-4. (7 June.)
2 Gamel J W, Vogel R L. A model of long-term survival following adjuvant therapy for stage 2 breast cancer. Br J Cancer 1993;68:1167-1170.
3 Gamel J W, Vogel R L, Mclean I W. Assessing the impact of adjuvant therapy on cure rate for stage 2 breast carcinoma. Br J Cancer 1993;68:115-118.
4 Gamel J W, Vogel R L, Valagussa P, Bonadonna G. Parametric survival analysis of adjuvant therapy for Stage II breast cancer. Cancer 1994;74:2483-2490.
5 Haybittle J L. Curability of breast cancer. Br Med Bull 1990;17:319-323.
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