Education And Debate

Fraction of normal remaining life span: a new method for expressing survival in cancer

BMJ 1997; 314 doi: https://doi.org/10.1136/bmj.314.7095.1682 (Published 07 June 1997) Cite this as: BMJ 1997;314:1682
  1. Jayant S Vaidya, surgical research fellowa,
  2. Indraneel Mittra, consultant surgeona
  1. a Department of Surgery, Tata Memorial Hospital, Parel, Bombay 400 012, India
  1. Correspondence to: Dr J S Vaidya Department of Surgery, Institute of Surgical Studies, University College London, Charles Bell House, London W1P 7LD, j.vaidya@ucl.ac.uk
  • Accepted 17 February 1997

Introduction

Conventionally, the time between diagnosis and date of last follow up or death is used to plot survival curves for patients with cancer. This ignores the patient's expected life span had the patient been healthy at the time of diagnosis. In human terms the impact of a projected prognosis of 10 year survival on a woman diagnosed as having breast cancer, for example, may be different depending on whether she is aged 30 or 70. Furthermore, oncologists have no answer to the question: “What is my chance of cure?” For this reason, we believe that survival is better expressed as a fraction of normal remaining life span expected at the time of diagnosis. We propose a new method which takes account of age at diagnosis in calculating survival.

The new method

To illustrate this concept we used a database of 1134 patients with breast cancer from Bombay who were operated on at Tata Memorial Hospital between 1974 and 1988. The patients were divided into three groups on the basis of the number of involved axillary lymph nodes (0, 1-4, and >4). The survival curves were plotted in two different ways: by the conventional method (fig 1) and by a new way that we call the real life expectancy method (fig 2). The difference in the two methods is not in the statistical handling of data but in the way period of survival is expressed. Both curves were plotted with the computer program SUREAL by the actuarial method.1

FIG 1

Conventional actuarial survival curves, with x axis indicating number of years from diagnosis and y axis indicating cumulative percentage surviving

FIG 2

Real life expectancy curves, with x axis indicating fraction of normal remaining life and y axis indicating cumulative percentage surviving

To plot real life expectancy curves we used data from the Life Insurance Corporation of …

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