Key points are not available for this paper at this time.
Sheila M. Gore, Stuart J. Pocock, Gillian R. Kerr, Regression Models and Non-Proportional Hazards in the Analysis of Breast Cancer Survival, Journal of the Royal Statistical Society. Series C (Applied Statistics), Vol. 33, No. 2 (1984), pp. 176-195
Gore et al. (Sun,) studied this question.