我已经能够使用以下R代码为我的数据绘制生存曲线：
radiation=read.csv("radiation.csv",header=T)
head(radiation)
attach(radiation)
summary(Treatment)
library(splines)
library(survival)
log_rank=survdiff(Surv(radiation$Time)~radiation$Treatment)
log_rank
sfit<-survfit(Surv(radiation$Time)~radiation$Treatment)
plot(sfit,lty=c("solid","dashed"),col=c("red","green"),xlab="survival time in hours",ylab="survival probability")
title("Survival post lethal dose radiation")
legend("topright",c("GF","Placebo"),lty=c("solid","dashed"),col=c("red","green"))
我现在坚持的是如何在生存概率（y）为50％时找到x值（时间）。
这是否有R代码？
谢谢！