[R] Survival analysis and comparing survival curves

[R-girl]

Hei,

I have a one simple question which does not seem to be that simple as I
cannot find any solution/answer:

Is it possible to compare multiple survival curves in R with
survdiff-function when there is interaction term involved in predictor
variables (and this interaction is significant)?

Example:

survdiff(Surv(death,status)~treatment*gapsize)

R is making "problems" with it ie.e. it does not want to perform the test.
And all the examples I have found so far, may involve multiple predictor
variables but in additive format (e.g. treatment+gapsize).

If survdiff is not the way to go, are there any other solutions in order to
compare statistically the curves i.e. if they differ significantly from each
other?

I would really appreciate if someone can answer!

Cheers, Minna

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