I am trying to do a cox-regression on a data set with patients treated for meningioma (a kind of brain tumor). I have varying follow-up times after surgery (time variable), whether the tumor recurred (status variable) and a number of predictors. My sample size is about 40 patients, nicely distributed over categorical variables (treatment, gender)

When I run the analysis I get the error message "since coefficients did not converge, no further model will be fitted."

I think the problem is that the most important predictor (treatment strategy) has no events in the new treatment strategy. As a clinician it's a huge success because the recurrence rate in the patients receiving standard treatment is quite high, so we're obviously doing the right thing, but I can't get the cox regression to work.. If I enter one event in the treatment group that has no events, the analysis works fine..

I'm suspecting that this outcome is inherently logical from the way the cox regression works, but still curious if there's anything I could do - I mean, there's an obvious difference between the groups! Wait for an event in the treatment group? :)

Any suggestions greatly appreciated!