[R] a question

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Hi, I read on a paper the below statement, don't know how that was calculated. Basically, there are 2 continuous variables x1 and x2, as independent variable for predicting cancer recurrence. So this is a survival analysis. Now the author try to check the correlation between x1 and x2. He calculated Spearman rand correlation (~0.22), then he had the following statement:

"Only approximately 5% of the variability in the estimates of recurrence using either of these scores was explained by the other".

How was that done? with a Cox model including both x1 and x2, can we get that number (5%)?

Thanks for any hints

John