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The curves eventually collapse because Provenge does not provide immortal life! Eventually, everyone (both placebo and those on Provenge) will die, either due to PC, or due to other causes.>

The curves also may collapse because there are too few patients follwed out that far. Survival curves often are displayed to reflect the numbers of patients out that far. Sometimes its just a matter of one patient dying if the numbers are small that causes the curves to meet.