A DMC can stop the trial early. Here is what the fount of knowledge Wiki says.
" There are typically three reasons a DMC might recommend termination of the study: safety concerns, outstanding benefit, and futility.
Safety concerns
The primary mandate of the DMC is to protect patient safety. If adverse events of a particularly serious type are more common in the experimental arm compared to the control arm, then the DMC would have to strongly consider termination of the study. This evaluation has to be made in consideration of risk/benefit. In many cases, the experimental arm could cause serious adverse events (chemotherapy, for example), but the resulting improvement in survival outweighs these adverse events.
Overwhelming benefit
In the rare, but fortunate, situation that the experimental arm is shown to be undeniably superior to the control arm the DMC may recommend termination of the trial. This would allow the company sponsoring the trial to get regulatory approval earlier and to allow the superior treatment to get to the patient population earlier. There are cautions here though. The statistical evidence needs to be very high indeed. Also, there might be other reasons to continue, such as collecting more long-term safety data.
Futility
Futility is not as widely recognized as safety and benefit, but actually can be the most common reason to stop a trial. As an example, suppose a trial is one-half completed, but the experimental arm and the control arm have nearly identical results. It's likely in no one's interest to have this trial continue. It is extremely unlikely that the trial, should it continue to its normal end, would have the statistical evidence needed to convince a regulatory agency to approve the treatment. The company sponsoring the study could save money for other projects by abandoning this trial. Also, current and potential trial participants could be freed to take other treatments, rather than this experimental treatment which is unlikely to benefit them."
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