0.5% false positive rate is actually quite high. I don't have the numbers, but I'd be surprised if more than 1% of the people that would receive such a screening actually already have cancer, which would mean that it produces about as many false positives as false negatives.
Picture a population of a million people, all receiving the test. 1% of them have cancer (unknown to them) - half of those people get a negative screening result and half get a positive one (51% successful identification), and 0.5% of the full population get a negative screening result. Those numbers are roughly equal.
That doesn't make it useless by any means, but it's not nearly as 'specific' as it sounds on paper.
Picture a population of a million people, all receiving the test. 1% of them have cancer (unknown to them) - half of those people get a negative screening result and half get a positive one (51% successful identification), and 0.5% of the full population get a negative screening result. Those numbers are roughly equal.
That doesn't make it useless by any means, but it's not nearly as 'specific' as it sounds on paper.