We keep them forever. It is important to note they do not need to be
kept on site. Any reasonably temperature controlled storage facility would
be fine. (Unless of course there are some guidelines that say otherwise) I'd
be hesitant to just pitch perfectly good tissue. There are so many research
projects out there that would love to get a hold of this. As long as you
de-identify the reports you could have quite a resource on your hands.
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