There are two approaches I can think of which might help encourage
physician attendance at your tumor boards:

One is to try to convey to administration how important it is that
they actively support physician attendance at the conferences. If
you're accredited by ACoS, you can point out to them the importance
of demonstrating consistent, multidisciplinary attendance over the
course of the accreditation period (a copy of the survey application
form might bolster your argument here). Hopefully, administration
will do what they can to encourage attendance.

The other is to make sure that notices are posted at least a week in
advance, to remind physicians of the upcoming meeting while they
still have some wiggle room in their schedules.

You could also try e-mailing various specialists when a case within
their discipline comes up -- i.e., let the GI surgeons know that
there's going to be a colorectal case presented.

Hope this is some help.

Eileen