I do some OT in acute care and the doctors ALWAYS interrupt both OT and PT! LOL -- But the other staff (nurses, nursing assistants, social services, even respiratory) let us know they need the patient also, but usually don't interrupt, unless it is urgent or the patient has to go for a test. In acute care, you have to work as a team and sometimes the patient's medical condition takes precedence.
I might have nicely told the activity director that the patient was in the middle of her therapy treatment and would be available in -- minutes. If you don't stand up for yourself, believe me, nobody else will.
Maybe a little inservicing on OT would go a long way towards understanding and teamwork?