Cholecystectomy is a procedure. It doesn’t just appear, a surgeon performs it. Better said something like “Cholecystectomy since the prior exam” or just “Interval cholecystectomy”.

A complete sentence would be “There has been an interval cholecystectomy” but I think just the two words “interval cholecystectomy” is better. They can’t mean anything other than “Since a prior point in time the gallbladder has been surgically removed.” If the date of the prior exam has been specified in the report, that unambiguously establishes the starting point of the interval.

Interval is often used incorrectly as in “Interval cholecystectomy since the June 12, 2010 exam”. This is redundant use since omitting “interval”  doesn’t change the meaning: “Cholecystectomy since the June 12, 2010 exam.”