gynecological or gynecologic?

I often write medical copy and have always used 'gynecologic' as the descriptor, as in 'gynecologic oncologist' or 'gynecologic disorders.' However, I am editing something where the writer uses 'gynecological' in the same way, and cannot seem to find the rule that says that is incorrect. Can you guys help me?

Merriam-Webster gives both gynecologic and gynecological as an adjective. Therefore, whether you use one or the other is at your discretion, but I would advise being consistent with your decision.

However, the OED recognizes only gynaecological (note the added a), which suggests that gynecologic (or gynaecologic) is the nonstandard form of the word in British English.

Note that, in cases such as these where you are writing for a particular department or publication, it is important to ask the person(s) whom you are writing for which spelling is preferred.