CMS Guidelines for Modifier 79

I work in a surgical billing office. I have noticed that our coder is using modifier 79 on 36561 (port placement for chemo) when the patient is in global for something like a mastectomy due to breast cancer. I've always been told to use modifier 58 in this situation because the port placement is related to the mastectomy as far as CMS is concerned. Which would be the correct modifier to use? Any feedback is appreciated.