Ports put under arm or breast?

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I have a recurrence of breast cancer after 5 years and a lumpectomy. Had a mastectory May 19. Now I need chemotherapy, but don't know where they can put the port. I have a pacemaker on right side of chest, and scar tissue from previous port and mastectomy on left side. Does anyone have any experience with ports put under their arm or under their breast?