Have Lump Under Arm With Puss

I was diagnosed with stage 1B melanoma on the top side of left arm a year ago. Had excision, SNB, 1 Lympnode removed, and a PET scan. All came back clean. Ever since my surgery I've had a cyst in my left armpit that won't heal. I've had the derm look at it and she told me it was cyst and gave me some antibiotics (that I admit I've only taken sporadically because I forget). I've also had my oncologist look at it and he said the same thing. But I'm just wondering if anyone else has had this. I know it gross but I can squeeze and puss comes out-but it just won't heal.

Hi!  I work for a dermatologist and it is not unusual to develop cysts.  If they are asymptomatic (no symptoms) they can be left alone but it is bothering you which is understandable under the arm you should see about having it removed. It can be incised and drained as well but likely to fill again and you would need antibiotics.  The reason why this bothersome cyst won't heal is because the core is still there and it is symptomatic.  Tell the doctor how bothersome it is for you and ask if it can be permanently dealt with.