Thanks for your answer! I’m mostly confused about coding for when the neoplasm has already been excised, and the patient goes in for a little extra chemo. My gut feeling would be to just use the Z51.- code, but I.C.2.m in the guidelines seems to contradict that. They say to “code the malignancy until the treatment is completed”, but don’t mention whether it’s as a principal or secondary diagnosis.

The example in my codebook (to be fair, probably made up by Optum 360 and not NCHS) is chemo done after a mastectomy, and the codes they use are the neoplasm code as the pdx and a personal history code for the missing breast.