When can itching of the breast be an early cancer, or is itching unrelated?

Could be: Unilateral eczema excoriation, itching on nipple could be sign of breast cancer ( paget"s disease) need to your doctor.

Usually, unrelated: Itching of the breast would not make it on my top 20 list of common breast cancer symptoms. The only time I have seen itching with breast cancer is related to paget's disease, which causes an eczema-like rash on the nipple, which can itch. I hope this helps.

Does it persist?: Although itching is an unusual symptom in breast cancer, it can occur with paget's disease, inflammatory carcinoma of the breast, or due to irritation of local nerve endings. Usually, itching is from a benign condition - and should promptly resolve after proper treatment. If itching persists or there is any mass or a breast discharge, consult with a health professional.