How does radiation to the breast in a lactating woman affect milk production?

Additional information regarding radiation and the effect on lactation:

There is no evidence that breast fed babies are at increased risk of cancer. For women who have had radiation to the breast, in a published study, 80% had decreased lactation volume reported. Most women could produce mild, (55.6%), but 38.9% could not produce milk. For the women who did produce milk, production was decreased.

Also noted in a Yale study, after breast cancer treatment, 40% of the women could lactate, but with decreased volume. There was no effect in time to pregnancy after treatment. These women had median dose of 53 Gy to the breast. Babies may favor the untreated breast as they may have difficulty pulling on the nipple.

On pathology, tissue shows scle...

We generally use medication to suppress lactation during RT as otherwise it can cause symptomatic engorgement.