Some of the lupus medications are "chemo-like".  Actually I believe I
saw one lady here (some years ago) say she felt very well on chemo, but
not all breast cancer patients have chemotherapy. Some have Tamoxifen or
something else (which I forget), depending on the hormone receptor
status of their breast cancer.

This web site says radiation therapy a risk for a flare. (but they don't
explain whether skin or general). I know about two weeks after radiation
therapy starts, fatigue can set in and continues until about a month
after the end of treatments, but some radiation treatments for breast
cancer are just one or two treatments (IIRC).  Just so you know, since
Lupus can also include severe fatigue. So the person might not be able
to carry on their day, the way most can who go on to their work or
regular routine.
http://www.uklupus.co.uk/flare.html

This Cdn breast cancer website http://tinyurl.com/5a962
"Radiotherapy may pose an increased risk for some people with severe
heart or lung disease. Also, people with scleroderma or systemic lupus
erythematosus may be at  increased risk for severe side effects from
radiotherapy. "

Perhaps there's a preparation that can be done beforehand?
I would think it better (if the cancer doctors thought it prudent) to
take the chance, rather than have the leftover cancer cells spread),
yes?
ask on sci.med.diseases.cancer about Lupus and radiation therapy.
There's several doctors who treat cancer there.

Hope this helps
J - not an expert

um...Yes, serious chemo. What is it you're wanting to know? The effect
on the lupus?

G.