I haven't seen this elsewhere, but I've always had a pet theory about this that is rather dark ...
The whole linchpin to the theory is an exchange at the hospital, in the Season 5 episode "Shadow," between Buffy and a doctor:
DR. ISAACS: (OS) Well, just let me know as soon as possible. And I
could use some information regarding your mom's lifestyle and home
environment. For instance, does she use a cell phone?
Uh, I think so. Uh, yeah, she um, she-she has one of those ear things.
DR. ISAACS: OK, is your house near any power lines, chemical plants,
waste disposal facilities?
He's asking about (among other things) electromagnetic radiation sources like cell phones and power lines. And it's established throughout the season that the key is mystical energy. I think Dawn, being the key, was full of concentrated energy and that her mere presence in the Summers home is what gave Joyce her cancer.
(Quotes from BuffyWorld)