Is it absurd?

Is radiation really necessary?  Some would say that with Grade 3 DCIS that already had a microinvasion, it is.  But others would say that it depends on the size of the margin.  Personally I fall in the 2nd camp.  If your mother's margins after this 2nd surgery are at least 10mm all around, then radiation could be considered optional.  But if her margins are smaller (most docs consider margins to be "clear" after a lumpectomy if they are 2mm or greater in size), then radiation will be much more important to her treatment plan.

Your mother's oncologist and radiation oncologist should be able to provide her with information about her estimated recurrence risk without having radiation, and what her recurrence risk would be if she has radiation.  That should help with this decision.

Here are a couple of documents that talk about DCIS recurrence risk after a mastectomy.   You can see how important margin size is.

http://www.breastdiseases.com/dcispath.htm

http://theoncologist.alphamedpress.org/cgi/content/full/3/2/94/T2

I was going to skip radiation if I had wide margins, BUT, 1) I didn't have high-grade DCIS or a microinvasion, and 2) I had a 1cm clear margin and beyond that 1cm was more DCIS. I personally don't want to have my breast radiated already than it already has, so I'm opting for a skin-sparing mastectomy.

If your mom's clear margins this time are over 1cm, she's probably not taking a huge risk in skipping radiation (I personally would certainly skip it if I had that wide of margins and no further DCIS were still left). But, it's a personal decision and it's also worth a second opinion such as with Dr. Lagios that others have discussed here.

It's true - there are no guarantees.  Wide margins don't mean that we can be sure that all the DCIS is out.  And wide margins certainly don't guarantee that there won't be a recurrence.  I hope that my earlier post didn't suggest or imply that.  What wide margins simply mean is that the likelihood that there may be a recurrence is much less.  And with a lower recurrence risk, the benefit from the treatment is less and the risks associated with the treatment become a more significant consideration.  However it is important to always understand that a low recurrence is not the same as no recurrence risk.  And suggesting that a treatment might not be beneficial does not mean that there is no risk.  It simply means that the risks from the treatment may outweigh the benefits that the treatment will provide.

If you have two women with similar diagnoses - let's say that both have a 1cm single focus of grade 3 DCIS - and one has 2mm margins and the other has 10mm margins, then simply because of the difference in margin size, the first woman will have a higher recurrence risk and the second woman will have a lower recurrence risk. That's a fact. The first woman's risk might be in the range of 20%; the second woman's risk might be in the range of 5%.  So the first woman is quite a bit more likely to have a recurrence.  But it could turn out that the first woman doesn't have a recurrence, while the second woman does.  The risk estimates simply indicate what will happen across a group of 100 women who all share the exact same pathology - they do not say what will happen to any individual woman.  There is unfortunately no way to know what will happen to any one of us.  And even if there is only a 1 in 1000 chance that something might happen, one of us will be that very unlucky 1 in 1000. 

Unfortunately there is absolutely nothing that we can do to guarantee that we have no recurrence risk.  That's simply the reality we all have to live with after a diagnosis of breast cancer.