Multifocal DCIS - do SNB or not?

I don’t know the answer to your first question, but to answer your second question about waiting until after the mastectomy to see if a SNB is needed, they can’t do that. Once the breast is gone, there is no way of identifying the sentinel nodes. I believe there is a new technique used by Angelina Jolie where they do mark them prior to surgery just in case, but that is not really being done anywhere else yet. You could always ask your doctor about it, but as I said it is not in common use at this point. I suspect part of why it’s not is that the pathology would need to be completed before all the dye (which marks the nodes) passes out of the body and most places are not equipped to do it that quickly but that is just a guess.

Regarding your first question, my mother had a mastectomy with SNB and they only removed one node. She did have two separate areas of cancer - one DCIS and one IDC. I believe they were fairly close together though, and since she had a known IDC they may only have tried to identify it from that one spot - I really don’t know.

The "sentinel" node is the first node or first few nodes in a chain that lymph fluid drains to from the area around the tumor.  The theory is if cancer has spread it will show up in those lymph nodes.  They used to remove a lot more nodes but now usually just do the sentinel node(s) and while you are still at risk for lymphedema its less common.  A blue dye is injected into the nipple a few hours before the surgery and the surgeon removes any nodes that are blue for testing.  After the breast is gone as Annette 47 noted, the sentinel nodes cannot be identified.  I think the confusion may be that if you have DCIS and are having a lumpectomy they don't remove sentinel nodes because you still have your breast and it can be done in future if needed.  I had a mastectomy for widespread DCIS and 2 nodes removed.  Haven't had any problems so far! 

I have heard of a woman on this site that had her sentinel nodes marked during surgery so that they could be identified post-MX if anything invasive was found. I had 2 nodes originally taken at my BMX and have dealt with stage-0 LE since. I think the low probability of LE that they tell us (1-2%) does not include Stage 0, which involves small amounts of swelling and arm pain. I have to be careful with my arm and protect it from blood draws, blood pressure cuffs, bug bites, hang nails, cuts, sun burns, and other injuries. I do lymphatic drainage massage and wear a sleeve if I have problems, and the pain is usually gone in an hour. Anyone who's had lymph nodes removed should be careful.