A case for rads - or not?

Here are some studies on close margins after MX for DCIS:

Close or Positive Margins After Mastectomy for DCIS: Pattern of Relapse and Potential Indications for Radiotherapy

Impact of Margin Status on Local Recurrence After Mastectomy for Ductal Carcinoma In Situ

Is Radiation Indicated in Patients With Ductal Carcinoma In Situ and Close or Positive Mastectomy Margins?

Is there a role for postmastectomy radiation therapy in ductal carcinoma in situ?

So the answer as to whether or not rads is indicated is.... "maybe".

Turquoise, Liz is right.  The way that a recurrence develops is if any stray cancer cells remain alive in the breast or against the chest wall after surgery.  At the time of the surgery, there may be so few cancer cells left that they aren't visible on any imaging but over time, as these left-behind cancer cells continue to develop and multiply, they eventually show up as a recurrence.  This can happen even with DCIS cells, and in 50% of cases, DCIS recurrences are not found until the cancer cells have evolved to become invasive.  So leaving DCIS cells in the breast can in fact lead to an invasive recurrence.

Radiation has a track record of reducing recurrence risk by approx. 50%.  In other words, radiation is about 50% effective at killing off any rogue cancer cells that might remain in the breast after surgery.  For someone who has large clean margins after MX surgery, the likelihood that some cancer cells still remain in the breast is pretty slim.  If the entire area of cancer was nowhere near the chest wall or the skin, it's unlikely that any cells will be left against the chest wall or skin.  This is why those who have good margins after a MX generally have a very low recurrence risk.  If the risk of recurrence is only 2%, then the 50% benefit from rads is only 1%.  Rads, like all treatments, comes with risks.  And this is why rads is usually not recommended after a mastectomy (whether for DCIS or even node-negative invasive cancer) if the surgical margins are good.  The recurrence risk in this situation is already so low that the benefit from rads will be very small, not worth the risks from the treatment.

In your case, you know that you have positive margins in two areas.  So this means that it's almost a certainty that some DCIS cells remain in your breast.  What you don't know (and can't possibly know) is: How many DCIS cells remain; just a few or many?  If it's just a few, will they die off on their own, or simply go into remission and never bother you again?  Or are some of these DCIS cells healthy and in active growth mode? And if they are and they do continue to develop, will you catch the recurrence before it develops into invasive cancer?  If you have rads now, will the rads be successful at killing off all of the DCIS cells that remain in your breast?  Or will you have a recurrence regardless?

Unfortunately it's not possible to know the answers to any of these questions. Both of your options present risks and a bunch of unknowns. So you have to access the situation and decide which path is the best for you.  Rads now, to reduce your recurrence risk by 50%, or no rads, hoping that no recurrence develops but if one does, that it is caught while still DCIS.

As for what your recurrence risk might be, here is the info from the 4 studies I linked earlier:

• 5 (16%) of 31 patients with a surgical margin of ≤2 mm after a MX for DCIS developed a recurrence vs. only 1 (2%) of 49 patients who had margins >2 mm.  5 of the 6 patients who had a recurrence had high-grade disease and/or comedonecrosis and all 6 patients were less than 60 years old
• Of 142 patients, the overall recurrence rate after a MX for DCIS was 1.4%, with a recurrence rate of 4.8% for those with positive margins, 4.3% for those with close margins, and 0% for patients with negative margins.
• The overall rate of chest wall recurrence among 193 women who had a MX for DCIS was 1.7%.  The rate of recurrence was 3.3% for high-grade DCIS. There were too few patients with positive margins to draw any firm conclusions.
• Among 211 women, there was a 0.9% recurrence risk after a MX for DCIS, i.e. two women had recurrences.  Both of the women who recurred had surgical margins of less than 1mm.

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So from this, I would venture to guess that your recurrence risk, having high grade DCIS and positive margins, is at least 5%.  It could even be as high as high as 20% (based on the first study that showed a 16% rate for those with <2mm margins; your margins were positive).  Your new MO won't know what your recurrence risk is, but this is the discussion that you should have with him; hopefully he'll be able to provide an educated guess.