Ah, that's a tough one.  It used to be that rads was never given after a MX for DCIS, but these days it is becoming more and more common to see rads prescribed in cases where the chest wall margin is very close or positive.  This follows the release of some research (a very small study, mind you) that showed that recurrence rates where substantially higher after a MX if the margins were close or positive. 

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

"Results  With a median follow-up of 61 months, 6 (7.5%) of the
80 patients developed local recurrence. Of the 31 patients with a
margin of ≤2 mm, 5 (16%) developed local recurrence vs. only 1 (2%) of
49 patients with a margin of 2.1–10 mm (p = 0.0356). Of the 6
patients with local recurrence, 5 had high-grade disease and/or
comedonecrosis. All six recurrences were noted in patients <60 years
old.

Conclusion  The findings of this review suggest that patients with pure ductal carcinoma in situ
who undergo mastectomy with a margin of <2 mm have a
greater-than-expected incidence of local recurrence. Patients with
additional unfavorable features such as high-grade disease,
comedonecrosis, and age <60 years are particularly at risk of local
recurrence. These patients might benefit from postmastectomy
radiotherapy."

Another interesting article/study:

Analyzing the Risk of Recurrence after Mastectomy for DCIS: A New Use for the USC/Van Nuys Prognostic Index

"Methods  We reviewed a
prospective database of 1,472 patients with pure DCIS. All patients were
scored from 4 to 12 using the USC Van Nuys Prognostic Index, an
algorithm based on DCIS size, nuclear grade, necrosis, margin width, and
patient age. Probabilities of recurrence and death were calculated
using the Kaplan–Meier method.

Results  A total of 496
patients with pure DCIS were treated with mastectomy. None received any
form of postmastectomy adjuvant treatment. Average follow-up was
83 months. Eleven patients developed recurrences, all of whom scored
10–12 using the USC/VNPI. No patient who scored 4–9 recurred. All 11
patients who recurred had multifocal disease and comedo-type necrosis.
The probability of disease recurrence after mastectomy for patients
scoring 10–12 was 9.6% at 12 years, compared with 0% for those scoring
4–9. There was no difference in overall survival.

Conclusions  There were no
recurrences among mastectomy patients who scored 4–9 using the USC/VNPI.
Patients scoring 10–12 were significantly more likely to develop
recurrence after mastectomy. At risk were young patients with large,
high-grade, and multifocal or multicentric tumors. For every 100
patients with USC/VNPI scores of 10–12, 10 patients will recur by
12 years and 2–3 will develop metastatic disease."

.

And here are a couple of older threads from the DCIS Forum where this issue is discussed.  You might find the discussions helpful or you might want to follow-up with some of the other women who've been in the same position that you are now:

Topic: DCIS, BMX/MX, and radiation ladies

.

Topic: Chest Wall Margin 0.1mm after Mastectomy - radiation decision

TB90, I was told I would need radiation regardless of whether or not I had mastectomy or lumpectomy - so at the end of the day the surgical question for me was more cosmetic.  The area that needed to be removed was large, relative to the size of my breast (5 cm total - only 1 cm ended up being DCIS, the rest was benign ADH and other things - like a radial scar, etc.).  Luckily, it was all in one quadrant - so I decided to trust my surgeon.  He did a fabulous job (moved some stuff around after the surgery so I only have a "depression" and not a huge chunk .  The hardest part of the surgery was the amount of tissue that he took down against the fascia and chest wall.  

Anyway - the RO was the one who gave me my risk without radiation, and I decided it was worth the risk reduction to have radiation. That said, I did not have the option of Tamoxifen - so it might be that at the end of the day you could have the same risk as someone who had radiation if you choose Tamoxifen.  Regardless, a consult with an RO might help you a lot, you can get more information that you can use to decide.

Best wishes to you.

Hi TB, I believe that your radiation (post mx) would be to the chest wall vs. someone who had a lumpectomy for DCIS, where the radiation is to the breast. 

What I learned from my DCIS interactions with the different medical staff is that Oncologists specialize in oncology. Most surgeons specialize in surgery (cept for oncologist surgeons, of course.)

Although they  may know some about each others' fields, I learned that they don't know it all.

Example: My oncologist recommended that I have a BMX with node removal AND a total hysterectomy with ovaries and tubes removed at the same time. I discussed this with my surgeon, and she said that was a bad idea. I had the BMX w/node removal separately and holy cow I am glad I didn't get it all done at the same time!!!!

Another example: I had a BMX w/node removal with clean margins for DCIS. My breast surgeon wanted to put me on Tamoxifen. I talked to the medical oncologist and he explained that would be unnecessary and overtreatment (not mention dangerous with some of my other health issues) because in my case, the cancer was contained within the milk ducts and I had very clean margins after the BMX. Since the milk ducts were gone, the cancer was essentially gone.

In your case, you have dirty margins. If I were in  your shoes, I would go with what the radiation oncologist has to say. You can always get a second opinion from another radiation oncologist.

It sure seems like we have to make pretty important decisions rather quickly sometimes with cancer.

I wish you the best.

"Would you rather be chased by a bear or a shark?"  

It depends whether I'm on land or in the water.  

My understanding is that it's the role of the Medical Oncologist to look at the entire picture. 

MEDICAL ONCOLOGIST 

A doctor who specializes in diagnosing and treating cancer using
chemotherapy, hormonal therapy, biological therapy, and targeted
therapy. A medical oncologist often is the main health care provider for
someone who has cancer. A medical oncologist also gives supportive care
and may coordinate treatment given by other specialists.

Edited to try to fix the formatting

As Ballet says, the radiation and tamoxifen do essentially address different issues, although there is some overlap.   In my case (lumpectomy) I opted for both, for several reasons.

As far as my experience with both goes - some mild swelling and fatigue with radiation; no major skin issues at all, no noticeable effects to heart or lungs.     With tamoxifen, few if any side effects - the occasional mild hot flash that peaked at about 4-6 months in (haven't had one in the past couple months), some brittle nails and minor hair loss, both of which seem to have ended - again, those SE's peaked at about 4-6 months in and now seem to be going away.   I've actually had some nice benefits from it too, including an ending to the fibrocystic cyclical pain and swelling I've had most of my life.   My periods are lighter and more regular than they had been.   

That's not to say your experience would be the same, but just know that while people tend to be very vocal about the bad SE's they experience from various treatments, there are many women out there who do NOT experience those same SE's, but you are less likely to hear from them.   Part of your conversation with the docs should center around how likely YOU are to experience certain SE's, particularly the severe ones.     With the Tamoxifen, if the "quality of life" side effects as my onc refers to them are bad,  you can always quit the drug.   That's how I went into it - try it and see how I tolerated it, as it is not as essential for my survival as it would be with someone with more advanced cancer, but will provide some benefits.   Unfortunately there is no real "try it and see" with radiation!

Hi TB, do post that article, because I've never heard or read that, and I am treated at a world-class cancer center, and they said the typical thing, that the hormonals reduce the recurrence risk (of either DCIS or IDC) in the treated breast by 50 percent (the aromatase inhibitors but a slightly higher percent).  These are relative reductions.  If your actual estimated risk of recurrence is 12 percent, then the Tamoxifen reduces the risk to 6 percent (again for either DCIS or IDC).  Usually you have a 50 percent chance of a recurrence being DCIS and 50 percent chance of the recurrence being IDC (which is also bound to confuse you further). If you zap the narrow margin with radiation, I believe (but don't know for sure) that technically your recurrence risks should then be the same as someone who didn't have that margin and also had a mastectomy for DCIS, in other words, one to two percent.  Do ask the RO about this.

So you have to decide if you want to do a time-limited targeted treatment, or a more long term somewhat less targeted treatment.  I, personally, wouldn't "save" radiation for later.  I would probably want to do what I could now to reduce recurrence risk, while it is exclusively pre-invasive disease (at least as of the path report you have now), rather than wait to use radiation later, when you could have invasive disease that is potentially more difficult to control. You don't want a "later", if you can help it.  Now, if you have other reasons for not doing radiation, like concerns about side effects (e.g. left chest, etc.) that's something else entirely.  Do you have time to get a second opinion on the radiation? Is the DCIS high nuclear grade? with comedonecrosis? multi-focal?  Or is the DCIS low grade, and maybe Tamoxifen would be enough to reduce the risk? Or is it high grade, and potentially more aggressive?  I think everyone who has posted on these threads, with a similar situation, has agonized over it.

Hi TB, I realize that you want clarification on whether Tamoxifen prevents recurrence of either DCIS or IDC in patients with prior DCIS. The second study stated that there was a "trend" toward reduction in recurrence of IDC in the ipsilateral (treated) breast.  I'm sure that there are many studies showing a significant effect of Tamoxifen in preventing recurrence of IDC (not just DCIS).  I just don't have the time right now to find them. Maybe Beesie has some handy.

TB--glad to help in a small way.  Let us know what you decide and what the "team" recommends. I think you understand the difference between the radiation (targeted) and the Tamoxifen (more generalized effect) now. It's really like comparing apples to oranges.  That's why, with lumpectomies, the radiation is usually a must for local treatment, followed by the Tamoxifen for generalized protection in the long run.