Since several of y'all asked: There is the board answer and then there is the practice answer.
Presuming 2nd trimester and beyond, with invasive disease and not Stage IV-
Traditionally:
MRM was recommended and is still probably a "safe answer", however, it ignores the fact that sentinel node biopsy is standard of care (SOC).
More modern practices:
1) the use of radioisotopes in pregnancy is considered safe, therefore, one should consider simple mastectomy with sentinel node biopsy and possible axillary node dissection as SOC. Vital blue dye may not be safe (I do not use it in most cases anyway). This presumes that the woman wants a mastectomy and that she is clinically T1/T2 and N0.
2) for the woman who wants breast conservation, you *could* offer a lumpectomy and axillary staging followed by adjuvant chemotherapy and RT. She is no longer a candidate for brachytherapy due to the delay in RT intervention (even if she were, she would have to be done on protocol - NSABP-B39 and some Rad Oncs are uncomfortable, IMHO, with brachy in such young patients).
3) my preference is, regardless of what surgical intervention you plan, since most pregnant women with breast cancer are candidates for chemotherapy, is neoadjuvant chemo followed by whatever surgical intervention you and the patient agree upon.
The reason for my preference is (in no real order):
1) operating on the pregnant breast is painful - blood and milk everywhere. I used to be interested in it, until I did it.
2) very real risk of milk fistulae
3) these patients need systemic treatment - any post-op complications delay lifesaving systemic treatments
4) can see CCR and allow for breast conservation
5) gives the patient time to decide what she wants to do surgically, be tested for BRCA (which may change her surgical plan), see PRS
6) there is no good surgical reason to delay reconstruction in these patients generally
Bottom line:
1) no RT during pregnancy
2) first trimester breast ca - termination still offered, but if late first trimester, can delay neoadjuvant until 2nd trimester (by the time you do workup, get port in, etc.)
3) no methotrexate, only anthracycline based chemo
4) no vital blue dye but Tech 99 is considered safe
5) MRM is traditional answer but probably not best answer *in practice*
6) if any question includes RT during pregnancy, it is wrong
7) the boards are always several years behind - for the written boards there will be a right and a wrong answer according to the ABS; for the orals, as long as you defend your actions, YOU can be the determinant of what is right
Therefore, either MRM or lumpectomy followed by chemotherapy starting 2nd trimester and RT after delivery are appropriate. You can also induce to deliver early and start treatments, but probably not before week 36.