Tamoxifen vs. mastectomy

My understanding is that tamoxifen is to prevent a future cancer, not to treat a current cancer. Tamoxifen was not recommended for me after BMX, but I had DCIS only. Best wishes in your treatment!

DCIS is Stage 0 pre-invasive cancer.  DCIS cells are confined to the milk ducts and cannot 'invade' into the breast tissue or beyond to the nodes or into the body.  DCIS cells can however evolve to become invasive cancer cells.  At that point the diagnosis and staging changes.  This is why DCIS cells usually are surgically removed - so that they never have the chance to become invasive cancer. 

LCIS is different. It's generally considered to be a marker for cancer rather than cancer itself.   This is because while LCIS confers a higher risk that breast cancer might develop in the future in either breast, the LCIS cells themselves usually don't develop to become invasive cancer.  I believe the exception is Pleomorphic LCIS.  If LCIS is staged, it is Stage 0.

So the question about Tamoxifen depends on your diagnosis.  If you have DCIS and LCIS, both being Stage 0, there is a school of thought (a very small school, mind you) that suggests that surgery might not be necessary.  More specifically, surgery is not required for LCIS but normally surgery would be recommended for DCIS.  However there are some who believe that Tamoxifen might be able to slow the development of DCIS and possibility even push it back a bit.  This is not an accepted or proven school of thought.   But to your question, is it possible that Tamoxifen might reduce the amount of cancer enough so that a MX isn't necessary?  Yes, it's possible but it's not likely and it would be a highly unusual approach to take.

If you do have the BMX and if your diagnosis is Stage 0 DCIS and LCIS, then there is no reason to take Tamoxifen after the surgery.  Tamoxifen is effective at reducing recurrence risk, but after a BMX your local (in the breast area) recurrence risk would be only about 1% - 2%.  And since these are not invasive cancers, you would have no distant recurrence risk.   So in a case like this (a BMX for a high risk condition or a pre-invasive cancer), the risks from Tamoxifen actually outweigh the risks from the disease.

On the other hand, if you do have a Stage I diagnosis (perhaps ILC and not LCIS?), then it's a completely different story.  First, I doubt that there is any doctor who would agree that taking Tamoxifen would be effective at reducing or eliminating an invasive cancer.  Surgery is necessary if you have invasive cancer.  And second, with an invasive cancer, you face the risk of distant recurrence.  And this is why Tamoxifen would be prescribed even after a BMX.  Although your local recurrence risk would still be only 1% - 2%, the reason for taking Tamoxifen would be to reduce the risk of a distant recurrence (i.e. metastasis of the breast cancer somewhere in the body).

Sorry for the long and rather complicated response.  I hope it makes sense!

I obviously didn't edit my comment and Beesie is right. I should have said it wouldn't affect the cancer that you already have (invasive) so you WOULD need a mastectomy....being that stage 1 is an invasive cancer....and given you already have an invasive cancer it makes any DCIS or LCIS suspicious for future outbreaks, which is probably why they recommend the BMX.