I don't know what to do with Radiation

This is a VERY personal choice and it's regrettable that you are facing it while still in shock with what's happening to you.

First, it's not always possible to avoid radiation, even if you have a mastectomy.  It may be recommended for you if: 1) the surgical margins are too close; or 2) the tumor(s) were close enough to the chest wall to make your treatment team nervous.  You need to ask your treatment team their reasons for recommending radiation, so that you understand YOUR situation clearly.  This is important for you, so that you can make an informed decision about your options.

Second, radiation - like surgery - is, primarily, for local disease control.  It's to help prevent a local recurrence.  There is, also, recent research that demonstrates that radiation, also, provides some protection against a metastatic recurrence, but it's primary benefit is for local disease control.

Third, chemotherapy - as you already know - is for systemic disease control.  It's, likely, being recommended because of: 1) your age; and 2) your nodal status.  Your cancer is, already, demonstrating it's desire to spread and, since medical imaging cannot find each- and every cancer cell that may be floating around, chemotherapy is the standard-of-care.  Herceptin is being recommended because you are HER2+, which can be a particularly aggressive form of breast cancer.

And, finally, hormonal therapy is being recommended because you are ER/PR+.  Tamoxifen is the hormone treatment of choice for pre-menopausal women.

While your chances of developing lymphadema are increased with the ALND and radiation, it's not always a given that you will develop full-blown lymphadema.  Again, you will have to talk with your treatment team about WHY radiation is being recommended for YOU.  Ask them what they are basing their decision on.  Ask them to explain all the benefits- and risks YOU will be taking if you agree- or decline.  Ask them what they are hoping to achieve with the radiation.  Ask them to demonstrate how much of a recurrence benefit it will be for you.  Write everything down if you need to.

Then, the hardest part: you will have to sit down and figure out if the benefit that radiation will provide (in combination with all the other treatment you will be receiving) out-weighs the risks.  You may find that radiation may be right for you; you may find that it isn't.  But YOU have to be comfortable with what you have decided.  And, once you have made that decision, try not to second-guess yourself.  Know that, whatever it is, it is right for YOU.

Good luck.

i had radiation up to collarbone and armpit post mastectomy and full axilla clearance including rad boosters, i made sure i exercised my arm regularly and stretched and kept it above heart level every evening and i havenot  had any LE in arms but a little truncal.......I am now 16 months post last rads 

Hope this helps