Radiotherapy after mastectomy and lymph node removal?

I am 78 years old and have Stage 3B breast cancer, T4, N0, M0. I first had a lumpectomy 3 weeks ago, diagnosis was Invasive Lobular Carcinoma, T4a (tumor infiltrates chest wall muscle). Resected margins were clear. ER postitive and c-erb B2 positive. I am now on Armidex.
Last week I had surgery to remove my axilliary lymph nodes and mastectomy of my left breast. The pathology report showed no residual tumor in my breast and no metastasis in any of my lymph nodes. My oncologist recommended that I undergo radiotherapy since I am T4a. However, my surgeon had mentioned he excised part of my chest wall muscle during the 1st surgery, and felt he had removed all of the primary tumor. Is it still necessary for me to undergo radiotherapy, especially since my lymph nodes and margins were clear?
If I do start radiotherapy and the side effects get to be too much, can I stop halfway?

[Cleveland Clinic]

Dear BelSmith:  Without evaluation, it is impossible to speculate on what should be done about this breast cancer.  You may benefit from a discussion with the radiation oncologist and oncologist about what the added value of radiation may be in light of risks.  This information may help you to make a better decision.  There is no research to tell us whether a "little bit" of radiation has any benefit, so careful consideration of risks vs benefits up front seems like a better way to proceed.