To calculate the risk of a relapse and the benefits of taking Tamoxifen or chemotherapy (e.g. AC or adriamycin-cytoxan or P paclitaxel) start with this table based on the size of the tumor and the number of nodes
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from  Understanding the Utility of Adjuvant Systemic Therapy for Primary Breast Cancer, Charles L. Loprinzi and Stephan D. Thomé .JCO Feb 2001: 972-979.
 
Then look at the odds of being cured at 10 years, assuming the tumor was estrogen receptor positive, comparing no chemotherapy, with T (tamoxifen) with T and AC (tamoxifen with adriamycin, cytoxan)
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If the tumor was estrogen receptor negative, compare the odds of being cured with no chemotherapy, versus AC (adriamycin, cytoxan) with AC and P (adriamycin, cytoxan and paclitaxel)
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