Is it okay for an oncologist to do breast cancer treatment without tamoxifen?

Yes: Tamoxifen is a very effective drug for the treatment of hormone-sensitive (estrogen receptor +) breast cancers. It may be used alone or in combination with traditional IV chemotherapy, depending on the cancer stage. It has no role in rx of estrogen (-) breast ca. Furthermore, aromatase inhibitors are an effective alternative to tamoxifen for estrogen (+) breast cancer after menopause.

Yes!: Tamoxifen is only useful in breast cancers which are estrogen receptor positive(er+). So tamoxifen is of no use if the tumor is er negative. Check your tumor's hormone receptor status and you will have the answer to your question>.