Are there any histologic subtypes of ER+, HER2- breast cancer that you would omit adjuvant endocrine in?

I agree that low grade, stage I, 'rare, less aggressive' histology has a great prognosis vis-a-vis distant metastasis. But, my decision on adjuvant endocrine therapy will not be based on just the histologic subtype. In other words, I would factor in traditional prognostic factors in deciding therapeutic benefit of anti-estrogen therapy.

There are clearly some patients with small tumor and favorable biology who don't benefit much from any adjuvant therapy. It has been hard to identify such patients but Mammaprint testing may help as they identify this "ultralow-risk group" where benefit of endocrine therapy is minimal

For the ultralow-risk patients, who received tamoxifen (n = 54), the 20-year disease-specific survival rate was 97% and for patients who didn't receive tamoxifen (n = 44), the 20-year survival rate was 94%.

Esserman LJ, Yau C, Thompson CK, et al. Use of Molecular Tools to Identify Patients With Indolent Breast Cancers With Ultralow Risk Over 2 Decades. JAMA Oncol. 2017;3(11):1503–1510. doi:10.1001/jamaoncol.2017.1261