Do you give adjuvant chemotherapy/trastuzumab to a premenopausal woman with a pT1aN0 HR+, Her2+ breast cancer?

Existing data are from convenience cohorts, but generally don't support infusional therapy for T1aN0 HER2+ tumors, who seem to do well with DRFS well above the usual threshold for intervention of 8-10%. An analysis from the MDACC database included about 250 untreated T1a-bN0 HER2+ tumors with long-term follow-up. In that analysis, adjusted for other prognostic features the cutpoint for seeing a benefit of intervention (chemotherapy or chemotherapy + trastuzumab) was 0.8cm (He X et al, CCR 2019). Generally, I don't treat T1aN0 with chemotherapy + trastuzumab.

We would certainly bring this case to our Multidisciplinary Case Conference and look at the pathology. If there is simply a <5 mm focus of invasive cancer, either alone or in a field of DCIS, and nothing else in the surgical specimen, then, I agree with Dr. @Dr. First Last's response that her prognosis is excellent and no further adjuvant therapy is needed - but preventive endocrine therapy can be considered. If there is an extensive field of DCIS with multiple foci of invasion or microinvasion (i.e. the volume of disease if greater than just T1a), then I would offer adjuvant therapy with the APT (weekly-paclitaxel and trastuzumab) regimen and adjuvant or preventive endocrine therapy.

A corollary question: For the above patient who declined any adjuvant therapy (chemo, trastuzumab, tamoxifen, radiation) after lumpectomy for a 6mm triple positive tumor and had a local recurrence (5mm w/ DCIS) 18 months later, would your recommendations be the same?