You need to separate the risk of a local/regional recurrence (in the breast or nodal area), which is affected by the type of surgery chosen, from the risk of a distant recurrence (i.e. metastasis), which for the most part is not affected by the type of surgery chosen.

From these two small studies, it appears that being node positive and not having radiation (necessary with a lumpectomy; not usually done with a MX unless node positive) are factors that may drive a higher local/regional recurrence rate.

Locoregional recurrence and survival outcomes by type of local therapy and trastuzumab use among women with node-negative, HER2-positive breast cancer.

Predictors of locoregional outcome in HER2-overexpressing breast cancer treated with neoadjuvant chemotherapy.

I was only able to have 3 months of Herceptin due to heart issues, however, 7 years later, I am still here. When I got that news, I was grateful I had a Mastectomy. There are many issues to look at ... my sister was not Her 2 Pos, but her first mammogram post cancer treatment was too stressful for her to handle and had a double mastectomy as she did not want to go through screening as it was way to stressful for her.

Always, I recommend to get a second opinion at an National Cancer Institute Designated Cancer Center. They are the best of the best in the USA. https://www.cancer.gov/research/nci-role/cancer-centers/find

Sending all my best to you!