Would you recommend post lumpectomy RT in an elderly woman with a T1N0M0 stage breast cancer ER/PR/HER2 positive?

As your question states, neither CALGB 9343 nor the PRIME II study assessed HER2 status of its patients as both of these trials commenced prior to the routine testing of HER2 status. So we cannot glean any information from these randomized trials of the omission of radiation in the ER+ HER2+ patient...