• The patient or a legally authorized representative must provide study-specific informed consent prior to study entry and, for patients treated in the U.S., authorization permitting release of personal health information.

• female and male patients who have undergone breast conserving surgery and completed a minimum of 4 cycles (12 weeks) of neoadjuvant or adjuvant chemotherapy in combination with HER2-targeted therapy.

⁃ ≥ 40 years of age

• ECOG performance status of 0 ,1, or 2/Karnofsky performance status above 60

• Histologically or cytologically confirmed invasive breast carcinoma.

• tumor must have been determined to be HER2-positive by current ASCO/CAP guidelines based on local testing results.

• Patient must have undergone axillary staging, either sentinel node biopsy (SNB) or axillary lymph nodal dissection (ALND). In neoadjuvant patients, SNB following neoadjuvant therapy is strongly recommended. SNB prior to neoadjuvant therapy is discouraged, but patients are permitted if node negative (pN0).

• The following staging criteria must be met according to AJCC 8th edition criteria:

‣ Adjuvant cohort : By pathologic evaluation, the patient's primary tumor must be \</= 2 cm and ipsilateral nodes must be pN0. Surgical lumpectomy margins must be negative for invasive cancer and ductal carcinoma in situ (no ink on tumor).

‣ Neoadjuvant cohort: Prior to neoadjuvant therapy, the patient's primary tumor must be \< 3 cm by imaging studies, with negative axillary nodes (cN0) based on axillary U/S, CT, PET or MRI. Physical examination is not sufficient documentation of cN0 status; • Must be ypT0N0 at surgery (lumpectomy); patients with residual non-invasive disease (DCIS) in the surgical specimen (ypTis), are NOT eligible.

• For the Adjuvant cohort, adjuvant therapy must have consisted of a minimum of 4 cycles (12 weeks) of chemotherapy in combination with HER2-targeted therapy.

• For the Neoadjuvant cohort, neoadjuvant therapy must have consisted of a minimum of 4 cycles (12 weeks) of chemotherapy in combination with HER2-targeted therapy.- ; Patients who did not receive chemotherapy in the neoadjuvant setting are not eligible, even if they achieved pCR with their preoperative treatment; nor would these patients become eligible by receiving chemotherapy after surgery.

• In patients assigned to radiation therapy, treatment should start ≤ 12 weeks from surgery on the Neoadjuvant cohort and ≤ 8 weeks from the completion of chemotherapy on the Adjuvant cohort. Patients should continue HER2-targeted therapy during assigned study treatment (radiation or observation).

• Bilateral mammogram or MRI within 52 weeks prior to randomization.

• HIV-infected patients on effective anti-retroviral therapy with undetectable viral load within 6 months of randomization are eligible for this trial.