Management of Stage IIA HER2+ Invasive Ductal Carcinoma Post-MRM and Neoadjuvant Chemotherapy

Adjuvant Therapy

The American Society of Clinical Oncology recommends adjuvant endocrine therapy with tamoxifen or an aromatase inhibitor (if postmenopausal) plus completion of HER2-targeted therapy with trastuzumab to complete one full year of treatment, along with radiation therapy to the chest wall and regional nodes based on pre-chemotherapy staging 1
Tamoxifen 20 mg daily for 5 years is recommended for ER+/PR+ tumors, with a strength of evidence level I 2, 3
For postmenopausal women, aromatase inhibitors (anastrozole or letrozole) are preferred over tamoxifen based on superior efficacy, with a strength of evidence level I 4

Complete up to 1 year total of trastuzumab therapy (including any given during neoadjuvant phase), with a strength of evidence level I 1
Consider trastuzumab emtansine (T-DM1) if there is residual disease after neoadjuvant therapy, based on the KATHERINE trial, with a strength of evidence level I 1
Dual HER2 blockade with pertuzumab may be considered for node-positive disease, with a strength of evidence level II 1

Radiation therapy indications should be based on pre-chemotherapy tumor characteristics, with a strength of evidence level I 5, 1
Post-mastectomy radiotherapy is recommended for patients with four or more positive axillary nodes, with a strength of evidence level I 3, 6, 7

The ideal initial follow-up is 7-14 days post-operatively to assess wound healing, drain output, and pathology results, with a strength of evidence level III 7
Annual mammography of the contralateral breast is recommended, with a strength of evidence level II 2, 3
No routine imaging (CT, PET, bone scans) is recommended in asymptomatic patients, with a strength of evidence level III 3, 6

The international working group guidelines recommend reassessment of HER2 status when there is a negative or equivocal result on pretreatment core biopsy, with a discordance rate of 6-9% between pre- and post-neoadjuvant therapy, in patients with Stage IIA HER2+ invasive ductal carcinoma 8, 9
If HER2 status is reassessed and found to be discordant, retesting should be performed with both immunohistochemistry and in situ hybridization, to determine the final HER2 status and guide treatment decisions 9, 10
The inclusion of trastuzumab in neoadjuvant treatment may increase the rate of negative conversion for HER2, and loss of HER2 amplification following neoadjuvant trastuzumab has been associated with worse outcomes in patients with HER2-positive disease 8, 9

Post-mastectomy radiotherapy indications must be based on pre-chemotherapy tumor characteristics, not post-neoadjuvant pathology, with radiation therapy to the chest wall and regional nodes recommended if there were four or more positive axillary nodes at initial presentation, in patients with Stage IIA disease 11

Omitting HER2 retesting when the initial result was equivocal is a critical mistake, as this reassessment is mandatory and will guide treatment decisions, including the use of HER2-targeted therapy 8, 9, 10