Pathological Complete Response with Neoadjuvant Trastuzumab, Pertuzumab, and Chemotherapy Followed by Modified Radical Mastectomy in a Patient with HER2-Positive Occult Breast Cancer.

BACKGROUND Occult breast cancer (OBC) is diagnosed when regional or distant metastases are found without evidence of a primary tumor. The low overall incidence is a great challenge for the management strategy of OBC. Aggressive diagnosis and personalized treatment are feasible treatment strategies for OBC. We report the case of an OBC patient who achieved pathological complete response (pCR) after neoadjuvant chemotherapy. CASE REPORT A 43-year-old woman was admitted to the hospital 6 months after detecting a lump in her left axilla, about the size of a quail egg, but not red or swollen, and the lump gradually grew. Mammography, ultrasound, and magnetic resonance imaging showed a visible left axilla lesion but no nodules in bilateral breasts. A core-needle biopsy of the axilla lesion revealed an invasive carcinoma of breast origin. The tumor cells were estrogen receptors (ER)-negative, progesterone receptor (PR)-negative, and HER2-positive (3+) by immunohistochemistry. The patient was finally diagnosed with HER2-positive, hormone receptor-negative occult breast cancer of the left breast, cT0N2M0, stage IIIA. The TCbHP regimen (docetaxel, carboplatin, trastuzumab, and pertuzumab) as neoadjuvant chemotherapy was given. She underwent a modified radical mastectomy, showing a pCR. Subsequent radiotherapy and HER2-targeted therapy were administrated. CONCLUSIONS This case highlights that even aggressive HER2-positive breast cancer can present as an occult primary tumor. Our clinical experience suggests that neoadjuvant chemotherapy followed by modified radical mastectomy can be effective for treating such rare cases. The patient achieved pCR, which can provide a therapeutic strategy for effective treatment of similar OBCs.