Occult breast cancer showing a marked response to pembrolizumab plus gemcitabine and carboplatin therapy complicated...

Occult breast cancer (OBC) is a rare condition presenting with axillary or distant lymph node metastases without a detectable primary tumor. Immune checkpoint inhibitors (ICIs) show promise in triple-negative breast cancer (TNBC) treatment; however, their efficacy in OBC remains unclear. The present study describes the case of a 71-year-old woman who presented with left cervical and axillary lymphadenopathy. ¹⁸F-fluorodeoxyglucose positron emission tomography-computed tomography (CT) and breast ultrasonography revealed no detectable primary breast lesion. Core needle biopsy of the axillary lymph node confirmed the diagnosis of TNBC (estrogen receptor, 0%; progesterone receptor, 0%; human epidermal growth factor receptor 2 score, 0). The programmed death-ligand 1 combined positive score was >10. Since the patient had stage IV disease with the biological subtype defined by lymph node biopsy, breast magnetic resonance imaging was not performed to avoid delaying systemic therapy. Pembrolizumab (Pembro) combined with gemcitabine and carboplatin was initiated. However, the patient developed a Grade 1 rash after the first cycle and diarrhea after the second cycle, followed by immune-related colitis requiring hospitalization. Notably, the emergence of immune-related adverse events (irAEs) paralleled a marked treatment response, with non-contrast CT after three cycles showing complete resolution of left axillary lymph node metastases. Pembro was discontinued, and the patient has maintained a complete response on eribulin monotherapy for >1 year. The present case suggests an association between irAEs and the clinical effectiveness of ICIs, highlighting the potential of Pembro-containing chemotherapy for OBC treatment, and emphasizing the importance of prompt irAE recognition and management. Lymph node-dominant disease may represent a particularly immunogenic context for ICI therapy.