Extended-stage small cell lung cancer with breast metastasis: complete response after pembrolizumab

What happened, in summary

A 65-year-old woman was evaluated for both a chest mass and a breast mass. Biopsies from the lung and breast showed small cell carcinoma, and the breast lesion was determined to be a metastasis from primary small cell lung cancer. Brain MRI and FDG-PET were used for staging, and the disease was classified as extensive-stage small cell lung cancer, cT2aN3M1c. First-line treatment began with carboplatin, etoposide, and durvalumab. Durvalumab was given first, followed by etoposide, but the etoposide infusion caused immediate anaphylactic shock during the first course. Etoposide was stopped, she was treated with cimetidine, dexamethasone, chlorpheniramine, and saline, and the reaction did not recur. Because the original regimen could not safely continue, her doctors switched treatment to cisplatin plus irinotecan. After 4 courses, both the lung and breast masses had significantly decreased. A FoundationOne tumor panel showed tumor mutation burden of 10 mutations/Mb, microsatellite-stable status, TP53 V172F, PTEN L128N, RB1 loss, and SOX2, PIK3CA, and FGF12 amplifications. The TMB-high result supported use of pembrolizumab, so she moved to pembrolizumab monotherapy. This treatment suppressed tumor progression and maintained a complete response for more than 18 months. About 1 week after pembrolizumab began, her pre-existing palmoplantar pustulosis flared with papules and pustules on the palms and soles, but topical steroids controlled the symptoms. The breast mass was followed as a response marker alongside the chest mass, not treated as a separate breast primary. This made the later complete response clinically meaningful across both known disease sites. As of March 2025, her lung cancer remained in complete response.