Two major phase 3 trials presented at ASCO 2026 tackled the same question: does adjuvant immunotherapy after surgery help patients with Merkel cell carcinoma (MCC), a rare but highly aggressive skin cancer? The answer, it turns out, is nuanced.

The ADAM trial randomized 100 high-risk stage III MCC patients to adjuvant avelumab or placebo. Avelumab produced a meaningful numerical improvement in 1-year relapse-free survival (87.2% vs. 59.6%), with roughly a 50% reduction in relapse risk in exploratory analysis—but the primary endpoint just missed statistical significance (P = .075). Meanwhile, the STAMP trial (293 patients, pembrolizumab vs. observation) also fell short on its primary RFS and OS endpoints, though a new exploratory analysis showed significant improvements in MCC-specific RFS and distant metastasis-free survival with pembrolizumab.

Crucially, both trials found that patients who relapsed on the control arm were often successfully salvaged with subsequent immunotherapy, keeping MCC-specific survival high across both groups—raising the question of whether upfront adjuvant therapy is necessary for all patients.

Key Takeaways:

• Adjuvant avelumab cut 1-year relapse risk by ~28% in absolute terms vs. placebo; adjuvant pembrolizumab showed significant benefit in MCC-specific RFS (HR 0.65) and distant metastasis-free survival (HR 0.58)
• Neither trial met its primary endpoints, so routine adjuvant immunotherapy for all MCC patients is not yet supported
• Clinicians should individualize treatment decisions based on recurrence risk, patient age, and tolerance for immunotherapy toxicity

Why it matters: MCC is rare but deadly, and these trials represent the largest randomized evidence base yet for adjuvant immunotherapy in this disease. While neither study delivers a definitive green light, both point toward meaningful benefit for high-risk patients—and set the stage for future work on neoadjuvant strategies and ctDNA-guided treatment selection.