Peeling the I-Onion to Demystify Pseudoprogression.

In this issue of Cancer Cell, Topp et al. analyze data from 799 patients treated with pembrolizumab beyond progression by RECIST 1.1 across six trials. Although 8.9%-24.4% of patients demonstrate a ≥30% reduction in target lesions, conversely 11%-18% of patients had a ≥20% increase. The benefits of treatment beyond progression must be carefully weighed against physical and financial toxicities.