Report of two cases of pseudoprogression in patients with non-small cell lung cancer treated with nivolumab-including histological analysis of one case after tumor regression.

The recent approval of nivolumab and other immune-checkpoint inhibitors for the treatment of certain solid tumors including non-small cell lung cancer (NSCLC) has transformed cancer therapy. However, it will be important to characterize effects of such agents not seen with classical cytotoxic drugs or other targeted therapeutics. We here report two cases of NSCLC showing so-called pseudoprogression during nivolumab treatment. In both cases, imaging assessment revealed that liver metastatic lesions initially progressed but subsequently shrank during continuous nivolumab administration, with treatment also resulting in a decline in serum levels of carcinoembryonic antigen. Histological evaluation of the liver metastatic lesion of one case after regression revealed fibrotic tissue containing infiltrated lymphocytes positive for CD3, CD4, or CD8 but no viable tumor cells, suggestive of a durable immune reaction even after a pathological complete response. Given the increasing use of immune-checkpoint inhibitors in patients with NSCLC or other solid tumors, further clinical evaluation and pathological assessment are warranted to provide a better understanding of such pseudoprogression.