Second primary cancers in non-Hodgkin lymphoma: Bidirectional analyses suggesting role for immune dysfunction

Second primary cancers (SPCs) account for an increasing proportion of all cancer diagnoses. It is unlikely that prior therapy is solely responsible for SPC risk. To investigate risk of SPC after diagnosis of non-Hodgkin lymphoma (NHL) and 10 of its subtypes we conducted a novel bidirectional analysis, SPCs after NHL and NHL as SPC. Using the Swedish Family-Cancer Database, we identified 19,833 individuals with primary NHL diagnosed between 1993 and 2015. We calculated relative risks (RRs) of SPCs in NHL survivors and, for bi-directional analysis, risk of NHL as SPC. The overall RRs were significantly bidirectionally increased for NHL and 7 cancers. After diagnosis of NHL risks were increased for upper aerodigestive tract (RR = 1.96), colorectal (1.35), kidney (3.10), bladder (1.54) and squamous cell skin cancer (SCC) (4.12), melanoma (1.98) and Hodgkin lymphoma (9.38). The concordance between RRs for each bidirectional association between NHL and 31 different cancers was highly significant (r = 0.86, p < 0.0001). Melanoma was bidirectionally associated with all 10 subtypes of NHL. The observed bidirectional associations between NHL and cancer suggest that therapy-related carcinogenic mechanisms cannot solely explain the findings. Considering that skin SCC and melanoma are usually treated by surgery and that these cancers and NHL are most responsive of any cancer to immune suppression, the consistent bidirectional results provide population-level evidence that immune suppressed state is a key underlying mechanism in the context of SPCs. Furthermore, the quantified risks for NHL subtypes have direct clinical application in the management of NHL patients.