Background/Purpose: Given suspected links between inflammation and malignancy, several groups have attempted to elucidate the baseline risk of cancer in various inflammatory or autoimmune conditions, including rheumatoid arthritis, where there is an increased risk of hematological malignancies (lymphoma in particular). However, there are very few studies of cancer risk in large pediatric-onset rheumatic disease cohorts.  The purpose of our study was to further investigate cancer risk in JIA, based on a very large multi-centre clinical population.   

Methods:  We examined data from 4 pediatric rheumatology clinic cohorts in Canada and 2 in the United States, across 1971-2011. We linked patients to regional cancer registries to detect observed cancers (O) occurring after cohort entry, defined as date first seen in the JIA clinic cohort. End of follow-up was defined as the date of cancer registry linkage (thus was not truncated once subjects reached adulthood).  The expected number of malignancies (E) was obtained by multiplying the person-years in the cohort (defined from cohort entry to end of follow-up) by the geographically matched age, sex, and calendar year-specific cancer rates, and summing overall person-years.

The standardized incidence ratio (SIR; ratio of cancers observed to expected) was generated, along with 95% confidence intervals (CIs), based on the assumption of cancer occurrence as a Poisson-distributed variable. Results were also stratified by age group of person-time (categorized into 0-19 and >20 years of age).

Results:  A total of 5,108 JIA patients were studied. The mean age at cohort entry was 8.9 years (standard deviation, SD 5.0). The majority were female (68.0%) and the sample was predominantly Caucasian, reflecting the source populations. The most common JIA subtype was oligoarticular.  The mean duration of follow-up (time from cohort entry to end of follow-up) was 6.7 years, resulting in 34,224 patient-years of observation. 

During total follow-up, 9 invasive cancers were observed, compared to 10.1 expected, for an over-all SIR of 0.89, 95% CI 0.41-1.69. Three of these were hematological (1 Hodgkin’s lymphoma, 1 Non-Hodgkin lymphoma, and 1 leukemia). For all hematological malignancies, the SIR was 1.33 (95% CI 0.27-3.88).  

When stratifying SIR estimates, for the 0-19 age group, seven cancers occurred, for an SIR of 1.69 (95% CI 0.68-3.48) for all cancers and a SIR of 2.01 (95% CI 0.41-5.87) for hematological cancers.

Conclusion: Our study is one of the largest examining the relationship between malignancy risk and JIA. Cancer was a relatively infrequent outcome, and most of the cancers observed occurred during the years when patients were aged 0-19 years. Further follow-up over time, of the subjects in this JIA cohort, would be necessary to capture accurately cancer risk for JIA in the adult years.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/cancer-risk-in-5108-patients-with-juvenile-idiopathic-arthritis-jia/