Multiple primary malignancy as a model for the study of cancer occurrence in rheumatoid arthritis.

Research approaches to the study of multiple primary malignancy can be helpful in the study of cancer occurrence in rheumatoid arthritis. Research on multiple primary malignancy suggests that an observed increased risk of cancer in rheumatoid arthritis could be due to shared host susceptibility to both diseases, shared risk factors, rheumatoid arthritis-caused changes predisposing to cancer, biased research, chance, or the therapy used in the treatment of rheumatoid arthritis. All of these factors should be considered before concluding that treatment was the cause. Multiple primary malignancy research suggests that sophisticated analyses are needed in order to evaluate cancer risks following therapy. There may be long latency periods between therapy and cancer onset. Drugs may have been given in various dosages, sequences, and combinations and for different durations. Persons with chronic diseases such as rheumatoid arthritis are often difficult to follow over time; hence, there may be variable follow-up intervals for subjects. All of these problems must be taken into account in analyses of cancer risk.