Evidence for a link between rheumatoid arthritis (RA) and cancer is far from definitive and further research is necessary, experts have said.
A recent meta-analysis indicated that RA increases the risk of certain cancers, especially lymphoma, which was shown to be twice as likely in those with RA as in generally healthy individuals, Daniel Solomon and Thorvardur Love (Brigham and Women's Hospital, Boston, Massachusetts, USA) noted in the journal Arthritis Research and Therapy.
In contrast, the same study showed that RA decreased the risk of both breast and colorectal cancers, they added.
Work by other scientists has suggested that the use of tumor-necrosis factor (TNF)-α antagonists in RA patients might account for the increased risk of certain cancers, perhaps by blocking a TNF-α-driven pathway that retards the growth of tumors. If this is the case, lymphoma incidence would be expected to increase dramatically shortly after TNF-α antagonists were introduced and future epidemiological studies should attempt to determine such a pattern, Solomon and Love stressed.
It has also been suggested that the chronic inflammation characteristic of RA encourages the differentiation of normal immune cells into malignant cells. Similarly, the increased risk of lung cancer in RA patients may be explained by smoking, which increases the risk of both RA and lung cancer.
On the other hand, the authors pointed out that the use of non-steroidal anti-inflammatory drugs (NSAIDs) may account for the reduction in colon cancer risk in RA patients. Indeed, major trials have shown that specific NSAIDs can significantly reduce the recurrence of adenomatous polyps and adenomas, the authors said.
They concluded: "The important meta-analysis into the relationship between RA and cancer risk pushes our understanding forward but raises many important questions for future research."
Most importantly, Solomon and Love recommend that future studies be performed in more genetically diverse populations and control for the influence of potentially confounding factors, such as smoking.
Based on the outcome of such studies, the ultimate question will be whether or not patients with RA should be screened for malignancies, the authors said.