Background: Chronic cancer-related pain remains underdiagnosed and undertreated, although it affects 40% of cancer survivors. Recent insights suggest that cytokine signaling between immune, neuro, and glial cells contributes to chronic pain.
Objectives: This study systematically reviewed cytokine levels and their relation to chronic cancer-related pain and, additionally, investigated differences in cytokine levels between cancer survivors with and without chronic pain.
Study design: Systematic review.
Methods: This systematic review was conducted and reported following the Preferred Reporting Items for Systematic Reviews and Meta-Analysis guidelines (PRISMA). The study conducted a systematic literature search in the databases PubMed, Web of Science, and Embase for articles examining cytokine levels and pain experience at a time point of a minimum of 3 months post-cancer diagnosis. Pain experience was categorized into a total pain score, pain intensity, and pain interference. The risk of bias was assessed using the Newcastle Ottawa Scale.
Results: Eight articles were included, investigating 6 cancer types and 30 cytokines. Moderate evidence was found for pro-inflammatory cytokine IL-6 to be correlated with pain intensity, of which higher levels are observed in cancer survivors experiencing chronic pain compared to pain-free survivors. Moderate evidence was found for TNF-alpha to be not correlated with any pain experience, which is similar for anti-inflammatory cytokines IL-8 and IL-10 with pain intensity. For the remaining 26 cytokines and pain outcomes, only limited evidence was found for an association or alteration.
Limitations: The number of included studies was small. Overall, studies showed a moderate risk of bias, except one indicated a high risk of bias.
Conclusion: More standardized post-cancer treatment studies are warranted to confirm these results and explore associations and alterations of other cytokines. Nonetheless, moderate evidence suggests that elevated levels of IL-6, in contrast with TNF-alpha levels, are correlated with pain intensity in cancer survivors experiencing chronic pain compared to pain-free survivors.
Keywords: IL-6; TNF-alpha; cancer pain; chronic cancer-related pain; chronic pain; interleukin; persistent pain; Cytokines.