Evidence is accumulating that cancer therapy leads to changes in brain structure and metabolism that are associated with inflammation and symptoms. Kesler et al. [
48] reported an association between lower left hippocampal volume as measured by MRI and higher levels of circulating TNF-α and lower levels of IL-6 in breast cancer survivors, but not in controls. Verbal memory performance in survivors was predicted by an interaction between hippocampal volume and TNF-α. Most of the breast cancer survivors in this study (>80%) had received a chemotherapeutic cocktail including doxorubicin, which was shown in an animal model to increase TNF-α (and IL-1β) in the hippocampus [
49]. Thus, the association between
circulating TNF-α and hippocampal volume found in the clinical study possibly reflects associations between
central TNF-α and hippocampal volume, leading to cognitive symptoms. A small pilot study that included eight breast cancer patients who underwent chemotherapy during the study [
50] found associations of higher sTNFR2 and IL-6 levels with decreased gray matter volume in specific regions. However, fatigue and cognitive functioning were not related to the inflammatory markers. Two preliminary reports from an ongoing longitudinal study showed associations between plasma levels of inflammatory mediators and reduced brain metabolism (as measured by FDG PET-scan) after chemotherapy for breast cancer: Ganz et al. [
51] showed an association between plasma sTNFR2 and increased memory complaints, along with
diminished resting-state metabolism in the inferior frontal regions in a subsample of 12 breast cancer patients who had undergone chemotherapy. Declines in sTNFR2 over the 12-month study period were associated with fewer memory complaints. Interestingly, associations between plasma sTNFR2 and memory complaints diminished when controlled for fatigue, implying that the report of memory complaints was partly due to inflammation-induced fatigue. Conversely, Pomykala [
52], reporting on the same longitudinal study but with a larger sample (23 chemotherapy-treated patients), observed associations between higher plasma levels of inflammatory mediators (IL-6, sTNFRII, IL-1ra, and CRP) and
increased resting-state metabolism in prefrontal and anterior temporal cortex shortly after chemotherapy completion as well as 1 year later. Metabolism in these brain regions was associated with patient-reported memory complaints. These apparently contradictory findings might be explained by a compensatory system in which reduced activity in some brain regions is compensated by increased activity in other regions. Zick et al. [
53] found that both IL-6 and specific brain metabolites (higher glutamate + glutamine to N-acetyl-aspartate ratio in the posterior insula) predicted fatigue in breast cancer survivors. However, these brain metabolites did not correlate with IL-6, suggesting that not all changes in brain metabolism are inflammation-related, although we note that only one inflammation marker was tested.