Persistent dysbiosis of intestinal and oral microbiota after neoadjuvant radiotherapy for rectal cancer: Implications for surgery and microbiome intervention

Background: Neoadjuvant radiotherapy represents a conventional approach for managing locally advanced rectal carcinoma. Although this regimen has many benefits, radiation injury is associated with intestinal mucosal injury and gut microbiota dysbiosis. The long-term dynamics of inflammation and microbial recovery, including oral microbiota alterations, remain unclear. Methods: Intestinal mucosal specimens were obtained from patients with rectal cancer who underwent surgical treatment without radiotherapy or at predetermined time intervals (4 ± 1 weeks, 8 ± 1 weeks and 12 ± 1 weeks) after radiotherapy. The levels of TNF-α, IL-4 and IL-6 in the intestinal mucosa were determined using ELISA. Meanwhile, the intestinal mucosa and saliva of 18 recruited rectal cancer patients were subjected to 16S rRNA gene sequencing (radiotherapy group, n=9; non-radiotherapy group, n=9) for analysis of the gut and oral microbial communities. Results: Mucosal IL-4 levels were still significantly elevated at 12 weeks post-radiotherapy with respect to the non-radiotherapy group (p<0.05), while TNF-α and IL-6 were back at baseline levels. The gut microbiota composition of radiotherapy patients differed from that of non-radiotherapy patients, with a significant reduction in Chao1 diversity index (p<0.01) and distinct β-diversity clustering (ANOSIM R = 0.614, p=0.001). These differences were still present at 12 weeks post radiotherapy. The oral microbiota changes were limited, with only a few taxa that showed significant differences (ANOSIM R = 0.38, p=0.009). Conclusion: Neoadjuvant radiotherapy is associated with persistent intestinal mucosa inflammation and prolonged disruption of gut microbiota and this dysbiosis does not fully normalize within 12 weeks after treatment. Changes in oral microbiota appeared less pronounced. These findings provide the biological context for understanding intestinal recovery after radiotherapy and indicate that more studies are needed to direct future efforts towards the peri-operative manipulation of gut microbiota.