Background Mistletoe extract, which is usually used as a complementary agent for cancer patients, provides an anticancer effect on various malignancies. The present study aimed to evaluate the effect of mistletoe extract (Abnoba Viscum Q®) on tumor responses in neoadjuvant chemoradiotherapy for locally advanced rectal cancer. Methods The rectal cancer patients who underwent neoadjuvant chemoradiotherapy were analyzed from Jan 2018 to Jul 2020. In the mistletoe group (MG), the patients were

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... istered Abnoba Viscum Q® subcutaneously during chemoradiotherapy, and it was maintained just before surgery. Patients' demographics, clinical outcomes, and histopathological outcomes were compared between chemoradiation with the MG and nonmistletoe group (NMG). Results A total of 52 patients were included. There were MG of 15 patients and NMG of 37 patients. Baseline demographics were statistically similar between the two groups except the CA19-9 and tumor location levels from anal verge. There was no difference in the clinical stage for both groups. We also observed better tumor response in MG in terms of TRG, T stage, and overall TNM stage. Tumor response was significantly better in MG comparing NMG in terms of pathologic complete response rate (53.3% vs 21.6%, p = 0.044), good responder of tumor regression grade (66.7% vs 32.4%, p = 0.024), T downstaging (86.7% vs 43.2%, p = 0.004), and overall downstaging (86.7% vs 56.8%, p = 0.040). The toxicities during NCRT in both groups were minimal. Conclusion MG treated with chemoradiation combined with mistletoe extract showed better outcomes than NMG in terms of tumor responses. This diversity in treatment may elevate the method to hope for better oncologic outcomes. Prospective and randomized studies with long-term follow-up are warranted to confirm and extend this study.