Abscopal Effect of Radiotherapy Combined with Immunotherapy in Advanced Gastrointestinal Cancer: A Retrospective Study

Corresponding author: Zhanglin, stepinghuns2@163.com

DOI: 10.12201/bmr.202606.00027

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  Abstract: Objective: To evaluate the incidence of abscopal effect in non-irradiated lesions of patients with advanced gastrointestinal cancer treated with stereotactic body radiotherapy (SBRT) combined with PD-1 monoclonal antibodies, analyze its clinical predictors, and assess the safety and survival benefits of the combined therapy. Methods: This retrospective study included 68 patients with advanced gastric cancer or colorectal cancer who received SBRT combined with PD-1 monoclonal antibody therapy at the Eighth Medical Center of Chinese PLA General Hospital from January 2020 to April 2025. Baseline patient characteristics, SBRT parameters, immunotherapy regimens, and clinical outcome data were collected. The primary endpoint was the incidence of abscopal effect in non-irradiated lesions (defined as partial response/complete response [PR/CR] of non-irradiated lesions). Secondary endpoints included local control (LC) of irradiated lesions, progression-free survival (PFS), overall survival (OS), and incidence of grade ≥3 adverse events. Univariate and multivariate logistic regression analyses were used to identify independent predictors of the abscopal effect, and the Kaplan-Meier method was used for survival analysis. Results: Among the 68 patients, 19 developed an abscopal effect, with an incidence rate of 27.9% (95% CI: 17.6%–38.2%). The local control rate of irradiated lesions was 73.5% (50/68). Univariate analysis showed that BED10 ≥80 Gy, first-line immunotherapy, liver/lung metastasis, and microsatellite instability-high (MSI-H) status were significantly associated with the abscopal effect (P < 0.05). Multivariate logistic regression analysis confirmed that BED10 ≥80 Gy (OR = 4.215, 95% CI: 1.523–11.657, P = 0.006), first-line immunotherapy (OR = 3.782, 95% CI: 1.386–10.325, P = 0.009), and MSI-H status (OR = 5.103, 95% CI: 1.674–15.552, P = 0.004) were independent favorable predictors of the abscopal effect. The incidence of grade ≥3 adverse events in the entire cohort was 17.6% (12/68), primarily including radiation pneumonitis, immune-related enteritis, and myelosuppression, with no treatment-related deaths. Survival analysis showed that the abscopal effect group had significantly better median PFS (10.2 months vs. 4.1 months, HR=0.32, 95%CI: 0.18–0.58, P<0.001) and median OS (18.5 months vs. 9.3 months, HR=0.28, 95%CI: 0.15–0.52, P<0.001) compared to the non-abscopal effect group. Conclusion: SBRT combined with PD-1 monoclonal antibody therapy can induce a considerable abscopal effect in advanced gastrointestinal cancer patients with a manageable safety profile. High biologically effective dose (BED10 ≥80 Gy), first-line combined immunotherapy, and MSI-H status are key clinical factors predicting the abscopal effect, providing real-world evidence to optimize the combination strategy of radiotherapy and immunotherapy for advanced gastrointestinal cancer.

  Key words: advanced gastrointestinal cancer; stereotactic body radiotherapy; PD-1 monoclonal antibody; abscopal effect; predictive factors; prognosis

  Submit time: 12 June 2026

  Copyright: The copyright holder for this preprint is the author/funder, who has granted biomedRxiv a license to display the preprint in perpetuity.
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