Effects of incidental low-dose radiation on abscopal responses after stereotactic body radiotherapy in patients with metastatic cancer: a retrospective real-world analysis.

Although the abscopal effect is a rare phenomenon, preclinical and clinical studies have suggested that low-dose radiation may modulate the tumor microenvironment and enhance immune activation. We investigated the effect of incidental low-dose radiation on abscopal responses in patients with metastatic cancer treated with stereotactic body radiotherapy (SBRT). We retrospectively analyzed 39 patients with metastatic cancer who underwent SBRT for a single lesion with additional non-target lesions and received incidental low-dose radiation between 2010 and 2022. Tumor responses in these non-target lesions were evaluated within 3 months of SBRT and predictors of the abscopal effect were explored. At a median follow-up of 13.9 months, patients experiencing the abscopal effect (23.1%) showed significantly improved 1-year progression-free survival compared to those without it, while the 1-year overall survival showed a favorable trend without statistical significance. Among the 79 non-target lesions, those receiving incidental very low-dose radiation (< 0.5 Gy) demonstrated a significantly higher likelihood of an abscopal response than those receiving no radiation (odds ratio [OR] 9.707, p = 0.041). Higher pre-SBRT lymphocyte counts (> 2,000/μL) were also associated with an increased abscopal response (OR 7.782, p = 0.048). Conversely, pre-SBRT systemic therapy was inversely associated with abscopal response (OR 0.182, p = 0.034). Unintended low-dose radiation may exert an abscopal effect in patients with metastatic cancer treated with SBRT. This effect was linked to improved progression-free survival, indicating a possible clinical benefit. Further studies are required to confirm these results, which may inform SBRT planning strategies.