Non-targeted effects of stereotactic radiotherapy: a review of the evidence coming from the clinical field.

Background: Preclinical animal studies have demonstrated that radiation treatment (RT) can induce effects beyond the anatomical site of irradiation. Non-targeted effects of RT (NTER) have been sporadically reported in clinical settings. However, with the advent of high-dose stereotactic radiation techniques (SRT) and immunotherapy (IT), renewed attention has been given to NTER. This systematic review aims to summarize current knowledge about NTER across various malignancies, with a focus on cases involving SRT. Methods: A systematic database search was performed, and records were screened following PRISMA guidelines. Only full-text original articles written in English and reporting clinical studies involving NTER after SRT were included. The results are categorized by cancer type, with separate general and critical analyses. Results: Sixty-three studies were reviewed, including 32 case reports/case series, 18 retrospective studies, and 13 prospective studies, predominantly published after 2018. NTER was most frequently observed in melanoma and lung cancer and commonly reported as the abscopal effect (AE), albeit with varying criteria. In most cases, IT with suboptimal response was ongoing at the time of SRT, and the median time to NTER onset was 3 months. Overall, NTER was documented in 297 patients: 34 from single cases and 263 from a pool of 1,212 evaluable patients (22%) across other studies. Prospective trials reported an NTER rate of 36%, rising to 56% in lung cancer. Discussion: In prospective clinical studies, the phenomenon of NTER following SRT has been observed in a significant proportion of patients. Nevertheless, the literature is limited, with small patient cohorts. Interest in NTER has grown, particularly in the context of IT. Standardization of definitions and reporting, along with the conduct of more clinical trials, is essential to better understand how NTER can be induced by SRT.