Inclusion criteria for the review were defined. Several relevant data sources were searched. Attempts were made to reduce reviewer error and bias during data extraction and quality assessment, but it was not clear if the same attempts were made for study selection.
Quality assessment was undertaken according to specific quality factors, which indicated that the reliability of the evidence base was sub-optimal (most of the studies were observational). A narrative synthesis was presented for most outcomes. Meta-analysis was used to pool overall response rate data, although the analysis may have not been appropriate as observational and RCT data were pooled together. The analysis also had a high level of statistical heterogeneity, which indicated that the data may not have been suitable for pooling. The authors noted that the reasons for re-irradiation were not always specified, which may have introduced selection bias. The authors also noted that the timing of irradiation was not standardised in any of the studies, which may have biased the results.
The suboptimal quality of the evidence base, and the potential for biases in the included studies and in the review, limits the reliability of the authors’ conclusions.