Dose-volume histogram analysis of slit-beam irradiation: biological implications in mouse ex vivo testes

Spatially fractionated irradiation (SFI), particularly slit-beam irradiation (SI), is gaining attention in radiation oncology for its potential to spare normal tissue via non-target effects (NTE), such as the bystander effect. However, it remains unclear whether the observed tissue-sparing effects (TSE) result from biological NTE or simply from physical dose-volume characteristics. In this study, we investigated the theoretical biological consequences of SI by analyzing the dose-volume histogram (DVH) of a previously reported ex vivo mouse testis experiment. Using Monte Carlo simulations (PHITS), we calculated three-dimensional micro-scale dose distributions for slit widths of 12.5, 50, and 200 μm. We evaluated the Equivalent Uniform Dose (EUD) based on the linear-quadratic (LQ) model and performed a sensitivity analysis on LQ parameters. Our results indicated that while EUD remained robust across different α and β values for SI, it varied drastically for conventional uniform irradiation (UI), suggesting that standard EUD comparisons may be inappropriate when biological parameters are uncertain. Furthermore, we developed "artificial radiography" to visualize non-slit non-uniform irradiation (NSNUI) configurations that possess DVHs identical to those of SI. We conclude that the TSE observed in previous SI studies may be attributable to DVH differences rather than specific biological NTEs. To rigorously verify the existence of NTEs in vivo, we propose that NSNUI with a matched DVH represents the rational "control irradiation," eliminating the confounding influence of physical dose distribution.