Geographic access to radiotherapy facilities in Japan.

Daily attendance requirements for radiotherapy (RT) make geographic accessibility a critical determinant of treatment adherence and completion. Although facility surveys indicate that advanced modalities such as intensity-modulated radiotherapy (IMRT) and brachytherapy are concentrated in urban centers, precise nationwide quantification of the resulting patient travel burden remains lacking. This study provides the first nationwide, modality-stratified assessment of geographic access to RT in Japan. Using the Open Source Routing Machine and a high-resolution dataset of 176 964 household-weighted 1-km mesh centroids, we calculated driving times to the nearest external-beam radiotherapy (EBRT), IMRT and brachytherapy facilities. Beyond standard distribution metrics, we generated high-resolution 'penalty maps' to quantify the incremental time tax imposed by advanced modality requirements. Although EBRT access was uniformly short nationwide, with a median travel time of 6.48 min, this increased to 8.26 min for IMRT and 14.06 min for brachytherapy. Crucially, the proportion of the population facing poor access (≥120 min) doubled from 0.24% for EBRT to 0.48% for brachytherapy. The spatial analysis identified specific 'newly poor-access' areas-regions that are accessible for EBRT but become remote when advanced care is needed-forming coherent geographic clusters in mountainous and island zones. These findings demonstrate that modality requirements introduce meaningful inequities despite strong national EBRT infrastructure. These indicators provide a vital evidence base for spatially optimizing resources to mitigate travel burdens for Japan's aging, mobility-limited population.