Robustness of interstitial photodynamic therapy treatment planning under power and positional uncertainties in light delivery.

While computer simulations can accurately model light dose delivery in interstitial photodynamic therapy (iPDT), predicting individual clinical outcomes remains difficult. In addition to biological uncertainties, inaccuracies in light delivery must be considered. Using simulations on virtual brain tumour (glioblastoma) models, we analyze two sources of uncertainty: light source power variations of [Formula: see text], [Formula: see text], and [Formula: see text], and positional deviations during source insertion, modelled as angular errors producing up to [Formula: see text] displacement. Simulated outcomes show minimal impact from power uncertainty, even at worst-case [Formula: see text]: the percent difference between maximum and minimum [Formula: see text] does not exceed [Formula: see text], with tumour coverage only dropping from the targeted [Formula: see text] to [Formula: see text]. Using a new power-uncertainty-aware option in the PDT-SPACE planning tool improves the worst-case minimum coverage from [Formula: see text] to [Formula: see text], eliminating the risk of under-treating. Position uncertainty was simulated by discretizing the space and randomizing source placements, showing a larger negative effect. Power re-optimization on measured post-insertion positions restores tumour coverage to [Formula: see text], while PDT-SPACE source-position optimization reduces average healthy tissue damage by [Formula: see text]. Combining both yields the most robust performance and minimizes sensitivity to positional deviations, thereby limiting light-delivery errors in iPDT.