Robustness of Interstitial Photodynamic Therapy Treatment Planning under Uncertainties in Light Delivery

While computer simulations can accurately model the light dose delivered by interstitial photodynamic therapy (iPDT) treatmentplans, predicting individual clinical outcomes remains difficult. Besides biological unknowns limiting the correlation between a plan and its clinical effect, uncertainties in the accuracy of light delivery must also be considered. We simulate and analyze two aspects: light source power uncertainties at ±5%, ±10%, and ±20%; and positional deviations during source insertion, assuming angular errors that displace source points by up to 3 mm. Simulated outcomes show minimal impact from power uncertainty, even at worst-case ±20%: the percent difference between maximum and minimum v100 does not exceed 9%, with tumour coverage only dropping from the targeted 98% to 96.9%. Using a new power-uncertainty–aware option in the PDT-SPACE planning tool improves the worst-case minimum coverage from 96.9% to 97.3%, 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 98%, while PDT-SPACE source-position optimization reduces average healthy tissue damage by 36%. Combining both yields the most robust performance and minimizes sensitivity to positional deviations, thereby limiting light-delivery errors in iPDT.