Abstract To establish the efficacy of interstitial photodynamic therapy (I-PDT) for solid cancer treatments, the treating physician needs to be able to tailor the treatments to each patient. During I-PDT in the clinic, there is a need to adjust the irradiance (light dose rate) and fluence (light dose) to account for patient-specific tissue and tumor optical properties and to account for changes in fiber placements that occur after an initial pretreatment plan is generated. This can be accomplished with the novel, near real-time computational software, DOSIETM, developed by Simphotek, Inc. The DOSIE key advantages are: (i) it is a dedicated single-package software package for computing dose metrics of intratumoral light irradiance and fluence for I-PDT; and (ii) it includes a fast algorithm for updating the laser light settings according to optical properties of the tumor and actual fiber placement in near real-time. To demonstrate that DOSIE can effectively guide I-PDT, Simphotek and RP propose to conduct a pilot Phase II clinical study (Phase IIa) on patients with solid malignancies in the lung causing central airway obstruction involving extrinsic tumor growth and related airway compression. The proposed trial will utilize endobronchial ultrasound (EBUS) with a transbronchial needle (TBN) that will be used to guide optical diffuser fiber insertion for intratumoral illumination in I-PDT. The DOSIE treatment planning system that will be employed in this trial is vital to calculate the intratumoral fluence and irradiance, which will impact tumor response in I-PDT of the locally advanced cancers.