A comprehensive mathematical model of microscopic dose deposition in photodynamic therapy

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Errata

This article is corrected by:

  1. Errata: Erratum: “A comprehensive mathematical model of microscopic dose deposition in photodynamic therapy” [Med. Phys. 34, 282–293 (2007)] Volume 35, Issue 9, 4278–4280, Article first published online: 26 August 2008

Abstract

We have developed a comprehensive theoretical model for rigorously describing the spatial and temporal dynamics of oxygen (O23) consumption and transport and microscopic photodynamic dose deposition during photodynamic therapy (PDT) in vivo. Previously published models have been improved by considering perfused vessels as a time-dependent O23 source and linking the O23 concentration in the vessel to that within the tissue through the Hill equation. The time-dependent photochemical O23 consumption rate incorporates sensitizer photobleaching effects and an experimentally determined initially nonuniform photosensitizer distribution. The axial transport of O23 is provided for in the capillaries and in the surrounding tissue. A self-sensitized singlet oxygen (O21)-mediated bleaching mechanism and the measured, initially nonuniform distribution of meso-tetrahydroxyphenyl chlorin at 3h after intravascular administration were used to demonstrate the capabilities of the model. Time-evolved distributions of O23 concentration were obtained by numerically solving two-dimensional diffusion-with-reaction equations both in the capillary and the adjacent tissue. Using experimentally established physiological and photophysical parameters, the mathematical model allows computation of the dynamic variation of hemoglobin-O23 saturation (SO2) within the vessels, irreversible sensitizer degradation due to photobleaching, and the microscopic distributions of O23, sensitizer concentration, and O21 dose deposition under various irradiation conditions. The simulations reveal severe axial gradients in O23 and in photodynamic dose deposition in response to a wide range of clinically relevant treatment parameters. Thus, unlike former Krogh cylinder-based models, which assume a constant O23 concentration at the vessel, this new model identifies conditions in which O23 depletion and minimal deposition of reacting O21 exist near the end of axial segments of vessels and shows that treatment-limiting O23 depletion is induced at fluence rates as low as 10mWcm2. These calculations also demonstrate that intercapillary heterogeneity of photosensitizer contributes significantly to the distribution of photodynamic dose. This more rigorous mathematical model enables comparison with experimentally observable, volume-averaged quantities such as SO2 and the loss of sensitizer fluorescence through bleaching that have not been included in previous analyses. Further, it establishes some of the intrinsic limitations of such measurements. Specifically, our simulations demonstrate that tissue measurements of SO2 and of photobleaching are necessarily insensitive to microscopic heterogeneity of photodynamic dose deposition and are sensitive to intercapillary spacing. Because prior knowledge of intercapillary distances in tumors is generally unavailable, these measurements must be interpreted with caution. We anticipate that this model will make useful dosimetry predictions that should inform optimal treatment conditions and improve current clinical protocols.

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