Diverse optical characteristic of the prostate and light delivery system: implications for computer modelling of prostatic photodynamic therapy

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Abstract
To explore the use of photodynamic therapy (PDT) as a minimally invasive form of treatment for organ-confined prostate cancer, for although there are several therapies, ablative treatments are associated with significant morbidity. Using the photosensitizer tin etiopurpurin, dogs were treated with interstitially placed laser fibres in an effort to validate PDT for treating prostate cancer. Earlier models assumed a uniform distribution of light output from a cylindrical fibre and a uniform attenuation coefficient throughout the prostate. Subsequent observations show that this model was too simple and that light radiance is not linear. To overcome under-treatment, a computer program to complement real-time fibre placement was developed. As light radiance from interstitially placed laser fibres varies significantly from the commonly assumed ideal cylindrical emission, a predictive mathematical model of prostate PDT needs to consider the real emission. Also, the optical properties of the prostate, e.g. absorption and scattering of light, are anisotropic. Differences in the attenuation coefficient (combining absorption and scattering of light) also varied among different animals. Incorporating all these variables into a computer program produced a virtual model of the photo-ablated zone within +/- 2 mm of that observed in animals. PDT of the prostate is not trivial and should benefit from computer-aided methods as it is developed for clinical use.