PDT dose dosimetry for pleural photodynamic therapy.

PDT dose is the product of the photosensitizer concentration and the light fluence in target tissue. Although existing systems are capable of measuring the light fluence in vivo, the concurrent measurement of photosensitizer in the treated tissue so far has been lacking. We have developed and tested a new method to simultaneously acquire light dosimetry and photosensitizer fluorescence data via the same isotropic detector, employing treatment light as the excitation source. A dichroic beamsplitter is used to split light from the isotropic detector into two fibers, one for light dosimetry, the other, after the 665 nm treatment light is removed by a band-stop filter, to a spectrometer for fluorescence detection. The light fluence varies significantly during treatment because of the source movement. The fluorescence signal is normalized by the light fluence measured at treatment wavelength. We have shown that the absolute photosensitizer concentration can be obtained by an optical properties correction factor and linear spectral fitting. Tissue optical properties are determined using an absorption spectroscopy probe immediately before PDT at the same sites. This novel method allows accurate real-time determination of delivered PDT dose using existing isotropic detectors, and may lead to a considerable improvement of PDT treatment quality compared to the currently employed systems. Preliminary data in patient studies is presented.