In Vivo Monitoring of Photodynamic Therapy: from lab to clinic

Photodynamic therapy (PDT) is an emerging clinical treatment modality, which utilizes light, oxygen and a light sensitive drug (the photosensitizer), for curative and palliative treatment of a variety of malignant and non-malignant conditions tumors. PDT is frequently used in the clinic for treatment of superficial skin lesions by superficial illumination of the lesion after the topical administration of a photosensitizer or its precursor. However, PDT is also under investigation for treatment of larger tumors volumes in regions such as the head and neck1,2 and the prostate3 by inserting opticfibers in the tumor volume for the delivery of the treatment light. The therapeutic effect in PDT is induced by the interaction between the tissue and reactive oxygen radicals. These reactive oxygen radicals, predominantly singlet oxygen, are formed by the interaction of photosensitizer, light (of an appropriate wavelength) and oxygen in the tissue. The deposited PDT dose is the amount of light that actually interacts with the photosensitizer that leads to formation of reactive oxygen species responsible for inducing tissue response. Note that this is different from the actual amount of light delivered to the tissue since not all of the light delivered interacts with the photosensitizer scattering and absorption of the tissue. In addition, based on the photosensitizer’s ability to form reactive oxygen species (ROS) only a percentage of light that interacts with the photosensitizer lead to formation of ROS. Also there is a difference between the actual delivered light dose and the intended delivered light dose. Where the intended delivered light dose is set by the clinician to be delivered. Only by measuring the amount of light in situ it is possible to determine the actual delivered light dose. So although this intended light dose is kept the same in individuals undergoing treatment, the actual delivered light dose and the deposited PDT dose can vary due to biological variation and the dynamic interaction between light, photosensitizer and oxygen in tissue. Inter individual variations in deposited PDTdose yield variations in induced tissue response and treatment outcome. For this reason it is necessary to determine and monitor the deposited PDT dose during therapeutic illumination

Phosphorescence-Fluorescence ratio imaging for monitoring the oxygen status during photodynamic therapy

The effectiveness of photodynamic therapy is strongly dependent on the availabilty of oxygen. In the present paper we show that the ratio between photosensitiser phosphorescence and fluorescence is a parameter that can be used to monitor the competition between singlet oxygen production and other processes quenching the photosensitiser triplet state. We present a theoretical basis for the validity of this approach and a series of in vitro imaging experiments

In vivo quantification of photosensitizer fluorescence in the skin-fold observation chamber using dual-wavelength excitation and NIR imaging

A major challenge in biomedical optics is the accurate quantification of in vivo fluorescence images. Fluorescence imaging is often used to determine the pharmacokinetics of photosensitizers used for photodynamic therapy. Often, however, this type of imaging does not take into account differences in and changes to tissue volume and optical properties of the tissue under interrogation. To address this problem, a ratiometric quantification method was developed and applied to monitor photosensitizer meso-tetra (hydroxyphenyl) chlorin (mTHPC) pharmacokinetics in the rat skin-fold observation chamber. The method employs a combination of dual-wavelength excitation and dualwavelength detection. Excitation and detection wavelengths were selected in the NIR region. One excitation wavelength was chosen to be at the Q band of mTHPC, whereas the second excitation wavelength was close to its absorption minimum. Two fluorescence emission bands were used; one at the secondary fluorescence maximum of mTHPC centered on 720 nm, and one in a region of tissue autofluorescence. The first excitation wavelength was used to excite the mTHPC and autofluorescence and the second to excite only autofluorescence, so that this could be subtracted. Subsequently, the autofluorescence-corrected mTHPC image was divided by the autofluorescence signal to correct for variations in tissue optical properties. This correction algorithm in principle results in a linear relation between the corrected fluorescence and photosensitizer concentration. The limitations of the presented method and comparison with previously published and validated techniques are discussed

Optical Scattering Measurements of Laser Induced Damage in the Intraocular Lens

This study optically determines whether the amount of light scatter due to laser-induced damage to the intraocular lens (IOL) is significant in relation to normal straylight values in the human eye. Two IOLs with laser-induced damage were extracted from two donor eyes. Each IOL had 15 pits and/or cracks. The surface area of each pit was measured using a microscope. For 6 pits per intraocular lens the point spread function (PSF) in terms of straylight was measured and the total straylight for all 15 pits was estimated. The damage in the IOLs was scored as mild/moderate. The total damaged surface areas, for a 3.5 mm pupil, in the two IOLs were 0.13% (0.0127 mm2) and 0.66% (0.064 mm2), respectively. The angular dependence of the straylight caused by the damage was similar to that of the normal PSF. The total average contribution to straylight was log(s) = −0.82 and −0.42, much less than the straylight value of the normal eye