Photodynamic therapy

A treatment in which a systemic photosensitzer is activated by a laser light of specific wavelength resulting in the generation of singlet oxygen that results in the destruction of targeted tissues.

Photodynamic therapy is a form of phototherapy using nontoxic light-sensitive compounds that are exposed selectively to light, whereupon they become toxic to targeted malignant and other diseased cells.

From the excited singlet state the photosensitzer must undergo electron spin conversion to its triplet state.

Singlet oxygen formation can cause cell death via apoptosis or necrosis.

Anti tumor effects from direct anti tumor cytotoxic effects, damages vascular tissues, and induces inflammation and a systemic immunity.

Photodynamic therapy has two stages: one following the administration of light-sensitive photosensitizer, and two tumor sites are irradiated with a light of appropriate wavelength.

The light is the delivered by flexible fiber optic devices.

The region between 600-1200 nm is ref2242ed to the optical window of tissue..

Light up to approximately 800 nm can generate O2 because longer wavelengths have sufficient energy to initiate photodynamic energy.

Clinical efficacy of PDT depends on dosimetry, total light dose, light exposure time, and light delivery mode.

Treatment is ineffective against metastatic lesions.

PDT can be used before or after chemotherapy, radiotherapy, or surgery without compromising these other therapeutic modalities.

Photosensitizers that are presently used do not accumulate in cell nuclei and limit DNA damage such that they are not carcinogenic or lead to development of resistant clones.

The use of PDT allows retention of functional anatomy, mechanical integrity of organs and is associated with minimal fibrosis.

In patients with the inoperable tumors PDT therapy can achieve improvement in quality of life.

Not affected by radioresistance or chemoresistance in terms of sensitivity.

Excellent cosmetic outcomes from PDT for patients with skin cancers.

The only adverse effects of PD therapy relate to pain and persistent skin photosensitization.

Most photosensitizers utilized in cancer therapy have a tetrapyrrole structure, similar to in hemoglobin.

Absorption peak of photosensitizers optimally ranges between 600 and 800 nm to excite oxygen to its singlet state and to form a substantial yield of reactive oxygen species.

Photosensitizers should have no dark toxicity and should have rapid clearance from normal tissues to minimize phototoxic side effects.

Porfimer sodium (Photofrin), 5-aminolevulinc acid (Levulan) , methyl ester of ALA (Metvix) and meso-tetra-hydroxy-phenyl-chlorin (Foscan) are sensitizers approved for clinical use.

Photosensitivity is a common side effect, due to long-lasting skin photosensitivity and a relatively low absorbency at 630 nm.

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