Light Sources for PDT

There are two main light sources used in PDT, lamps for topical treatments and lasers when transmission via optical fibres is required. 

---

Early studies with PDT employed complex mixtures of poorly defined porphyrins known as hemato-porphyrin derivative (photofrin I) or a partially purified mixture known as porfimer sodium (PHOTOFRIN II) that was administered parenterally with subsequent irradiation using polychromatic light sources. The major problem with this approach was the prolonged period (4-6 weeks) of photosensitivity caused by skin retention of the porphyrin formulations. This led to a search for compounds that could be administered topically and that were eliminated more readily from the skin. The porphyrin precursor S-aminolevulinic acid (ALA) is converted to various porphyrins, particularly protoporphyrin (proto), in tissues including the skin (see below). Protoporphyrin subsequently is eliminated rapidly from the body, thereby minimizing the period of skin photosensitivity to a few hours. Topically applied ALA HCl (20% wA>) and, more recently, the methyl ester of ALA have been used successfully for the PDT of various types of nonmelanoma skin cancers and premalignant lesions. 

Incoherent (nonlaser) and laser light sources have been used for PDT. The wavelengths chosen must include those within the action spectrum of protoporphyrin and ideally those that permit maximum skin penetration. Light sources in use emit energy predominantly in the blue portion (maximum porphyrin absorption) or the red portion (better tissue penetration) of the visible spectrum. Nonhypertrophic actinic keratoses and superficial basal cell carcinomas and Bowen s disease seem to respond best to PDT. Topical ALA products for PDT approved by the FDA include LEVUIAN KERASTICK, BLU-U blue light, and METVIX. 

There are currently three main classes of light source used in PDT lasers, light emitting diodes (LEDs) and wavelength-filtered lamps. The basic requirements for PDT light sources, and how these are met by the different source types, are as follows. 

Figure 5. Light Emitting Diode (LED) sources for clinical PDT. (a) array for skin lesion treatments (courtesy EXPO Inc, Canada), (b) small array for intra-oral application (courtesy PRP Optoelectronics, England), (c) interstitial linear arrays (courtesy Light Sciences Inc., USA).

Figure 6. Filtered lamp sources for clinical PDT. (a) arc lamp coupled into a liquid light-guide (courtesy EXPO Inc., Canada), (b) bank of fluorescent tubes (courtesy DUSA Inc, USA). Note the use of the blue part of the spectrum for treating superficial actinic keratosis lesions with topical aminolevulinic acid.

Figure 12. Treatment planning for PDT. This example is for interstitial treatment of prostate cancer, using multiple cylindrical diffusing fiber sources. The treatment volume is defined by trans-rectal ultrasound, (a) light fluence (rate) distribution for five fibers at specific locations and with equal power to each fiber, (b) corresponding threshold-dose boundary, (c) treatment boundary with the light fluences from each fiber adjusted to reduce the dose to the urethra. (Images courtesy CADMIT Inc, Canada).

The first generation of NP platforms for combined PDT/PTT used two different light sources to excite photosensitizers and photothermal nanomaterials separately, due to their absorption mismatch. 

PDT is gaining support as an alternative for noninva-sive treatment of cancers. PDT is a concept whereby a nontoxic photosensitizer is delivered to an organism and then activated by an appropriate, harmless light source. SWNTs serve as intracellular transporters that can quench fluorescence probe designs as well as protect DNA probes from nuclease digestion. As shown in Figure 6, an aptamer developed by SELEX is covalently bonded with a photosensitizer. The aptamer is able to wrap itself around the... 

---