4. Photodynamic Therapy: How to Choose a Radiating Source?
Presently, malignant diseases are among first for their death rate, with photodynamic therapy being among first for its therapeutic efficiency. Today’s photodynamic therapy makes rapid advances. It consistently tackles the problem of irradiating an introduced photosensitizer. Tumor irradiation is the major photodynamic procedure, which can be performed in a number of ways:
1. Sunlight exposure;
2. Color-filter lamp exposure;
3. Light-emitting diode array exposure;
4. Nonsemiconductor laser exposure;
5. Semiconductor-based laser exposure.
Let us consider each of the ways. We shall start by comparing the cost of the radiating sources, which are typical of Russia. We shall also consider the most typical case where a photosensitizer absorption wavelength corresponds to 662 nm under the radiation power of 3 W per tumor.
It is evident that although solar radiation is free, it cannot ensure a requisite exposure. The solar radiation efficiency is very poor: 98 % of the radiation does not cause a requisite photoeffect. As a result, the treatment will produce an overload on the patient, which might cause burns. It is therefore clear that expensive optics is needed to provide a power of 3 W per tumor at a wavelength of 662 nm. However, this is not the 21st century’s technology.
Color-filter lamps, which cost US$ 1,000 to 2,000, are rarely employed in photodynamic therapy. As far back as 1998, Professor Oleg Skobelkin showed that such lamps were not effective. The point is that extremely sophisticated optics is needed to ensure a requisite power and spectrum (the spectrum width should not be more than ±10 nm). Furthermore, devices based on color-filter lamps show poor efficiency. They also require water cooling, which makes the entire system inconvenient for the physician. In this case, the physician needs to position the lamp and then to adjust the patient’s location such that the radiation would be incident on the tumor. This therapeutic procedure relies on the use of expensive special-purpose beds or armchairs.
Today’s photodynamic therapy employs devices based on light-emitting diodes. Such devices cost around US$ 5,000. We do not apply them for a few reasons:
1. The devices produce an extremely low power density. They are virtually unable to generate a radiation power of 3 W per tumor. Furthermore, they are almost unable to create a power density more than 300 mW/cm2.
2. The devices employ large numbers of light-emitting diodes, which is associated with a poor efficiency of light-emitting diodes. They have plenty of soldered contacts, which makes the devices very unreliable.
3. The devices cannot be positioned in a thermostat. As a result, the light-emitting diodes function at room temperature, which can show considerable variations.
4. Light-emitting diodes emit wide-spectrum radiation. So, the therapeutic efficiency of light-emitting diodes is much poorer than that of lasers. In order that light-emitting diodes and lasers would produce the same therapeutic effect, devices based on light-emitting diodes should generate a much higher power (as compared to those based on lasers). This leads to two side effects during photodynamic therapy: first, the patient’s body is heated and, second, the patient receives a useless extra light dose.
5. None of optical systems can make the light distribution of light-emitting diodes be as uniform as that of lasers.
6. As far back as 1998, Professor Oleg Skobelkin demonstrated that 90 % of photodynamic therapy operations could be performed using a hair-like fiber inserted into a cavity, special attachment, or needle. In other words, all the devices described will be useless in 90 % of photodynamic operations.
We do not make use of nonsemiconductor lasers in photodynamic therapy either. The reasons are as follows:
1. The cost of nonsemiconductor lasers is much higher than that of the semiconductor ones. In our case, their cost ranges from US$ 30,000 to 200,000.
2. As described above, there is no sense in using lasers without light-guiding fibers. Nonsemiconductor lasers are attributed to nonportable devices because they have very delicate and sensitive optics. The point is that the lasing media of semiconductor lasers is about 100 µm. As a result, all their radiation can be coupled into a light-guiding fiber of a diameter of 150 µm. The lasing media of other types of lasers are much larger. So, in order to couple their radiation into a light-guiding fiber of a diameter of 150 µm, one needs to use optical systems, which can be easily deranged during movements.
3. An overwhelming majority of nonsemiconductor lasers shows a poorer reliability as compared to semiconductor lasers. Note, the reliability of semiconductor lasers is rapidly growing.
4. Currently, industrial semiconductor lasers reveal the highest efficiency among optical sources. Moreover, the effort of many scientists headed by Nobel Prize Winner Jores Alferov allows the efficiency to tend to its theoretical limit of 100 %.
5. Semiconductor lasers alone can operate at any wavelength of photosensitizer sensitivity.
It can thus be concluded that semiconductor lasers are the most effective radiating sources for photodynamic therapy. This type of lasers is currently defying competition with other radiating sources (which can be also used in photodynamic therapy).