Question

P3.5. All of the ideas you have learned about regarding laser surgery apply equally well to the therapeutic technique called photodynamic therapy (PDT). In PDT, the patient is first fed a lightsensitive chemical (or photosensitizer), which tends to segregate naturally into tumors, but which is poorly retained in normal tissues. Light is then delivered to the tumor through a fiber optic guide. A chemical reaction initiated by absorption of the light by the light-sensitive chemical sets off a chain of cell-killing processes, killing off tumor cells without disturbing even nearby normal cells. A photosensitizer dye called phthalocyanine has been considered for use in photodynamic therapy (PDT). Assume that this photosensitizer can be concentrated in a malignant tumor, and that a physician wishes to irradiate the tumor with laser light that would be selectively absorbed by phthalocyanine, but not well absorbed by blood or melanin. The absorption spectra of hemoglobin, melanin, and phthalocyanine are shown in Figures P3.5 and 3.23 in the text. a. One source gives typical values of power densities used in laser PDT in the range of 80 milliwatts/cm2. What spot size would you use to achieve this power density for a laser with a continuous wave (CW) power of 1 watt? (Assume you can either focus or spread out the laser beam using some unspecified configuration of lenses.)

b. The total fluence (light energy deposited per square centimeter) of tissue for this procedure was in the range 100 to 120 joules/cm2. How long must the exposure time be to deposit this much energy? (Assume you only need to irradiate a region the size of the laser spot size you computed above, and assume that the light is 100% absorbed to get this estimate.) Is this reasonable for a therapeutic procedure?

c. Which of the lasers described in Chapter 3 would be appropriate for treating someone using PDT with phthalocyanine as the light-sensitive chemical? Assume that you want to both maximize absorption by phthalocyanine and minimize absorption by hemoglobin molecules in nearby normal cells. Explain your reasoning.
Lasers in medicine 113

d. Why do you think lasers are preferred to ordinary lamp sources for this technique? Be as complete as possible in answering.

Answer 1

a )

the spot size is " d "

0.08 = 1 / d² / 4

d = ( 4 / x 0.08 )^1/2

d = 3.99 m

b )

t = 100 / 0.08 to 120 / 0.08

t = 1250 to 1500 sec

c )

the Krypton laser is a gas Laser is operated at 7-- nm

and Ruby Laser is operated at 694.3 nm

so the desired wavelength is nearly 700nm

d )

the main differences between ordinary light and laser light is

Ordinary Light                                         Laser Light

1) Polychromatic light                            1) monochromatic light

2) high divergent                                    2) low divergent

3) low intensity                                      3) high intensity

4) low coherence                                   4) high coherent

5) less industry applications                   5) high industry applications,

because of the above reasons maximum are preferring Laser light instead of Ordinary light.

so same way used this in the above photosensitizer.