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54 Veterinary Laser Therapy in Small Animal Practice

Table 6.2 Solar irradiance data from National Renewable Energy Laboratory. [195]
Wavelength range Irradiance (power density) Hourly dosage
2
2
(mW/cm ) (J/cm /h)
Total (280–4000 nm) 100 360
UV (280–400 nm) 6.4 23
Visible (400–700 nm) 46 160
Infrared (700–4000 nm) 48 170
635 ± 20 nm 5.8 21
800 ± 20 nm* 4.3 16
“Therapeutic window” 33 120
(700–1100 nm)
Note: * Very few people would argue convincingly that 780 or 820 nm radiation has a significantly different therapeutic effect from 800 nm, hence
the bandwidth listed here.

experimentation, where there is (intentionally) a lack of cell death) in a way that is characteristic of cells that
thermal diffusivity to maintain cell viability. The body, absorb ionizing radiation (even though they didn’t).
on the other hand, is very well suited to deal with both Accordingly, we have to narrow the scope of individ-
internal and external heat or cooling sources. After ual cell and single cell monolayer studies to the search
all, we live in an environment that ranges from much for absorption sites and the cellular functions affected
cooler to marginally warmer than our internal tem- by these sites, and stay away from making broader tis-
perature; we also have the ability to drink hot coffee or sue-scale generalizations.
hold an ice cube in our mouths without experiencing I think by now you’ll have had enough of “if you
hyper- or hypothermia. To properly simulate this effect point light into the tissue,” and you’re ready for the
in vitro, some microfluidics designed to measure real- more hands-on chapters that discuss what happens
time temperature and simultaneously carry away heat clinically when you do.
would have to be employed.
In any case, it is clear that while in vitro experi- Summary from a
mentation is absolutely necessary to isolate individual different perspective
chromophore absorption characteristics and cellular
mechanisms of action, the Petri dish environment is
quite different from our bodies. This idea resonates Keep in mind the type of absorption we talk about
throughout the entire biological community: the reac- in laser therapy (Fig. 6.4), but also how much this
tion of a macroscopic matrix of cells that form tissue is is influenced by wavelength, and the type and
NOT the sum of the reactions of each of the individ- color of the tissue. Remember to keep the beam
ual cells. One of the great mysteries of biology involves perpendicular! (See Fig. 6.5.) Pulses have some
the complexity of cell–cell signaling and the ubiquity advantages, but don’t just focus on their peak
of bystander effects. A prime example of this intrin- power. Average power is always what defines
sic communication is in radiation oncology, where how much energy a laser will deliver in a given
researchers have used X-ray needles (microscopically time. We need enough power density to have a
narrow beams of X-rays) to irradiate individual cells therapeutic impact, but not so much as to cause
growing in a monolayer. Amazingly, cells far away from any thermal damage to cells or make the patient
the irradiated region received information from the feel any discomfort. Laser therapy should always
irradiated cells and underwent apoptosis (programmed feel pleasant to neutral for the patient.

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