It is important to understand the order of magnitudes involved when it comes to applications in radiation. Far too often, people make overdosage generalizations without at least a relative scale. A prime example of such hand-waving arguments is the ubiquitous citation of the Arndt-Schulz law, which refers to “U” shaped dose response curves for external agents: below a threshold there is no effect, a small amount of something has small effect, a moderate amount has a large effect, and a large amount has either no effect or an adverse effect.
This “law,” which was originally formulated in the world of pharmacology, has come in and out of favor several times, and now serves as one of the foundations of homeopathy. There is no doubt that there are issues relevant to laser therapy in which this idea applies; the creation of reactive oxygen species (ROS) or free-radicals is an obvious example.
Radiation oncology takes special advantage of free radicals as they are potent DNA breakers; in fact, the hydroxyl radical that comes as a by-product of ionized water accounts for about two-thirds of all radiation-induced mammalian DNA damage**.
In lower levels, however, ROSs serve as cell-signal carriers as well as to induce an endogenous response that leads to an increased long-term defense capacity against exogenous radicals and other foreign toxins.
But, it is crucial to remember that this is not a “law” at all, nor is it based on fundamental principles or cellular processes, and so to claim that more than X amount of radiation is inhibitory because the Arndt-Schultz law says so is completely unfounded.
Virtually all the empirical investigations that attempt to narrow the optimal treatment parameters have been performed in vitro. These studies have the advantages that the majority of the parameters can be easily measured and well controlled, and many of the results of these experiments have indeed shown an optimal dose region for biostimulation above which inhibition takes place.
There are, however, inherent limitations in extrapolating these results to conclusions on the effects in bulk tissue, as well as some fundamental shortcomings in the breadth of their investigations.
The first is simply the range of doses used and the a priori assumption that there is only one peak in the biostimulatory spectrum. Tiina Karu, among others, has shown this to be an invalid assumption, and that for a given cell line, there may be several peaks of similar biostimulatory effect separated by several orders of magnitude of doses**. So the “U” shaped dose response curve cited by a particular study may illustrate only one of the several potential peaks in a curve whose full range has not been measured.
The second major shortcoming of extrapolating in vitro results to in vivo conclusions is the idea that the reciprocity rule (i.e., the idea that the biological effect of treatment is directly proportional to the dose irrespective of the administration technique or treatment time) is simply not valid, in general.
There have been several studies, even on the same cell line with the same laser, that show that in different dose regions of the same response curve, the reciprocity rule is obeyed and then broken**. This speaks again to the idea that studies that claim this rule is strictly obeyed probably have not investigated the full dose domain.
When studies do attempt to explore the higher dose range, a third limitation presents itself: thermal accumulation.
Whatever energy of radiation is absorbed in the monolayer of cells and the serum environment is converted to heat, and in a petri dish thermal diffusivity is extremely low. Dose is defined as energy density and so the higher the dose, the more energy is absorbed, and thus the higher thermal accumulation.
To get a real idea of what contribution this has to the cellular environment, imagine we are testing the viability of 100 J/cm^2 of 980 nm radiation on a monolayer of cells in a petri dish. If you were to calculate how much heat resulted from the absorption of that much radiation exposure, you would find that at a minimum, the serum of the petri dish would rise to above 46 degrees C.
It is well known that bulk tissue can undergo irreversible tissue damage above 40 degrees C, never mind in a monolayer of cells with only two degrees of freedom to dissipate heat. In fact, this thermal accumulation is often taken advantage of, and clinical hyperthermia is an increasingly popular technique in oncology.
In hindsight then, testing any more than about 30 J/cm^2 of 980 nm beam of radiation on cell cultures will never yield positive results (without some micro-fluidic design to remove the heat).
Remember, this effect is simply an artifact of in vitro experimentation, where there is (intentionally) a lack of thermal diffusivity to maintain cell viability. The body, on the other hand, is very well suited to deal with both internal and external heat or cooling sources. After all, we live in environment that ranges from much cooler to marginally warmer than our internal temperature; we also have the ability to drink hot coffee or hold an ice cube in our mouths without experiencing hyper- or hypothermia.
This ability to handle heat is also very fortunate for another reason.
If you were to add up all of the sun’s radiation that falls into the near infrared (NIR: from 700-1000 nm), you would find a constant power density of 33 mW/cm^2 of NIR light. Add this up for 3,600 seconds and realize that the sun delivers 120 J/cm^2 to every square centimeter of our body in every hour of sunshine. This should give some peace of mind as well as some insight next time you hear about some laser overdosage claims.
We Are Not Petri Dishes
In any case, it is clear that while in vitro experimentation is highly necessary to isolate individual chromophore absorption characteristics and cellular mechanisms of action, the petri dish environment is quite different from our bodies.
This idea resonates throughout the entire biological community: the reaction of a macroscopic matrix of cells that form tissue is NOT the sum of the reactions of each of the individual cells.
One of the great mysteries of biology involves the complexity of cell-cell signaling and the ubiquity of bystander effects. Accordingly, we have to narrow the scope of individual cell and single cell monolayer studies to the search for absorption sites and the cellular functions affected by these sites, and stay away from making broader tissue-scale generalizations.
Bryan J. Stephens, Ph.D., is the director of research and development for K-Laser USA. He is an expert in radiation’s interaction with biological matter, specifically in radiation dosimetry and photobiology.
**For individual citations referenced here or a more complete bibliography of laser therapy studies, contact the author directly at firstname.lastname@example.org.
This Education Series article was underwritten by K-Laser USA of Franklin, Tenn.