Internal dosimetry of laser therapy is far too often overlooked or guesstimated but is crucial information for the design of treatment protocols and prediction of biological efficacy. In vitro studies give a general idea of the biostimulatory dose range, but their results should not be directly extrapolated to form conclusions in vivo.
It is universally understood qualitatively that the body is a turbid medium, which attenuates radiation penetration through a combination of absorption and scattering. For laser therapy to be incorporated into mainstream medicine we must predict quantitatively the transmission losses and beam shape augmentation at depths through the variety of tissue types involved.
“Some” or “enough” are not good answers to the question, “If you laser a dog’s hip, how much gets to the center of the joint?”
One study’s methodology comes from radiation oncology, not only in radiobiology, but also in understanding how radiation is absorbed and scattered within living tissue. This is the first detailed investigation of laser therapy’s internal dosimetry, that is, a modeling and measurement of what the beam looks like more than a few millimeters inside the body. We utilize three techniques of approximation and measurement: water phantom measurements, Monte Carlo simulation, and ex vivo detection.
The mammalian body is like a tub of water. Cells are 80 percent water, so the first step was to measure the transmission of radiation through different depths of water. A detector is placed face up and the laser is at a fixed distance above it, pointing down, and then increasing amounts of water are added between them. The data is illustrated below in Figure 1.
With 100 percent at the surface, the intensity decays exponentially with depth in the water. The beam also flattens as it spreads out at depth. Neglecting the skin and all the other absorbers and scatterers, at a 6-cm depth (just over 2 inches) we are left with just about 30 percent of the beam intensity remaining. This is a good start but is just an approximation, so let’s go one step better and model it.
Figure 2 is an MRI of a canine shoulder from which topography of the different tissue-types is obtained. For Monte Carlo simulations (the gold standard in any field where accurate models are necessary), the input parameters for each tissue type are absorption coefficient, scattering coefficient, anisotropy factor (tells you in which direction the beam is likely to be scattered), and the refractive index. With these data a three-dimensional matrix is created with each voxel (3-D pixel) containing all four of those values for the corresponding tissue type in that volume.
The simulation initiates 1 billion photons that are tracked until their full absorption. Each originates at the red arrow moving in that direction, and the dose deposition is tracked. Observe how the beam spreads out radially and decays with depth in the patient.
Next the simulation data is overlaid back onto the MRI to see exactly where the dose is deposited. If you as the veterinarian know where the pain originated, you can see how much of what is exposed to the surface was actually delivered to the target area. This is a very accurate method of dosimetry and is used daily in every radiation oncology clinic. In our industry as well, this technique can be used as both a predictive estimation and a retrospective tool to analyze past treatments.
Modeling is quite precise, but as always the most accurate data comes from measurement. Wendy Baltzer, DVM, a board-certified surgeon at Oregon State University, secured six fresh canine cadavers with a variety of breed, coat color, coat length, skin color and obesity represented. Utilizing a series of silicon detectors sensitive to microwatts of power, she measured the penetration through several tissue types at several depths.
First the skin was resected, a detector placed inside, and the skin was folded back over (Figure 3). Laser therapy was delivered to the surface, and the amount reaching the detector was measured. Next, the detector was placed beneath a layer of fat, then some muscle, then the front side of the bone, then in the middle of the joints, then the far side of the joint, and so on.
With these measurements (over 2,000 data points recorded) on several different positions on the cadavers, the delivered dose and optimal handpiece positioning were mapped out for all anatomical positions.
As expected, the first-order experiments underestimated the beam attenuation, but Monte Carlo results served as an accurate prediction of ex vivo observation. Doses delivered at therapeutic depths are up to two and three orders of magnitude less than those delivered to the surface. With enough data on different species using a variety of skin, tissue and bone thicknesses, this type of analysis will yield a full dosimetric profile.
Much more work remains to be done in quantitative internal dosimetry of laser therapy. This study, however, is a necessary step on the path of extrapolating the amazing cellular mechanisms to useful clinical insight.
Once further enlightened, we will be able to review both existing and future studies to better understand the biological effect of the delivered dose that came from the reported treatment prescriptions, and eventually converge on the optimal treatment parameters for clinical success.
Needless to say, words like “some” and “enough” are slowly disappearing from our vocabulary. Unfortunately, not everyone is paying close enough attention, but in truth, any laser company that attempts to build protocols without due attention to penetration analysis is simply guessing.
Bryan J. Stephens, Ph.D., is the director of research and development for K-Laser USA. For a more complete bibliography of laser therapy studies, contact the author at firstname.lastname@example.org.
This Education Series article was underwritten by K-Laser USA of Franklin, Tenn.