Discussions of Dosing in Therapeutic Lasers: Moving Away from the “Class Wars”
Vast information is available about various therapy laser devices in the veterinary and human medical fields. It is easy to see how a practitioner trying to decide which laser is the best fit for his or her practice may become skeptical, frustrated, and confused when examining this “complicated” technology.
Along with marketing materials with conflicting claims, the scientific literature on the science itself or practical information on dosing can be less than helpful. Antiquated terminology, such as cold laser or low level laser therapy is often used, and low quality, poorly controlled studies and non-peer-reviewed literature can often be inaccurate or misleading. Background information about light/tissue interaction and physics is important, as is understanding that so-called “hot lasers” (as opposed to cold lasers) are intended for surgical application not non-invasive therapy.
To try to clarify the situation, the term photobiomodulation therapy (PBMT) was coined as an indexing term in 2014, and is defined as “the therapeutic use of light absorbed by chromophores found in the body, to trigger non-thermal, non-harmful biological reactions that result in beneficial outcomes.1-3”
Unfortunately, the above “original” terminology has now also been confused with the Code of Federal Regulations (CFR) classification of types of lasers: Class I-IV, which is itself obsolete and being revised. The CFR classification has nothing to do with the mechanism of effect on tissues. For example, any objective reviewer would agree that, all other parameters being equal, a therapeutic device delivering 510 mW (Class IV) will not be less safe or more effective than a device delivering 490 mW (Class IIIb). While the FDA classification system is based on safety, it applies to ALL lasers, from those used in bar code scanners to industrial or fiber-optic systems and medical lasers. Therefore, it should be well understood that these classifications do not relate to any laser device’s potential for effectiveness in a therapeutic sense.
Calculating Laser Doses
A meaningful discussion of what doses of light are appropriate to safely and effectively utilize PBMT in veterinary practice must therefore separate itself from the Class IIIb vs Class IV laser system. It must also always include mention of therapeutic indication (type and location of tissue[s] being treated), whether the dosimetry was derived through preclinical in-vitro testing or clinical in-vivo studies, and the parameters of the device. The device parameters should minimally include:
wavelength(s) (nm)
temporal format (i. e. pulsed or continuous wave and, if pulsed, pulsing frequency and duty cycle)
irradiance (mW/cm2)
treatment (exposure) time
surface area over which the light’s energy is being applied (cm2)
relative depth of tissue (ie, nerve cells in a petri dish vs a superficial wound in vivo vs a musculoskeletal disorder in a well-muscled patient)
characteristics of in vivo tissues (heavily pigmented vs light skinned)
application technique (with the treatment head in contact and blanching superficial tissues vs off contact).
Without clarifying these factors, one might as well try to determine whether to use a 250- or 500-mg antibiotic tablet without knowing the size of the patient, the condition being treated, the route of administration, or the number (or portion) of tablets being used and how frequently.
Similar to calculating the necessary dosage of a prescribed medication, the recommended fluence, or energy density, measured in joules (J)/cm2 can be determined for a given condition with the information above. In theory, the dose can be administered in a nearly infinite number of ways by adjusting the treatment time and/or the irradiance (mW/cm2)—or, if the laser device beam dimensions are fixed, the power (W) of the device. For example, in treating an area that is 100 cm2 with a target fluence of 10 J/cm2, the total energy that should be applied to that area would be 1,000 joules. Given that 1 watt (W) will deliver 1 joule/second, treating at a power of 10 W, a laser would deliver that dose in 1 minute, 40 seconds. Treating at a power of 0.5 W (or 500 mW), the laser would require 33 minutes, 20 seconds to deliver the same dose.
Threshold of Light Intensity
There is, however, a certain threshold (minimum value) of light intensity, or irradiance (mW/cm2) at the target tissue that must be met to appreciate a therapeutic result. Below this threshold, no matter how long the treatment time, no clinical effect will be seen. Thus, for cells in a petri dish, or those in vivo in superficial tissues, which can be directly exposed to laser light without significant loss of light intensity, consistent therapeutic results may be appreciated at very low irradiances.
However, the assumption that one may simply increase time of administration, without also increasing the intensity (irradiance) of the light, to appreciate consistent results with deep tissue (musculoskeletal) conditions, is not necessarily true. Proper dosing of musculoskeletal cells in vivo requires understanding of irradiance losses as light travels from the outer epidermis, where applied, through the epidermis, dermis, fat, muscle, and other tissues to the “target” tissues. A given amount of light, depending upon the parameters described in the paragraphs above, will be reflected, scattered, or absorbed by the various tissue layers, and a small percentage of the original light’s intensity will reach deep target tissue.
For any given tissue, the rate of irradiance loss with tissue depth depends on both wavelength and tissue type; however, the irradiance at any depth is always proportional to the irradiance applied at the skin surface. Low intensity light applied at the skin surface may not be able to achieve a therapeutic dose at target tissue depth. For this reason, practitioners utilizing lower-power devices may appreciate a clinical benefit treating wounds or superficial musculoskeletal conditions (eg, tendon injuries that are not beneath layers of muscle and fat), but will see less consistent or no results treating deeper orthopedic conditions.
Low Power vs Higher Power
Manufacturers of low-power lasers will often claim that higher-power devices are dangerous. As with any medical equipment designed to be used with proper training and correct technique, the potential always exists, even with low-power devices, for misuse and adverse events. However, with proper device design, hands-on training, and education as well as proper use of clinical resources, this risk is minimal, and significantly outweighed by the potential therapeutic benefits.
It is important to recognize that the technique for application of laser light with lower-power devices has always been different than that for higher-power devices. The two should not be used interchangeably. For devices with lower irradiances point-to-point treatment is usually used, with multiple discrete points within the target area being treated to address the appropriate target tissues beneath the surface. For large areas, or when treating multiple sites on the same patient, this may become challenging from a time perspective, particularly when trying to achieve a target dose (joules) that is quite large.
While the ability to treat multiple areas efficiently is certainly a capability of higher-irradiance devices, more important is their ability to meet the required threshold of irradiance at the target tissue needed to appreciate a therapeutic result. For higher-irradiance devices a continuously moving, scanning technique should always be used. This technique, coupled with the fact that these lasers generally provide the option of a larger spot size (depending on the treatment head used), mitigates some of the thermal effects of increased irradiance. Manufacturer recommendations on treatment head selection and scanning speed should always be followed.
Irradiance, Light Penetration, & Effectiveness
More and more published studies have examined the relationship of irradiance and light penetration to effectiveness in treating deep tissue conditions ranging from peripheral nerve injury to traumatic brain injury and even ischemic skeletal and cardiac muscle. Such studies have elucidated the importance of optimization of irradiance for different wavelengths and the critical parameter of actual dose delivered to target tissues versus that applied at the skin surface. To make examination of the literature as relevant as possible, practitioners must understand the parameters reported and move away from an over-simplified and largely irrelevant classification of IIIb or IV and differentiate between dosages being used in laboratory studies vs those necessary in clinical practice.
Accordingly, the versatility of a laser device should be more critically examined when choosing a device than its FDA class: a Class IV device can always be used to deliver Class IIIb parameters. More relevant questions to answer might include:
Does the device allow for adjustments of power within a range of irradiances that would effectively treat both superficial and deep tissue conditions in both small and large animals?
Does the device allow selection of various treatment heads for optimizing delivery of dose to both superficial and deep tissue conditions by treating in contact with the skin surface or in an off-contact manner?
Does the device allow for adjustment based on tissue type (pigmentation and relative depth)?
Does the device software show all the dosage parameters for a given protocol or treatment such that the dose is repeatable and may be recorded appropriately in the medical record?
Does the device come with both hands-on training for veterinary staff as well as additional ongoing educational resources?
Does the device manufacturer support ongoing veterinary research?
Does the device manufacturer offer clinical consultation and support?
What type of device testing has been done to verify the stated output parameters?