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FAQ - Dosage and Dosimetry
QUESTION: What additional factors influence the calculation of dosage timing, and how?
One of the most important factors is the depth of the tissue that you want to treat within the body. Jan Tunér and Lars Hode have proposed a very useful working solution to this, which is represented in the equation below.
I have revised the terms used to describe each parameter of the equation to more closely reflect the intention of calculating the total irradiation time required to deliver the dose 'D' at the Target site in the tissue:
Irradiation Time [secs] = ((D x A) / P) x (1 + d)
Where:
- D = Desired Dose at Target Tissue [Joules/cm2]
- A = Area of Target Tissue [sq cm]
- P = Power of Incident Beam (Watts)
- d = Depth of Target Tissue (cm)
Note: the parameter 'd' is limited to a range of 0-4cm, with values 1-4 only applicable to the deeper-penetrating wavelengths (approx. 760-860nm GaAlAs and super-pulsed 904nm GaAs).
Ref: Tunér & Hode 'The Laser Therapy Handbook' pp 75-76
There are no hard-and-fast rules regarding other factors such as skin color, tissue type, hair cover (or fur), and so on, other than to understand that, for example, oxygenated blood more readily absorbs near-infrared wavelengths, and adipose tissue is more transparent to light than muscle.
Also, for tissue repair, you must take into account the stage of the healing process (inflammatory response, fibroblastic repair, or maturation remodelling) and, therefore, the desired clinical outcome of each particular treatment session during a course of treatment for a particular injury.
You may, for example, wish to apply an inhibitory dose for managing pain and inflammation at the very acute stage of an injury, and then to reduce the dosage and the frequeny of treatment to promote healing and effect beneficial remodelling of scar tissue during the later stages. (NB. 'frequency' in this instance relates to the number of treatment sessions over a given time period, not to the pulsing or modulation of the laser beam).
In relation to skin color, I developed the following rule of thumb some years ago, based upon the Fitzpatrick Skin Type (phototype) scale, and it seems to work well.
Light penetration of darker-pigmented skin is reduced due to the absorption of light by melanin. Absorbed light energy is converted into heat. The amount of heat so generated, and therefore its potential to cause discomfort or injury, is greater when the power/power density is sufficiently high that the rate of energy delivery and absorption exceeds the tissue's capacity to dissipate heat.
So, to both reduce the possibility of thermal injury/discomfort when using higher-powered devices, and to ensure the desired dose is delivered to the target tissue, I recommend:
- a) decreasing the power density at the skin by treating in non-contact mode (if the beam is divergent), or reducing the source power (if collimated); and,
- b) increasing the irradiation duration by adding a nominal amount (0.1-0.8cm) to 'd' in T&H's equation.
As a guide, for wavelengths between 760nm and 800nm, I suggest adding:
- 0.2 for Skin Type 3 (darker white skin)
- 0.4 for Type 4 (light brown skin)
- 0.6 for Type 5 (brown skin)
- 0.8 for Type 6 (dark-brown to black skin)
For wavelengths longer than 800nm, add:
- 0.1 for Skin Type 3 (darker white skin)
- 0.2 for Type 4 (light brown skin)
- 0.4 for Type 5 (brown skin)
- 0.6 for Type 6 (dark-brown to black skin)
So, for example, when treating a patient with Type 4 skin with an 810nm laser, one would add 0.2, as follows:
Irradiation Time [secs] = ((D x A) / P) x (1 + d + 0.2)
A similar approach can be used when treating animals to account for the amount and color of hair/fur covering the treatment area, and its effect on dosage delivery to the target tissues.
Of course these suggestions are of a very general nature, and so the skill and experience of the practitioner will play a large role in determining their most effective application.
Note: Recently published research confirms the importance of accounting for skin type in dosimetry.
Melanin Density Affects Photobiomodulation Outcomes in Cell Culture
Philip Brondon, Istvan Stadler, Raymond J. Lanzafame
Photomedicine and Laser Surgery. 2007, 25(3): 144-149.
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