Calculating Laser Dosage at Target Tissues

Many differing opinions are discussed about high power, low power, cold laser and hot laser. What really counts is not the labels, but rather dosage.

Many claims are made that a certain laser is superior to others. These claims include that multiple hand pieces allow the practitioner to treat multiple areas and that very large treatment heads treat larger areas more quickly. What matters is how much energy reaches the target tissue.

Defining a few terms will let us examine the claims with a common language.

Watt (W): The unit of power; equivalent to one joule of energy per second.

Joules (J): A unit of energy describing the work required to produce one watt of power for one second (one watt-second).

Joule per cm² (J/cm²): A unit of radiant exposure used in measuring the amount of energy per unit area of absorbing surface. Also called the dose.

Peak Power: The maximum pulse power output of a laser and it is fixed by the laser manufacturer, measured in watts (W) or milliwatts (mW).

Average Power: Refers to the average power per second that a laser emits during the treatment, measured in watts (W) or milliwatts (mW).

Power Density: The intensity of the laser beam; average power divided by the size of beam (“spot size”); relates to the dose.

Dose equals Energy Density.

Watt per cm² (W/cm²): The unit of irradiance used in measuring the amount of power per area of absorbing surface. Also called power density.

Energy Density: The amount of joules emitted to the tissue per cm2 listed in J/cm2. This is often listed over a period of time such as one second or 60 seconds.

What Is A Sufficient Dose?

Dose equals Energy Density. Examined another way it is Power Density multiplied by time expressed in seconds. Keys to the equation are the Average Power, the diameter of the handpiece aperture and time. Common aperture diameter sizes are 8mm, 15mm, 25mm and 50mm. Remember that the radius “r” is one half the diameter. Applying the formula Dose = (Power {in watts} X time {in seconds} )/ n r2 {area in cm2} results in the following chart for each laser firing for one minute (60 seconds).

Aperture Size

Average Power (watts)	8mm	15mm	25mm	50mm
5 mW (.005 watt)	.6	.17	.06	.02
25mW (.025 watt)	2.98	.85	0.31	.08
100mW (.100 watt)	11.94	3.40	1.22	.31
500mW (.500 watt)	59.68	16.98	6.11	1.53
1,000mW (1 watt)	119.37	33.95	12.22	3.06
10,000 mW (10 watts)	1193.66	339.53	122.23	30.56
30,000 mW (30 watts)	3580.99	1018.59	366.69	91.67
60,000 mW (60 watts)	7161.97	2037.18	733.39	183.35

Research by Tuner, Hode and Karu have established recommended Average Dose Per Tissue Type AT THE TARGET TISSUE. The World Association for Laser Therapy (WALT) has also provided guidelines as well. These recommendations range from 2 J/cm2 to 10 J/cm2 with the most common ranges from 4 J/cm2 to 8 J/cm2. In the authors experience the Average Dose At The Target Tissue shown below are reasonable guidelines.

Average Dose At The Target Tissue

Body Tissue	10 J/cm2
Tendon	10 J/cm2
Ligament	10 J/cm2
Fascia	10 J/cm2
Bone	7 J/cm2
Muscles	5 J/cm2
Nervous Tissue	5 J/cm2
Skin	5 J/cm2
Bursa	5 J/cm2

The key is that laser energy loses roughly 50% of its power for each centimeter that it enters the body. There needs to be sufficient power at the skin to accommodate the reduction in power as the photons flow into the body. Expanding the prior chart to account for depth of penetration shows the amount of dose necessary at the skin. Specific conditions are shown to illustrate the types of tissues. The highlighted boxes show the typical depth to reach the target tissue and the necessary average dose at the skin to reach the pathology.

Beware of stated average power for lasers when more than one diode or multiple LEDS are involved. Verily the dose that will be delivered.

Example 1: Consider a 100-milliwatt diode firing at the maximum power so that the Peak Power and Average Power are the same. Therefore, the Average Power is 100 milliwatts. If three 100-milliwatt diodes were place in the same handpiece but spread out to cover a larger area, the Average Power is still 100 milliwatts, not 300 milliwatts. Why? The Peak Power could be listed as 100 mW times 3 diodes which equals 300 milliwatts of Peak Power. But, for the area where each diode enters the body there is only 100 milliwatts of Average Power. Therefore, the dose would be same with both lasers.

Example 2: Consider another laser where the handpiece Peak Power is 100 milliwatts. There is a 25 milliwatt laser diode and the 75 milliwatts is composed of multiple LEDs. The Average Power will not be 100 milliwatts at all the target tissues. Why? Because the LEDs will only penetrate about 5mm. In addition, they are spread out and not concentrated in one area.

Example 3: Consider another laser that has multiple diodes that can fire independently through the same aperture and at the same time. The total of each diode’s Average Power can be added together when calculating the dose.

The Takeaways

1. Look beyond Peak Power to Average Power when evaluating lasers.
2. Calculate the dosage of a laser to determine what tissues it will successfully treat.
3. LEDs are not laser diodes and light therapy is not laser therapy.

References

1. Huang YY, Sharma SK, Carroll J, Hamblin MR. “Biphasic dose response in low level light therapy – an update. Dose Response. 2011 ;9(4): 602-18.
2. Hode,L, Tuner J. (2014) Laser Phototherapy Clinical Practice and Scientific Background. Sweden: Prima Books
3. Karu, T. (2007) Ten Lectures on Basic Science of Laser Phototherapy. Sweden: Prima Books.

Rob Berman is a partner at Berman Partners, LLC, a medical device sales, service, and marketing company. Berman Partners specializes in new and preowned therapeutic lasers. He helps doctors improve patient outcomes while increasing physician income.

Rob can be contacted by phone at 860-707-4220 or by e-mail at [email protected].

Dr. Michael Mathesie has practiced in Coral Springs, Florida for 28 years. He has taught and utilized laser therapy in his practice since 2005. He is board certified in physical rehabilitation and forensic science. He can be reached at 954-755-1434.