What is a “dose” - really?

By Jan Tunér

An alarming feature in the laser phototherapy literature is the frequent ignorance about the meaning of the term ”dose”. As a reviewer, I see this misunderstanding only too frequently and in published studies the misunderstanding pops up frequently as well. How is it possible that persons with an academic background can miss one of the most important aspects of laser phototherapy (LPT)?

So, what is a “dose”? In general medicine we tend to think that “One tablet in the morning and one in the evening” or “5 ml four times a day” is a dose. But when we move to LPT, there is a semantic trap. The “dose” is not the applied energy – the number of joules. The applied energy is only the ENERGY and is calculated as power in mW x times in seconds. So 100 mW applied during 10 seconds = 1000 millijoules (mJ) = 1 joule (J). 500 mW applied during 2 seconds is also 1 J. Same energy, but different biological responses.

Now, where is the dose? It is dependent upon the size of the irradiated area. You can call it “local intensity” if you like, but the professional term is dose or fluence or energy density. It is obvious that the intensity in the tissue is quite different if you apply 100 mW over an area of 10 cm2 or concentrate it over an area of 1 mm2. Most of us have experienced this by using a magnifying glass to concentrate the pleasant rays of the sun into a point of heat.

So the calculation is as follows. You apply 1 J over an area of 1 cm2. The calculation reads: 1 divided by 1 = 1. One what? Yes, the DOSE – J/cm2! Now, let us reduce the irradiated field to 0.5 cm2 and again apply 1 J. The calculation reads: 1 divided by 0.5 = 2 J/cm2. And let us suppose that the irradiated area is 0.25 cm2, then the calculation reads: 1 divided by 0.25 = 4 J/cm2. Same energy, different doses!

In order to know the dose then, you must know the size of the irradiated area. If you treat in contact and you know the size of the laser aperture, you can do an easy calculation. But you need to calculate the area of the aperture, not the diameter. So the old a = Do it once, and you are ready. Some manufacturers let the instrument calculate the dose, but that is then only true for contact mode. If you treat from a distance, the divergence of the beam will increase the irradiated area, and if you are using invisible light, the calculation becomes problematic, to say the least. But if you know the divergence, it is possible.

At this point, things are getting complicated. But don’t worry. If you are a clinically oriented person, you only need to know what the energy is and what the dose is. You can even for your records use “energy per point”. This excludes the dose, of course, because your colleague has a different size of his laser aperture and your “points” discussions become awkward. But at least you will be able to log the number of points and the energy applied to each point and by experience find a reasonable and successful protocol. But if you are a scientist, beware! And from here you should continue to read, the clinicians can rest.

Too many papers have only stated J/cm2 without indicating the applied energy. And too many papers have stated J/cm2 and believing this was the energy. And too many have read other papers and found values of J/cm2, believing that this was the energy. And vice versa. This leads to a situation when, for instance, one author applies 4 J per point, believing and reporting that this was 4 J/cm2 as in another paper – where the actual energy was very different. Apples and bananas.

When I do reviewing, I can find co-reviewers with a brilliant knowledge about the condition treated, statistics, the medical literature and so forth – but with no comment at all on the laser parameters. So I suppose that that is the reason for inviting me, at least to get the laser parameter basics right. But too many scientific journals do not have reviewers competent in the LPT field, and too many published papers are poor or even incorrect in this aspect. And this is even reflected in literature reviews and Meta analyses later on. So let us get it right from the beginning.

And by the way: do you have a power meter? If not, you are still out of control. Some manufacturers have an integrated power meter. If not, check for fairly inexpensive ones on the web. Take control!

Recommended reading – free reading on PubMed:

Chung H, Dai T, Sharma SK, Huang YY, Carroll JD, Hamblin MR. The nuts and bolts of low-level laser (light) therapy. Ann Biomed Eng. 2012;40(2):516-533.

And just as good, but not free:

Jenkins PA, Carroll JD. How to report low-level laser therapy (LLLT)/photomedicine dose and beam parameters in clinical and laboratory studies. Photomed Laser Surg. 2011;29(12):785-787.