Calculating Dosage?

There has been a lot of discussion about the best method to quantify dosage.
However, it is my opinion that:

  1. There is no agreed method of defining beam area
  2. Dosage expressed as J/cm2 is inadequate

No agreed method

Calculating the area of a laser beam should be simple:
But laser beams are rarely round:
And laser beams are rarely of uniform density:
Some diode laser beams appear
very distorted
Area of a circle = r2
Area of a circle = r1r2
Where is the edge of the beam? What is the beam area?

So beam area is hard to define and there is no agreement in our industry for defining it.
[I propose 1/e2 - will explain this soon].

Dosage expressed as J/cm2 is inadequate

“Dosage” is usually calculated as Power / Beam Area x Time = J/cm2. However, to consider that dosage should equal J/cm2 is, in my opinion, inadequate.

Let me explain:

Assume there is a well-defined beam area and thus a quantifiable dosage.

  • A 500mW laser with a beam area of 0.25Cm2 used for 20 seconds delivers 40 J/cm2
  • A 200mW laser with a beam area of 0.1Cm2 used for 20 seconds delivers 40 J/cm2
  • A 30mW laser with a beam area of 0.015Cm2 used for 20 seconds also 40 J/cm2

Each of these probes apparently apply the same "dosage". However, the total energy delivered is clearly different [10 Joules, 4 Joules and 0.6 Joules respectively].

While dosage appears consistent using J/cm2, I suggest that the clinical results would be quite diverse. So I say that J/cm2 is an inadequate method of expressing dosage.

We are currently working on these issues and will be presenting our conclusions at NAALT in April 2003.
We expect to have have an update for this page in May.

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