# Understanding percentage dose-depth curves

I just have a little question regarding dose-depth curves. When you look at them, you have on the y-axis the dose, and on the x-axis the depth into your material.

What I'm not sure of is how I interpret it ? Does it just mean, that if I want to give, let's say 2 Gy, to a point in a body or something, I have to look at where there is a 100% dose peak, or else combine more fields in order to add up to total of 100% ?

Basically: I'm not quite sure what the percentage means ?

Example for different particles at different energies:

• Maybe the context makes it obvious to people better qualified than I, but could you link to an example? I think I know what you're asking about but I'd like to see a little better. Apr 6, 2015 at 20:37
• Sure, I've added an image Apr 6, 2015 at 20:59

Percentage Depth Dose Curves (or PDDs) are used to determine how many Monitor Units (MU) a treatment machine needs to give (or how long the machine needs to be on) to deliver a particular dose to a particular depth.

The depth at which the PDD curve peaks is referred to as dmax. Treatment machines are often calibrated so that 1MU = 1 cGy at dmax in water.

Data: 6MV photon beam PDD has 100% at 1.6cm depth and 66% at 10cm depth.

Example 1: A doctor wants Tumor A to receive a dose of 200 cGy, and it is located at a depth of 1.6cm. This means that the machine needs to deliver 200 MU.

Example 2: A doctor wants Tumor B to receive a dose of 200 cGy, and it is located at a depth of 10cm. This means that the machine needs to deliver 200/66% = 303MU

You see the curves normalized to dmax because they are typically used in ways similar to the above examples, and as input data to Treatment Planning Systems. I suppose you are correct about integrating the curves, but I don't believe I have ever seen them used that way.

-DS

As far as I understand it, the curves indicate the fraction of energy released at each point by each type of (ionising I suppose) particle thrown at the material.

It can be interpreted as the percentage of energy lost in the neighbourhood of a point $M$ located at depth $d$ in the material.

If you were to integrate the curve over the whole depth, you would obtain 100% of the energy you put in.

Physically, these curves are related to the scattering cross section of the particles with the medium. This is betrayed for instance by the fact that more energetic photons (X-rays at 20 Mev) can go deeper in the medium than those with smaller energy (X-rays at 4Mev).

• This is actually what I was hoping to get out of it. If the integration gives the total energy of whatever I put in, then I understand it. But often the peak of them all is set to 100%, so I couldn't figure out what good I can take from that. Obviously it has been normalized somehow, I just don't see what, let's say about 50% dose at 7 cm for protons, gives me, that I can use for anything ? Apr 6, 2015 at 21:33
• @DenverDang would you happen to have a figure with the percentages you mention? Apr 7, 2015 at 8:45