Radiotherapy Hand Calculations: Percent Depth Dose (PDD)

[@MalayaC-tn2rl]

Nice and concise, thank you. Good pdd explanation, I often see this graphed incorrectly (basically the opposite for higher SSDs)

[@chuanwu6118]

Excellent demonstration!~~ Thank you very much. I am a first-year MP graduate student!

[@aristotle29]

This doesn't make sense as to why the PDD increases as SSD increases. Just pointing at a chart and saying "this chart says so" doesn't help. If the inverse square law states that an increase in distance decreases dose, then an increase in distance should also decrease PDD. This doesn't make sense that the law is only a law in some instances.

[@medphyCo60]

The PDD is defined as dose at depth d / dose at dmax. If you look at what the inverse square law does to both points (d and dmax), it should be clear that the PDD will be affected much more for small SSDs than it will for large SSDs. Think about the extreme case of a brachytherapy source, in those cases the inverse square is a very large effect because you are so close to the source. If you measured the PDD from a brachytherapy source, it would fall off very quickly due almost entirely to inverse square.

[@medphyCo60]

You can actually calculate the contribution of the inverse square law to the PDD as being: (SSD + dmax)^2/(SSD + d)^2. Notice that for very small SSDs, this factor will be very significant. However as SSD gets larger, this factor approaches one and the inverse square component becomes much less important.