Answer to Question #5891 Submitted to "Ask the Experts"

Q

What is the purpose of the SAD (source-axis distance) and the SSD (source-skin distance) in terms of the dose in radiation therapy? In other words, let's say the SSD were 75 cm for a patient, then the next day you set it to 80 cm; what effect would that have on the dose to the patient?

A

Strictly speaking, SAD (source-axis distance) is fixed when the machine is designed. Modern linear accelerator x-ray sources rotate only in one plane, so they rotate about one axis and that axis is usually 100 cm from the source; that is, the SAD = 100 cm. For simplicity (and simplicity makes mistakes less likely), the patient is usually positioned so that the center of the tumor is 100 cm from the source.

SSD is the distance from the x-ray source to the patient's skin (or surface). Since the x-ray source is designed to be effectively a point source and since every centimeter of tissue it passes through attenuates it (that is, the beam gets weaker as it spreads out from a point source and also when it passes through tissue), there are really two quantities that can change the dose (once the size and shape of the beam are set to match the target and any tissues that should be protected): (1) the sum of SSD and depth (or thickness of tissue between the x-ray source and the center of the tumor) and (2) the depth.

The sum of SSD and depth is the distance from the x-ray source to the point in the center of the tumor where the dose is prescribed, so it's called SPD (source-point-distance). Usually, the patient is positioned so that SPD = SAD. Because the x-ray source is a point, the dose is proportional to the inverse square of SPD. If SPD doubles, the dimensions of the beam double too and that means the area quadruples. The same amount of energy is spread over an area four times as big so the dose (which is the energy deposited in each kilogram of tissue) goes down by a factor of four.

The depth is the number of centimeters of tissue the beam has to go through to get to the prescription point. Under most conditions the attenuation (the amount of energy lost) is just proportional to depth.

By the way, the medical physicist at the radiation therapy center has measured all these numbers and checks regularly to make sure they are still accurate.

Hope this is not too technical, but you sound like someone who can handle a little technical stuff.

A good source of information on radiation therapy is the National Cancer Institute Web site.

Dennis M. Duggan, PhD, DABR