radiotherapy Radiation therapy or radiotherapy, often abbreviated RT, RTx, or XRT, is a therapy using ionizing radiation, generally provided as part of cancer treatment to control or kill malignant cells and normally delivered by a linear accelerator. Ra ...

, a percentage depth dose curve (PDD) (sometimes ''percent'' depth dose curve) relates the

absorbed dose Absorbed dose is a dose quantity which is the measure of the energy deposited in matter by ionizing radiation per unit mass. Absorbed dose is used in the calculation of dose uptake in living tissue in both radiation protection (reduction of harmf ...

deposited by a radiation beam into a medium as it varies with depth along the axis of the beam. The dose values are divided by the maximum dose, referred to as d_max, yielding a plot in terms of percentage of the maximum dose. Dose measurements are generally made in water or "water equivalent" plastic with an

ionization chamber The ionization chamber is the simplest type of gas-filled radiation detector, and is widely used for the detection and measurement of certain types of ionizing radiation, including X-rays, gamma rays, and beta particles. Conventionally, the ter ...

, since water is very similar to human tissue with regard to radiation scattering and absorption.
Percent depth dose (PDD), which reflects the overall percentage of dose deposited as compared to the depth of maximum dose, depends on the depth of interest, beam energy, field size, and SSD (source to surface distance) as follows. Of note, PDD generally refers to depths greater than the depth of maximum dose * PDD ''decreases'' with increasing depth due to the

inverse square In science, an inverse-square law is any scientific law stating that a specified physical quantity is inversely proportional to the square of the distance from the source of that physical quantity. The fundamental cause for this can be understoo ...

law and due to attenuation of the radiation beam * PDD ''increases'' with increasing radiation field size due to greater primary and scattered photons from the irradiated medium * PDD ''increases'' with increasing SSD because inverse square variations over a fixed distance interval are smaller at large total distance than small total distance

References

* ''Radiation Therapy Physics'', Hendee W., Ibbott G. and Hendee E. (2005) Wiley-Liss Publ., {{ISBN, 0-471-39493-9 Radiation therapy Medical physics

See also

References