In
radiotherapy, 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 term ...
, 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 understo ...
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
See also
*
Dosimetry Radiation dosimetry in the fields of health physics and radiation protection is the measurement, calculation and assessment of the ionizing radiation dose absorbed by an object, usually the human body. This applies both internally, due to ingested o ...
*
Dose profile
References
*
''Radiation Therapy Physics'', Hendee W., Ibbott G. and Hendee E. (2005) Wiley-Liss Publ., {{ISBN, 0-471-39493-9
Radiation therapy
Medical physics