Percentage depth dose curve
In radiotherapy, a percentage depth dose curve (PDD) (sometimes percent depth dose curve) relates the absorbed dose 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 dmax, 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, since water is very similar to human tissue with regards to radiation scattering and absorption.
-  Radiation Therapy Physics, Hendee W., Ibbott G. and Hendee E. (2005) Wiley-Liss Publ., ISBN 0-471-39493-9