A radiographer argues: "Since everywhere in a photon beam
PHYSICS · Multi-part · Fermi University Physics · Biophysics & Medical Physics · Radiation physics and diagnostic imaging · Nuclear medicine, dosimetry and radiotherapy
The Nucleus Resolved, Ch Dose: kerma, cavity theory, and what energy does to a cell
Problem
A radiographer argues: "Since D=ΦE(μen/ρ) everywhere in a photon beam, a detector placed exactly on the skin surface of a patient undergoing a 6MV therapy beam reads the same dose as the kerma computed at that point from the incident fluence and the coefficients of the material." For this beam the secondary electrons set in motion by the photons have ranges of up to about 3cm, and the depth-dose curve reaches its maximum, where dose and kerma track each other closely, at about 1.5cm depth.
((a)) Is the radiographer's claim correct at the skin surface? Identify the geometrical condition that is violated there.
((b)) State, with a reason, whether the true surface dose is greater than, equal to, or less than the kerma computed at the surface.