Medical Physics Education

I was teaching introductory physics to pre-med students when a discussion on EM waves turned into an explanation of their real-life applications. The energy of the different components of the EM spectrum increases with frequency according to the relationship

E = hf.

A chart of the spectrum is shown below:

Image result for em spectrum

X-ray is used on me at the denstist office, a student responded. Why should the dentist be using X-rays radiation if it very energetic? Of course, the answer is in the DOSE! The study of the dose of a radiation is the subject of dosimetry. We will introduce the dose of ionizing radiation with brief descriptions of exposure, absrobed dose, relative biological effectiveness (RBE), biological equivalent dose (BED), and biological effects of ionizing radiation.

Exposure is the total charge per unit mass of air. It shows the ionization produced in air ionizing radiation like X-rays. The SI unit is Coulomb per kg, but a unit known as roentgen (R) is still widely used. At applications with standard temperature and pressure (STP),

1R = 0.000258 C/kg.

The absorbed dose, RBE and BED are better suited for explaning the effects of radiation on biological tissues.

Absorbed dose = energy absorbed / mass of the absrobing material. The common units are gray (Gy) and rad. 1 Gy = 1 J/kg, and 1 rad = 0.01 Gy

RBE = (dose of 200 keV X-rays that produces a certaain biological effect) / ( dose of radiation that produces the same biological effect) . Typical RBE values are 1 for 200 keV X-rays, gamma rays, and beta particles (electrons). Protons, alpha particles, and neutrons have higher RBE values. The degree of damage a radiation could cause in biological tissues is a measure of its RBE.

The biological equivalent dose (BED) = absorbed dose in rad x RBE. The unit of BED is the rem (roentgen equivalent man)

From the perspective of radiation safety, 0 – 100 rem of dose in whole body single exposure will have no observable effect but a substantial risk in decreasing the white blood cell counts and temporary sterility. Other effects like nausea, significant decrease in blood counts, vomiting, hair loss, organ malfunctions, collapse of the nervous and gastrointestinal systems and fatality with dose ranging from 100 – greater than 2000 rem.