Dosimetry and Medical Radiation Physics (DMRP)

Hints and FAQs

This section is intended to provide hints and answers to some Frequently Asked Questions (FAQ) in relation with the activities of the International Atomic Energy Agency (IAEA) in Dosimetry and Medical Radiation Physics (DMRP). For further questions and answers please contact DMRP. We are open to suggestions and comments on these items, and any feedback is most welcome.

1. What is ionizing and non-ionizing radiation?

2. What is absorbed dose?

3. What is radiation dosimetry?

4. Can the body feel ionizing radiation?

5. Do all kinds of ionizing radiation have the same biological effect on human beings?

6. What are Standards for Radiation Measurements?

7. What is Medical Physics?

8. Radiotherapy Physics (Medical Physics) and Dosimetry

For more basic questions on radiation the reader is referred to simple and inexpensive books like "The Good News About Radiation", by John Lenihan (1993), published in the series Cogito Books by Medical Physics Publishing , Madison, Wisconsin (USA).

What is ionizing and non-ionizing radiation?

Radiation is the transport of energy by electromagnetic waves or atomic particles. There are two types of radiation, ionizing and non-ionizing.

Ionizing radiations carry enough energy to break chemical bonds and separate electrons from the parent atoms and molecules, thereby creating ions in the irradiated material. Ionizing radiation consists of directly ionizing and indirectly ionizing radiation. Directly ionizing radiations are charged particles (electrons, positrons, protons, alpha particles, heavy ions) with sufficient energy to ionize or excite atoms or molecules. Indirectly ionizing radiations are uncharged particles (photons, neutrons) that set in motion directly ionizing radiation (charged particles) or that can initiate nuclear transformations.

Radiation with less energy than that required to produce ions in the irradiated material, is called non-ionizing radiation. Ultraviolet radiation (except the high energy end of the UV-spectrum), visible light, infrared radiation, micro waves and radio waves, are all non-ionizing.

What is absorbed dose?

Radiation with less energy than that required to produce ions in the irradiated material, is called non-ionizing radiation. Ultraviolet radiation (except the high energy end of the UV-spectrum), visible light, infrared radiation, micro waves and radio waves, are all non-ionizing.

Absorbed dose is the amount of energy given to the medium per unit mass. It is measured in gray (Gy) defined as Joule/kg. The determination of the dose due to radiation is the task of radiation dosimetry.

What is radiation dosimetry?

Dosimetry has the task of absorbed dose determination and its physical interpretation. Absorbed dose is determined with radiation measurements and calculations.

Different instruments are used for absorbed dose measurements, all based on the detection of some of the physical and chemical changes caused by radiation. For example, ionization chambers, calorimeters or ferrous-sulfate dosimeters measure, respectively, the electrical charge produced by ionization of a gas inside a detector, the amount of heat produced by radiation in a piece of material, or the chemical changes in an aqueous solution of ferrous-sulfate. Most measurements therefore provide an indirect determination of the dose, and calculations are needed to convert the quantity measured into absorbed dose.

Radiation dosimetry is needed in many areas, such as in cancer treatment using radiotherapy, in clinical diagnostic radiology, in environmental radiation protection, and in industrial applications such as food irradiation and sterilization of health care products.

Can the body feel ionizing radiation?

Ionizing radiation cannot be detected by any of our senses, even if it produces the numerous physical and chemical changes that make possible to detect radiation. Many of these effects, for example, produce heat, but not enough to be sensed by the human body. A large absorbed dose of 10 Gy, which would be lethal if delivered in a short time to a human being, produces a temperature rise of only 0.0024 deg C; the total energy imparted to a person of 70 kg would only be 700 J. Compare this to the energy of 100 g of light yogurt or in 100 ml juice, about 200,000 J.

Do all kinds of ionizing radiation have the same biological effect on human beings?

The biological effect of ionizing radiation on a human being depends on the absorbed dose, the radiation quality (i.e. gamma, beta, alpha etc. and their energy), and on the organ(s) irradiated. Besides, all human beings are not equally sensitive to radiation, and additional differences in sensitivity are attributed to age and sex. If we restrict the comparison to the various qualities of ionizing radiation and keep all other parameters constant, the biological effect from a single exposure to alpha particle radiation is about 20 times more than from gamma or beta radiation for the same dose. The biological effect from neutron radiation varies between 2-10 times that from gamma or beta radiation depending on the energy of the neutrons. If the absorbed dose to the different organs in the body and the radiation quality are known, the effective dose equivalent can be calculated. Based on the estimates of the effective dose equivalent, the risk for the late effects such as cancer, can be compared for different exposure conditions and type of radiation.

What are Standards for Radiation Measurements?

In much the same way that the world needs an international agreement for everyone to adopt the same unit of mass (the kilogram), length (the meter), and time (the second), international agreement is also needed for the units used in radiation measurements. These units arise in the form of so-called standards, where "perfect samples" are built to become the most accurate representation of a quantity in the world (i.e. "the international prototype of the kilogram" at the International Bureau of Weights and Measures, BIPM, see the figure). Standards for measurements are established by the BIPM and the few Primary Standards Laboratories existing in the world. For radiation measurements, standards are distributed to scientists and engineers through Primary and Secondary Standards Dosimetry Laboratories (PSDL and SSDL respectively), building the so-called International Measurement System.

The IAEA's Dosimetry Laboratory is a Secondary Standards Laboratory, and its instruments are compared (calibrated) against the primary standards of the BIPM and the PSDLs. Within the Dosimetry and Medical Radiation Physics Subprogramme, the IAEA calibrates instruments for radiation measurements in radiotherapy, diagnostic radiology and radiation protection, which are then distributed to Member States through the IAEA/WHO network of SSDLs.

What is Medical Physics?

Medical Physics is primarily an applied branch of physics. It is concerned with the applications of the concepts and methods of physics to the diagnosis and treatment of human disease. From the different professional areas that have emerged for the medical physicist, the application of ionizing radiation to radiotherapy and medical diagnosis (diagnostic x-rays, nuclear medicine) is perhaps the most common. This is referred to as Medical Radiation Physics or Radiological Physics. The specialty is different from the so-called Health Physics, which mainly consists in the assessment and control of radiation hazards (Radiation Protection). Other fields of Medical Physics are nuclear magnetic resonance (magnetic resonance imaging), bioelectrical investigations of the brain and heart (electroencephalography and electrocardiography), and the medical uses of infrared radiation (thermography), ultrasound (sonography), heat (hyperthermia for cancer treatment) and lasers (for lasers surgery).

(Adapted from "The Medical Physicist", a brochure published by the American Association of Physicists in Medicine, AAPM)

Radiotherapy Physics (Medical Physics) and Dosimetry

It was the 16th century philosopher, Paracelsus, who said it is a matter of dosage as to whether a substance is a poison or a medicine, and nowhere is this more true than in radiotherapy, where the radiation dose is one of the main determinants of a cancer patient’s chance for survival and cure.

In radiotherapy, the radiation dose is prescribed by a radiation oncologist, and a medical physicist determines how much and in which way the dose is distributed inside the patient. Too high a dose damages healthy tissue in a way that makes recovery very difficult and causes complications that can be very severe; too low a dose will not destroy all cancer cells and the tumour will grow again, reproducing itself. The balance between high and low dose is sometimes very critical, and depends on the type of tumour and even on the patient. Generally speaking, if the dose given is in error by more than 5% above or below the correct dose, radiotherapy treatments will be inappropiate.

A very important component in radiotherapy dosimetry is the calibration of the radiation beam, that is, the accurate determination of how much radiation dose is delivered by the treatment machine. If the radiation beam is incorrectly calibrated, the consequences can be fatal, as some dramatic accidents reported in public media have shown. The IAEA has developed Codes of Practice for the calibration of the radiation beams used in radiotherapy; recommendations have also been published by various national organizations of medical physicists. In addition to the beam calibration the medical physicist is responsible for quality control measurements that guarantee the correct and safe operation of medical radiation treatment machines and associated equipment (this is part of the physical aspects of quality assurance in radiotherapy).

In radiotherapy, to ensure that the radiation doses given to patients are as near as possible to those prescribed by radiation oncologists, beams must be calibrated following the procedures (Codes of Practice) that rely on the standards for measurements in radiotherapy set by the BIPM and PSDLs. (The same is true for the fields of diagnostic radiology and radiation protection to determine the dose received by patients and population in general, as well as in radiation processing for the dose delivered to food and health care products for sterilization.) Dosimetry must always be consistent and traceable to international standards for radiation measurements.