Subramania Jayaraman

Clinical radiotherapy physics

1 Scope of Clinical Radiotherapy Physics.- 2 Atoms, Molecules, and Matter.- 3 Propagation of Energy by Electromagnetic Waves.- 4 Nuclear Transitions and Radioactive Decay.- 5 Radioactive Decay Calculations.- 6 Collision and Radiation Loss in Charged-Particle Interactions.- 7 Photon Interactions.- 8 Conventional X-Ray Machines.- 9 Equipment for Radioisotope Teletherapy.- 10 Particle Accelerators.- 11 Quantification of Radiation Field: Radiation Units and Measurements.- 12 Instruments for Radiation Detection.- 13 Basic Ratios and Factors for the Dosimetry of External Beam.- 14 Beam Dosimetry: Additional Corrections - Special Situations.- 15 Treatment Dose Distribution Planning: Photon Beams.- 16 Physical Aspects of Electron Beam Therapy.- 17 Physics of the Use of Small Sealed Sources in Brachytherapy.- 18 Radiation Safety Standards.- 19 Radiation Safety in External-Beam Therapy.- 20 Radiation Safety in Brachytherapy.- Appendix A and B.

Radiation Safety Standards

Ionizing radiation is a beneficial agent that contributes to improved health care. However, unnecessary exposure to ionizing radiation can be harmful [1, 2] and should be avoided. Because penetrating radiations cannot be contained entirely, some irradiation of staff members, visitors, and the public is bound to occur incidental to any use of radiation. All radiation work should be carried out in a preplanned and controlled manner so that the exposure to the workers and persons in and near sites of radiation use is kept as low as reasonably achievable (ALARA) and does not exceed the recommended limits. In the U.S., guidelines to ensure the radiation safety of radiation and non-radiation workers and members of the public are provided by the National Council on Radiation Protection and Measurements (NCRP) [3]. Recommendations are also provided by the International Commission on Radiation Protection (ICRP) [4, 5], the International Atomic Energy Agency [6], and the European Atomic Energy Community [7]. In some geographic regions, governmental regulatory agencies formulate the safety limits for radiation exposure and mandate that radiation users adhere to them. A radiotherapy department should plan and conduct its activities in such a manner that the safety recommendations (or regulations) are followed.

An overview of errors in line source dosimetry for gamma-ray brachytherapy

The generally used approach for routine brachytherapy dosimetry depends on a value of source strength provided by the source supplier, the theoretical concept of exposure rate constants for different nuclides, and the applicability of inverse square law to point sources. Corrections for filtration of the radiation within the capsule of the source and attenuation in tissue medium are effected based on attenuation coefficients and data published in the literature. Therefore there is no unique or ideal way of selecting the required data and there could be a spread in the dose values derived by different users. The errors inherent in the current practice of dosimetry of linear sources in brachytherapy are discussed. If the brachytherapy sources are specified in terms of exposure rate at a distance from the source by the source suppliers, the overall uncertainty in the dosimetry at regions of clinical interest around the source could be limited to about +/- 6%.

Basic Ratios and Factors for the Dosimetry of External Beam

Radiotherapy machines are of different kinds and are capable of producing beams of various qualities. Conventional X-ray machines, radioisotope teletherapy machines, and accelerators were discussed in the earlier chapters. Any radiotherapy clinic may equip itself with one or more of these machines, suitably selected to meet its requirements. The quality (meaning the type, energy, and penetration) of the beam produced by any particular radiotherapy machine should be understood so that the machine will be used properly on patients. The penetration characteristics of any beam should be evaluated by experimental measurements.

Instruments for Radiation Detection

Radiation is detected by measurement of the effect of its interactions with target materials. The radiation-induced effect on a detector produces a signal that can be interpreted to give the radiation quantity of interest. We already discussed the use of ionization chambers and calorimeters for this purpose in Chapter 11. In this chapter, we will provide more details on ionization chambers and also discuss other devices used for radiation detection.

Physical Aspects of Electron Beam Therapy

For the clinical applications of electron beams, the physical behavior of electrons has to be well understood. In this chapter, we discuss the fundamental features of electron transport and theways in which they influence the dose distribution patterns. Electrons are more complex than photons in their transport behavior. The dose distribution for electron beams can be well documented for standard conditions of a beam incident on a unit-density medium with a flat surface. For nonstandard situations that may be encountered in actual clinical contexts, the interpretation and prediction of electron beam dose distributions pose many challenges.

Can the AAPM task group 21 protocol lead to optimum ion chamber designs

The recently published AAPM Task Group 21 protocol for high-energy dosimetry is complicated in that it requires the physicist to obtain the values of about a dozen different physical variables by looking them up in tables or graphs. This should be compared with the procedure of earlier protocols using the concept of a single multiplier C lambda. We have investigated how the physical principles outlined in the improved AAPM protocol could be utilized for the redesign of the therapy-level ion chambers in such a way that one can reduce the number of factors that need to be looked up in tables or graphs for the calibration of high-energy teletherapy photon beams. In our analysis presented in this paper we found that one such design could be for an ion chamber having a wall acrylic or Bakelite of a thickness not exceeding 0.1 g/cm2 and having an inner diameter of 6 mm, and used in conjunction with a cobalt-60 buildup cap of thickness 0.35 g/cm2 made of acrylic, Bakelite, or Tufnol. If a chamber of such a design is used in a water phantom, the dosimetry practically reduces to the simplicity of the former protocols of depending on a single value of energy-dependent multiplier to be obtained from a table. With the above design parameters, it becomes possible to eliminate the explicit need to incorporate the factors Pwall, Prepl, Awall, beta wall, and the variable alpha, representing the fraction of ionization due to electrons from the wall material of the chamber.

Product representations of teletherapy dose distributions.

Product representations are frequently used for teletherapy dose distributions. For example, the dose in a central plane is often written as the product of two factors, one dependent on the depth and the other on the transverse variable. We have answered the following question: given a (two-dimensional) set of data, how closely is it possible to represent these data by a product of two such factors, and what factors would give a best-fit representation? We thus have developed a quantitative test with which to judge any proposed product representation, for a given set of data. As an example, we have applied our method in analyzing the accuracy of a model proposed by van de Geijn for representing central-plane Cobalt-60 dose data through a product representation on decrement lines intersecting the source.

Scope of Clinical Radiotherapy Physics

The word ”physician,“ rather than ”physicist,“ is commonly associated with the word ”clinic.“ It is natural that the term ”clinical physicist“ may provoke a reaction among lay people. They may ask whether indeed ”physicist“ and not ”physician“ was meant. Next could come the query, what role does a physicist play in a clinic? The reply is that medical physicists function in those clinics that need their expertise to ensure proper patient care. In general, medical physics includes all applications of physics to medicine, being concerned with the use of physical principles and techniques in any aspect of the prevention, diagnosis, and treatment of human diseases, and in medical research for the promotion of human health. Although medical physics encompasses this wider territory, rather than merely radiation therapy, radiation therapy remains one area in which the physicist has some influence on every individual patient treated. It is an area in which the physicist can interact with the physician and contribute to the better management of specific patients. For an understanding of this, it is necessary to discuss briefly what radiation therapy involves.

Atoms, Molecules, and Matter

One aspect of scientific inquiry is to try to perceive all materials as being made up of smaller components. Chemical experiments showed that compounds could be split into components, each of which has its own distinct chemical behavior. These individual components were given the name ”elements.“ Groups of elements have been identified as alkali, halogens, and inert gases, with elements in each group behaving alike chemically. In 1816, Prout suggested that all of the heavier elements might have been built from the lightest element, hydrogen. Soon afterwards, Dalton stated that samples of different sizes of any particular chemical compound contained the constituent elements in the same proportions. He referred to the smallest entity of any compound as a molecule of that compound, and to the smallest entity of any element as an atom of that element. A molecule and an atom are the smallest samples that display a characteristic chemical behavior.