Radiation Therapy Dosimetry

A Practical Handbook

Arash Darafsheh

• Publisher: CRC Press
• Year: 2021
• Print ISBN: 9781138543973
• eBook ISBN: 9781351005364
• Edition: 1
• Topic: Medicine
• Subtopic: Clinical Medicine

This comprehensive book covers the everyday use and underlying principles of radiation dosimeters used in radiation oncology clinics. It provides an up-to-date reference spanning the full range of current modalities with emphasis on practical know-how. The main audience is medical physicists, radiation oncology physics residents, and medical physics graduate students. The reader gains the necessary tools for determining which detector is best for a given application. Dosimetry of cutting edge techniques from radiosurgery to MRI-guided systems to small fields and proton therapy are all addressed. Main topics include fundamentals of radiation dosimeters, brachytherapy and external beam radiation therapy dosimetry, and dosimetry of imaging modalities. Comprised of 30 chapters authored by leading experts in the medical physics community, the book:

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• Covers the basic principles and practical use of radiation dosimeters in radiation oncology clinics across the full range of current modalities.

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• Focuses on providing practical guidance for those using these detectors in the clinic.

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• Explains which detector is more suitable for a particular application.

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• Discusses the state of the art in radiotherapy approaches, from radiosurgery and MR-guided systems to advanced range verification techniques in proton therapy.

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• Gives critical comparisons of dosimeters for photon, electron, and proton therapies.

I

Radiation Dosimeters and Dosimetry Techniques

CHAPTER 1

Fundamentals of Radiation Physics and Dosimetry

Blake R. Smith

University of Iowa

Iowa City, Iowa

Larry A. DeWerd

University of Wisconsin

Madison, Wisconsin

CONTENTS

• 1.1Absorbed Dose
• 1.2Charged Particle Transport
• 1.3Photon Interactions
• 1.4Quantities Used to Describe Ionizing Radiation
• 1.5Radiation Dosimetry
  • 1.5.1Cavity Theory
  • 1.5.2Overview of TG-21
  • 1.5.3Overview of TG-51
• 1.6Conclusion
• References

1.1 ABSORBED DOSE

One of the most important quantities that concerns the practice of medical physics is dose. This quantity is prescribed by physicians to treat tumors, set by radiation safety officers as exposure limits to workers, and referenced by radiobiologists while performing cell irradiation studies. This quantity, however, is rather arbitrary and ill-defined by itself. Dose, D, is defined as a point quantity from the fundamental quantities of energy and mass as:

D=dEdm→ΔEΔm,[Jkg]

(1.1)

In a theoretical sense, this point quantity refers to the energy deposited within an infinitesimal amount of mass (and thus volume). Realistically, energy deposition from atomic and subatomic events is discrete with respect to an infinitesimal volume for which the definition of dose is better represented as an average over a specified space leading to the adjacent expression in Equation 1.1. For example, a whole-body dose of 4 Gy has about a 50% chance of killing an adult human in 60 days [1]. On the other hand, patients undergoing radiation therapy often receive 40 Gy or more to their tumors, but in this context the dose is fairly localized with a large amount of energy deposited within a smaller volume.

It is also important to specify the medium that the dose is referred to. Dose to water versus dose to air imply subtle, but extremely important, differences in the amount of energy expended in the medium as well as how the energy was transferred. While different, both quantities are reported using the same units of Gray (Gy) which is defined as 1 Gy = 1 J kg⁻¹ = 6.24 × 10⁹ MeV g⁻¹. The subject of absorbed dose differences and dependences is paramount to the understanding of detector response, which will be a subject of further discussion later on.

Calculating dose during discrete particle transport through a medium can be difficult where only a handful of scenarios exist that can be solved analytically. However, more complicated problems can be solved using Monte Carlo methods where the path a particle experiences is simulated discretely. Dose is only delivered to matter through charged particles. Uncharged particles, such as photons and neutrons, will traverse through a medium unimpeded until either an elastic or inelastic interaction occurs. During inelastic interactions, energy is released from the uncharged particles to the medium potentially transferring kinetic energy to charged particles. Those liberated charged particles expend their kinetic energy to the surrounding medium putting other charged particles into motion or producing uncharged particles and the process repeats. Solving these radiation transport problems requires an in-depth understanding of the types of interaction that can occur, the probability of their occurrence, and the kinematics following these interactions.

Let us consider the definition of dose a little more closely. Of concern is the transfer of energy to matter within a medium from charged particles. Following the definition of dose provided in Equation 1.1, dose to a fragment of matter within the medium can be determined from the kinetic energy loss, ΔE, that a particle experiences across a fragment of the medium with mass, m. If we model the kinetic energy loss of the particle in discrete, straight-line steps of length Δx, then we can relate the total path, s, that the particle travels within the fragment of matter to the kinetic energy lost by that particle and imparted to the medium. This is referred to as the stopping power of the medium, (ΔEΔx), which is fundamentally related to the force acted on by the medium to slow the particle down. This loss of energy is then related to dose by

D=sm⋅(ΔEΔx)=sV⋅(ΔEρΔx)

(1.2)

→D=1A⋅(ΔEρΔx)=ϕ⋅(ΔEρΔx)for multiparticle scenario

(1.3)

where the normalization of the particle's stopping power to the density of the medium, ρ, is referred to as the mass stopping power. Dose is rarely defined or calculated from a single particle as a single particle's trajectory can vary substantially if the particle were to travel from the same initial conditions. The energy loss on this scale is largely stochastic in nature. The progression of Equation 1.3 illustrates how the calculation and definition of dose change from the single-particle scenario to a more familiar scenario of an incident fluence of charged particles, ϕ, upon an incident area, A, and volume, V, of the medium. The above example is referred to as the thin-film approximation, which subtly assumes no changes in the rate of energy loss of the particle as it traverses through the medium. While fine for our conceptual discussion of dose, more consideration is necessary to comprehensively describe the energy loss of charged particles. For further discussion, consider a general relation between dose and the transport of charged particles,

D=∫0Emaxϕ'(E)(dEρdx)cdE

(1.4)

For the calculation of dose, ϕ'(E) in Equation 1.4 is used for the differential fluence with respect to the energy of the charged particles and (dEρdx)c is the portion of the stopping power responsible for collisional energy losses from the incident charged particle undergoing multiple Coulombic scattering with the surrounding orbital electrons in the medium. Another assumption necessary to allow us to calculate dose accurately is to limit our energy fluence spectrum from energetic, knock-on electrons, known as δ -rays, which are produced within our region of interest and deposit their energy elsewhere. This restriction is also referred to as charged particle equilibrium (CPE) and is necessary for the calculation of dose. Simply, CPE requires that the charged particle fluence which enters our region of interest also leaves the region. In a sense, CPE is...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Contents
• Preface
• About the Editor
• Contributors
• Part I Radiation Dosimeters and Dosimetry Techniques
• Part II Brachytherapy
• Part III External Beam Radiation Therapy
• Part IV Imaging Modalities
• INDEX