Practical Radiation Oncology Physics E-Book

A Companion to Gunderson & Tepper's Clinical Radiation Oncology

Sonja Dieterich, Eric Ford, Daniel Pavord, Jing Zeng

• Publisher: Elsevier
• Year: 2015
• ISBN:
  9780323263757
• Topic: Medizin
• Subtopic: Onkologie

Perfect for radiation oncologists, medical physicists, and residents in both fields, Practical Radiation Oncology Physics provides a concise and practical summary of the current practice standards in therapeutic medical physics. A companion to the fourth edition of Clinical Radiation Oncology, by Drs. Leonard Gunderson and Joel Tepper, this indispensable guide helps you ensure a current, state-of-the art clinical practice.

• Covers key topics such as relative and in-vivo dosimetry, imaging and clinical imaging, stereotactic body radiation therapy, and brachytherapy.
• Describes technical aspects and patient-related aspects of current clinical practice.
• Offers key practice guideline recommendations from professional societies throughout — including AAPM, ASTRO, ABS, ACR, IAEA, and others.
• Includes therapeutic applications of x-rays, gamma rays, electron and charged particle beams, neutrons, and radiation from sealed radionuclide sources, plus the equipment associated with their production, use, measurement, and evaluation.
• Features a " For the Physician " box in each chapter, which summarizes the key points with the most impact on the quality and safety of patient care.
• Provides a user-friendly appendix with annotated compilations of all relevant recommendation documents.
• Medicine eBook is accessible on a variety of devices.

Part I

Building Blocks

Outline

Chapter 1 Reference Dosimetry for Ionizing Radiation

Chapter 2 Relative Dosimetry for MV Beams

Chapter 3 In-Vivo Dosimetry

Chapter 4 Quality Assurance and Commissioning of New Radiotherapy Technology

Chapter 5 Quality Assurance of Radiotherapy Dose Calculations

Chapter 6 Immobilization Techniques in Radiotherapy

Chapter 7 Image Guidance and Localization Technologies for Radiotherapy

Chapter 8 Brachytherapy

Chapter 9 Proton Radiotherapy

Chapter 10 Radiation Safety and Shielding in Radiotherapy

Chapter 11 Information Technology in Radiation Oncology

Chapter 12 Quality and Safety Improvement in Radiation Oncology

Chapter 1

Reference Dosimetry for Ionizing Radiation

1.1 Introduction

The key to the accurate delivery of radiation is the ability to establish the absolute dose delivered. In radiation therapy clinical practice the primary tool used to measure absorbed dose is the ion chamber. The use of ion chambers has been well described by international codes of practice. While some of the details may differ slightly, the basic concepts of the various codes of practice are the same. The ionization measured by the chamber (typically filled with air) is converted to absorbed dose by applying a calibration factor (determined by an accredited calibration lab) and other correction factors based on the chamber design. The calibration factor may be a direct calibration in water (megavoltage (MV) photons, MV electrons, protons) or a calibration based on air kerma (kilovoltage (kV) photons, brachytherapy sources). The end goal is to determine absorbed dose to water in either case.

The starting point for these calibrations is the absorbed dose standard developed at a primary standard dosimetry laboratory (PSDL). End users obtain a calibration factor for their equipment at a secondary standard dosimetry laboratory (SSDL), also known as an accredited dosimetry calibration lab (ADCL). The SSDL applies the standard developed by the PSDL using the available radiation sources at that lab. All SSDLs have ⁶⁰Co sources available for calibration but may not have linac-generated beams. Some labs do have other high energy photon beams available and can provide a calibration at multiple beam qualities. In the absence of this, correction factors must be applied to the calibration determined at ⁶⁰Co energy to determine the calibration for the beam quality of interest. Further corrections are needed if the beam of interest is protons or other heavy ions. For a more complete discussion of the interaction between PSDLs, SSDLs, and the end user, the reader is referred to section 2.1 of International Atomic Energy Agency (IAEA) TRS-398, Absorbed Dose Determination in External Beam Radiotherapy: An International Code of Practice for Dosimetry Based on Standards of Absorbed Dose to Water.¹

The regulations on the frequency of a full calibration in water may vary from country to country but are generally required at least once per year. The suggested regulations by the Conference of Radiation Control Program Directors (CRCPD) and implemented by many states in the United States require full calibrations at intervals not to exceed 12 months (section X.7.iii). More frequent constancy checks are required but can be done using solid phantoms and dosimetry equipment that is not calibrated by a calibration laboratory. These constancy devices should be compared with the calibrated system immediately after the annual full calibration. The equipment used for the full calibration should be sent for recalibration every 2 years. A constancy check should be performed before sending equipment to the calibration lab and after receiving it back to ensure that nothing happened during the process to change the response of the chamber. For new radiation therapy treatment machines, a second check of the absolute dose calibration should be obtained prior to treating patients.² This could be accomplished by using a mail-order reference dosimetry service or a second check by a colleague using an independent dosimetry system.

1.2 Standard Megavoltage Photon Beams

Several international codes of practice are used to determine absorbed dose for MV photon beams, the American Association of Physicists in Medicine (AAPM) TG-51, Protocol for Clinical Reference Dosimetry of High-Energy Photon and Electron Beams,³ IAEA TRS-398, Absorbed Dose Determination in External Beam Radiotherapy,¹ Deutsche Industrie-Norm (DIN) 6800-2, Dosimetry Method for Photon and Electron Radiation—Part 2 Dosimetry of High Energy Photon and Electron Radiation with Ionization Chambers,⁴ Institute of Physics and Engineering in Medicine (IPEM) 1990, Code of Practice for High-Energy Photon Therapy Dosimetry,⁵ and others. An addendum to AAPM TG-51 has been published containing new k_Q values.⁶ They are all based on absorbed dose in water calibrations of cylindrical ion chambers. The charge reading obtained from the ion chamber is corrected for temperature, pressure, ion recombination, and polarity. Corrections are then made to account for the perturbation to the medium (water) caused by the presence of the ion chamber. The calibration factor is then used to convert charge to absorbed dose. All of the protocols use a reference field size of 10 cm × 10 cm but the depth and source-to-surface distance (SSD) can vary among them. It is important to note that the reference depth for the calibration protocol is likely not the depth of absorbed dose specification in the clinic. For example, the protocol may specify measurement at 10 cm depth but the output of the machine is adjusted to 1.0 cGy/MU at depth of dose maximum (d_max). This will require the use of accurate percent depth dose to correct the readings taken at 10 cm depth to d_max depth. The general equation to calculate absorbed dose from a charge reading of an ion chamber measured with an electrometer is:

Dw(z)=M ND,wkpkskρkQ(z)

where D_w(z) = absorbed dose to water at depth z

M = the ion chamber reading corrected for the electrometer calibration

N_D,w = calibration factor for absorbed dose to water for ⁶⁰Co energy

k_p = polarity correction. The value is generally less than 1% from...