Practical Radiotherapy

Physics and Equipment

Pam Cherry, Angela M. Duxbury, Pam Cherry, Angela M. Duxbury

• Publisher: Wiley-Blackwell
• Year: 2019
• ISBN:
  9781119512745
• Edition: 3
• Topic: Medicina
• Subtopic: Tecnología y suministros médicos

Now in its third edition, Practical Radiotherapy continues to keep pace with current and emerging technologies, patient pathways, and the rapidly expanding role of therapeutic radiographers.

Extensively revised and updated, this accessible book examines all the essential aspects of radiotherapy, from the physics and mathematics of radiation beams, to in-depth descriptions of the equipment used by radiotherapy practitioners, to new and expanded coverage of MR-linac and Halcyon technology, proton therapy, stereotactic body radiotherapy, sealed-source verification and quality assurance for MV equipment.

• Covers all the core information essential to radiotherapy practice
• Describes the major aspects of therapeutic radiography in a practical context
• Includes images, diagrams, supplemental reading suggestions and more radiotherapy-specific examples
• Features expanded coverage of legislation, advanced treatment delivery, flattening filter free treatment and more

Practical Radiotherapy is a valuable resource for radiotherapy and medical physics students, radiotherapists, therapeutic radiographers, radiation therapists, clinical oncologists and oncology nurses.

CHAPTER 1
Introduction to Radiotherapy Practice

Angela M. Duxbury and Anne J. Jessop

Aim

The aim of this chapter is to provide a brief introduction and overview of the principles of current radiotherapy practice and to act as a guide for the other chapters presented in this text.

1.1 Introduction

Venturing into the field of radiotherapy physics is one of the most interesting and exciting aspects of radiotherapy practice. The rapid developments in computer and technological innovation continue to impact on changing and advancing practice.

1.2 What Is Radiotherapy?

Radiotherapy is a speciality that uses high‐energy ionising radiations to treat cancer and some benign conditions. In 2015, there were 359 960 new cases and 163 444 deaths recorded from cancer in the UK. Over 50% of cancer patients survive for 10 years or more and 27% of cancer patients will receive radiotherapy [1].

The intention of radiotherapy can be curative, known as radical treatment, or it can be given to reduce the symptoms of cancer, known as palliative treatment. It can be used as a treatment modality on its own and/or combined with cytotoxic (cell toxic) chemotherapy and/or surgery.

Radiotherapy delivered from outside the body is known as external beam radiotherapy, using X‐rays (photons) or electrons from a linear accelerator machine or protons produced by a cyclotron (see Chapter 8). It can also be delivered from within the body as internal radiotherapy, by placing sealed radioactive sources directly into tissue or cavities, known as brachytherapy (see Chapter 14), or by administering a fluid/capsule of radioactive material, an unsealed radionuclide, into the body (see Chapter 13).

Once a patient has been referred for radiotherapy, the aim of the treatment process is to undertake detailed imaging to visualise the tumour (see Chapter 6) followed by complex treatment planning (see Chapter 9) to ensure that accurate treatment delivery is achieved (see Chapters 7 and 10) in order to deliver a radiation dose that can destroy the tumour whilst minimising the dose to the surrounding healthy organs.

Radiation absorbed dose (see Chapter 4) is measured in Grays (Gy) and the therapeutic radiation dose administered varies depending upon: the curative intent of therapy; the radio sensitivity of the tumour; the volume of tissue to be treated; and the site of the tumour. To enhance the effectiveness of treatment and to allow normal tissue time to recover from the radiation injury, treatment is given in fractions over a specific period of time, for example, 45 Gy in 15 fractions over 21 days.

A combination of skill, accuracy, and complex technology are dedicated to delivering safe and effective radiotherapy in order to achieve the two competing goals – high tumour control and few treatment complications. Treatment failure to meet the treatment intent can result in the patient's clinical outcome being seriously affected in both the short and the long term. Many things can go wrong in this multi‐step/person/department process and error prevention and quality management (see Chapter 11) is essential to minimise catastrophic consequences for the patient [2].

1.3 Working with Ionising Radiations

The nature of ionising radiations means that they cannot be detected by the human senses therefore, in order to be able to detect and accurately measure the amount of radiation being delivered several different methods of radiation detection and measurements have been developed (see Chapter 4).

Working with ionising radiations is safe providing a raft of measures are adopted and followed. Safe working practices are a legal requirement and follow the Ionising Radiation Legislation, the Ionising Radiation (Medical Exposure) Regulations (IR[ME]R) 2017 (IR[ME]R NI 2018) [3] (see Chapter 12).

1.4 How Radiotherapy Works

There are several interaction processes that occur when ionising radiation interacts with matter. These depend on the nature and energy of the primary radiation beam and the structure of the medium through which the radiation beam passes. For X‐ray energies utilised in radiotherapy, these interaction processes are described in Chapter 5.

High‐energy radiation used for radiotherapy treatment can be lethal to both normal and abnormal tissue; this is due to either direct or indirect actions occurring when the radiation is delivered to the target volume within the patient.

Direct action occurs when the cells within the normal tissue or tumour are in the mitosis phase of the cell cycle and the DNA strands are exposed as part of the cell division. The X‐rays strike the DNA chain and cause either a single or double strand break; the result of a double strand break is cell death, however there is a possibility that following a single strand break cells can go on to have further cell divisions.

Indirect action occurs when the radiation ionises the water molecules within the cells and is not directly linked with the cell cycle. When the water molecule is ionised this leaves a H₂ element and an O element to restabilise and both these ions seek a partner to join with; some will become a water molecule again (H₂O) with no resultant effect. Other ions will combine as H₂O₂ (hydrogen peroxide), which is toxic to the cells’ internal environment, with the resultant effect that cell death will occur.

Both of these actions are based on the probability that radiation will come into contact with either the cell during mitosis, or water molecules along their path through the patient. As the radiation cannot discriminate between normal and tumour cells there is the likelihood that normal tissue will be affected, along with the tumour, as it is impossible to clearly define the tumour boundary. As a result of any tissue damage, cells in the vicinity will be stimulated to move into the mitosis phase of the cell cycle to repair the damage; this is true for both normal and tumour cells. With all of the tumour cells being included within the treatment volume during a course of radical treatment, the aim is to deliver a tumouricidal dose of radiation to the tumour whilst sparing as much normal tissue as possible; this is known as tumour control probability (TCP) and normal tissue complication probability (NTCP).

1.5 Radiotherapy Beam Production

Most commonly used radiotherapy beams are electronically produced using a linear accelerator; a machine consisting of a discrete number of components that function together to accelerate electrons before they strike the target to then produce high‐energy photons (X‐rays). These X‐rays are then directed towards the patient and subsequently the tumour through a series of collimation systems. Electron beams are produced using the same principles of accelerating electrons, however the target is removed from the exit window and the electron beam is then used to treat the patient (see Chapter 8).

Proton beams are produced using either a cyclotron or a synchrotron to accelerate the particles by magnetically pulling them through a circular path until the protons reach their maximum speed. The advantage of using a proton beam is that the Bragg Peak depth can be manipulated to more closely match the tumour shape by modulating the beam as it emerges from the head of the machine (see Chapter 8).

Kilovoltage machines were historically the main provider of external beam radiotherapy, until the introduction of Cobalt‐60 units, and subsequently linear accelerators; both of which have the capability to improve the delivery of dose at depth. However, kilovoltage machines (see Chapter 8) still have an important role within radiotherapy when treating superficial tumours, especially smaller lesions or lesions close to the eye.

1.6 Treatment Delivery and Planning

Radiotherapy can b...