Principles of radiotherapy

3 Principles of radiotherapy




Introduction


After surgery, radiotherapy is the most effective curative treatment for cancer, contributing up to 25–30% of cure. At least half of the patients with cancer require radiotherapy at some time in their illness of which about 60% are treated with curative intent, often in combination with surgery and chemotherapy. Radiotherapy involves use of various types of ionizing radiation, and X-ray is the commonest type of radiation used. Other forms of radiation include electrons, protons, neutrons and gamma radiations from radioactive isotopes. This chapter intends to review the principles of practical radiotherapy (Box 3.1) and a detailed discussion on radiobiology and mathematical modelling are beyond the scope of this chapter.



Box 3.1
Steps in practical radiotherapy














Methods of delivery of radiotherapy


Radiotherapy is either delivered by a radiation source placed away from the body (teletherapy or external beam radiotherapy), by placing a radiation source into the tumour (interstitial brachytherapy, e.g. small tongue cancer) or in a body cavity containing the tumour (intracavitary brachytherapy, e.g. cervical cancer) or by intravenous or oral administration of nonsealed radionuclides. Depending on the site and type of cancer, radiotherapy is delivered by either one of these techniques or a combination. Sometimes radiotherapy is delivered concurrently with chemotherapy or biological agents to improve the chances of cure (see below).



External beam radiotherapy (EBRT)


EBRT commonly utilizes X-rays and electrons. Before the 1950s, radiotherapy units were kilovoltage machines which produced X-rays with limited penetrability. These machines, which are still used in some centres, can be one of the following:




Modern radiotherapy is based on megavoltage X-rays (photons) and electrons. Megavoltage X-rays are produced by artificial acceleration of electrons through a vacuum to impact on a target in machines called linear accelerators (LINACs) (Box 3.2). The energy of the X-rays is proportional to the speed of electrons. Electrons are produced when the target in a linear accelerator is removed from the path of the electron beam (Box 3.2). Modern LINACs have facilities to produce both photons and electrons. Electrons have a predictable penetrability and hence are useful when it is important to limit the radiation dose to a deeper organ. Characteristics of photons that are useful in their clinical use are:











Informed consent


Once a decision is made to treat with radiotherapy, this decision should be communicated to the patient. The patient should be informed of the intention of treatment, potential benefits and side effects, both short term and long term. It is also an obligation to explain to the patient the potentially serious short and long term consequences of treatment in order to obtain truly informed consent. Often patients would like to know details of how radiation works (Box 3.4), dose and length of treatment, rationale for a number of treatments (fractionation – the process of giving the total dose of radiation as small doses over a period of time) (Box 3.5) and side effects (p. 348). Side effects of radiotherapy depend on the area of treatment, total dose and dose per fraction. These can be acute (occurring during radiotherapy and within 3 months of completion of radiotherapy) or chronic (occurring 3 months after completion of radiotherapy).





Box 3.5
Rationale for fractionation


Radiotherapy usually results in equal damage to both normal cells and cancer arising from them. However, normal cells and cancer cells differ in their ability to recover from radiation damage. This differential capacity of normal cells to repair and re-grow is exploited in eradication of tumour cells. Radiation sensitivity of cells is dependent on the phase of cell cycle they are in: G2 and M are radiosensitive whereas S phase is radioresistant. At the time of radiotherapy, the cancer cells are in various phases of cell cycles; some of them will be in the sensitive phase when there is a good chance of cell kill and some will be in the resistant phase.


Rationale of fractionation is explained by 5 Rs. Some of the Rs lead to more cancer cell kill, whereas others lead to better repair of normal cells.



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Jun 18, 2016 | Posted by in ONCOLOGY | Comments Off on Principles of radiotherapy

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