How radiotherapy works
Radiotherapy is the use of ionising radiation for the treatment of malignant disease. The high-energy electromagnetic waves have sufficient energy to displace an orbital electron from an atom and thus create free radicals. The radiation can be in electromagnetic form such as high-energy photons, or in particulate form such as an electron, a proton, a neutron, or an alpha particle.
The delivery of energy to tissues results in damage to DNA and this diminishes or eradicates the cell’s ability to replicate indefinitely. DNA damage caused by free radicals includes damage to nucleotide bases and the carbohydrate backbone, as well as cross-linkage between strands, and single and double strand breaks. Early reactions that occur in tissues that divide rapidly are seen during the course of radiotherapy and are usually reversible. The late effects of radiation are not immediate and may occur many months or even years after the radiation therapy, and are expressed when the cells attempt mitosis and fail. Such effects are less likely to be reversible. This accounts for the apparent delay in tumour response and the timing of radiation reactions in normal tissue.
Malignant cells generally do not have the ability to repair radiation damage and this differentiates them from normal cells. Cells that remain after radiation treatment have access to a greater relative blood supply in addition to other biological factors within the microenvironment of the tumour, such as cytokines or growth factors, and this can lead to cellular repopulation.
Fundamentally, radiation therapy requires oxygen in the target tissues to generate free radicals, which in turn damages the DNA of the tumour cells. Therefore, relative tissue hypoxia will reduce the killing effect of the radiotherapy. Low levels of haemoglobin can further reduce the effect of radiation treatment for the same reason.
High-energy photons
This is the most common form of radiation used in practice, where photons are released from the nucleus of a radioactive atom and are known as gamma rays. When photons are created in a linear accelerator, they are known as X-rays. Therefore, the only difference between the two terms is the origin of the photon.
Photoelectric effect
Incoming photons collide with a tightly bound electron in the target tissue and transfer the majority of their energy to the electron and cease to exist. The electrons begin to ionise the surrounding molecules and this interaction depends on the energy level of the incoming photons, as well as the atomic number of the target tissue. For example, the atomic number of bone is 60% higher than that of soft tissue and therefore bone is seen with more contrast and detail than soft tissue. The energy range in which the photoelectric effect predominates in tissue is about 10–25 keV.
Compton effect
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