Radiotherapy in oncology
In human medicine, radiotherapy / irradiation is the second most important method of treating malignant neoplasms after surgery. It is estimated that about 40% of the cured people were treated with radiotherapy. In veterinary medicine, this percentage is lower due to the much lower availability of the method for patients. We hope to change it permanently in our country.
Radiotherapy is a local treatment, similar to surgery, but it can involve a much larger area of tissue. It can be used as the only treatment, or as an element of the entire treatment process, and even improve the results of a surgical procedure.
What are the goals of radiation therapy in animals?
Depending on the type of cancer and its spread / size, location, and the intentions of the patient caregiver, radiation therapy may be aimed at:
- therapeutic (radical / definitive radiotherapy), when it is possible to cure the disease completely or to control symptoms in a long term; a high total dose is then administered in a relatively large number of small fractions / doses (15-25), within 3-5 weeks or in 1-3 very large fractions, within 1 week (stereotaxic radiotherapy),
- palliative (palliative radiotherapy), when the neoplastic disease is advanced and it is only possible to inhibit tumor growth, alleviate the symptoms of the tumor and thus significantly improve the patient’s quality of life, without the possibility of complete cure or long-term control of the disease; only a few fractions are given, most often at weekly intervals.
How does radiation therapy work?
The essence of radiotherapy is the ability of the radiation beam to ionize tissues, and thus the ability to knock out electrons from the structures of tissue atoms. The released electrons in turn:
- they directly damage cellular DNA (one or both of its strands) and, to a lesser extent, cell membrane structures,
- by hitting water molecules, they generate the formation of highly chemically reactive, free hydroxyl radicals that damage DNA, causing biochemical and biological damage.
What radiation is used in radiotherapy?
The following bundles have the ability to ionize the tissue:
- photons (X-rays, gamma rays) with energy from 250 kV (kilovolts) to 25 MV (megavolts),
- molecules (electrons, protons, helium nuclei, heavy ions, …) with energy up to 220 MeV.
How is the radiation dose delivered?
Generally speaking, radiotherapy can be divided into:
- teleradiotherapy, i.e. external beam radiotherapy, when the radiation source is distant from the irradiated tissue; it is the most commonly used method of radiotherapy,
- brachytherapy, when a radiation source is placed in close proximity to or inside the irradiated tissue; in Europe, there are several veterinary centers that use brachytherapy devices, in humans it is often used in the treatment of cancers of the genitourinary system,
- systemic radiation therapy, where the patient is administered radioactive isotopes; Iodine 131 is used to treat feline hyperthyroidism in many centers in Europe.
What are the types of radiation therapy?
Due to the energy of the radiation beam, radiotherapy is divided into:
- megavolt radiotherapy,
- orvoltage radiotherapy.
More about types of radiotherapy – TYPES OF RADIOTHERAPY.
What tissues are sensitive to radiation therapy?
The sensitivity to radiotherapy in different tissues can vary considerably. Lymphocytes, bone marrow cells, stem cells and neoplasms originating from these cells are subjected to programmed death (apoptosis) after irradiation with even small doses. In other tissues, the percentage of cells undergoing apoptosis may vary and determines the tissue radiosensitivity.
The most sensitive to radiation are dividing cells (in the mitotic phase) and in the G2 phase of the cell cycle, while those in the later S (synthesis) and G0 (resting) phases usually turn out to be radio-resistant. In other words, tumors that are intensely dividing / growing tend to be more radiosensitive, but not always radially curable.
The supply of oxygen to the tissue is of key importance for the effectiveness of radiotherapy. Since 25-30% of the therapeutic effect of radiation is dependent on reactive oxygen species produced in tumor tissue as a result of radiation therapy, hypoxic tissues will respond poorly to such treatment.
Why are radiotherapy patients generally anesthetized?
Radiotherapy requires planning how to deliver specific doses of energy. This is often associated with a CT scan, on the basis of which the treatment plan is made. Therefore, the positioning for this tomography must be exactly the same as during the subsequent irradiation. Special immobilizing masks are used to position the patient, and often also other equipment – they make it easier to later reproduce exactly the same position of the patient. The radiotherapy plan must always be presented to the patient’s caregiver and approved by him. Delivering individual doses / fractions requires short general anesthesia of the patient – for subsequent doses it must be arranged in exactly the same way, must not move during the administration of the fraction itself. In humans, similarly, general anesthesia during the delivery of each fraction is used in young children and people with claustrophobia.
How is computed tomography for radiotherapy planning different from diagnostic computed tomography?
In diagnostic computed tomography, the patient lies on a concave table, usually without any additional accessories. During treatment planning computed tomography, the patient is placed in the appropriate position, which is accurately reproduced later during the irradiation. For proper positioning, immobilization accessories (special vacuum pads, thermoplastic masks, etc.) are used to help maintain the correct position during subsequent treatment. The table in such a CT scan is flat, as is the therapeutic table of a linear accelerator. Additionally, MRI images, especially for brain tumors, can be used to plan treatment. Generally speaking, it takes 4 to 6 hours to prepare a radiation therapy plan.
Why is the total dose divided into fractions?
Although administering a high dose of radiation at one time does heal the tumor, it also causes extensive necrosis in the surrounding, healthy tissues. Dividing the total radiation dose into smaller but more frequently administered doses (i.e. the so-called dose fractionation) allows to maintain the therapeutic effect while minimizing side effects, making the treatment safe. The method of fractionation depends on the type of tumor and the purpose of the treatment.