Nerdy Science Blog

Radiation therapy (also called x-ray therapy, irradiation, radiotherapy) is the use of ionizing radiation to kill cancer cells and shrink tumors (to relieve symptoms). It destroys cells in the targeted area by damaging their genetic material. The radiation will ionize atoms which make up the DNA chain, damaging them and causing them to die or reproduce more slowly.

Radiation therapy has been in use as a cancer treatment for more than 100 years. The concept of therapeutic radiation was invented by German physicist Wilhelm Conrad Rontgen. With the discovery of computed tomography (CT), physicians are able to directly measure the dose delivered to the patient’s anatomy based on axial tomographical images. With the creation of magnetic resonance imaging (MRI) and positron emission tomography (PET), radiation therapy now have better treatment outcomes and fewer side effects.

Radiation therapy can be used alone or in combination with other cancer treatments such as chemotherapy or surgery.

The goal of radiation therapy is to damage as many cancer cells as possible, while limiting harm to nearby healthy tissues.

The amount of radiation absorbed by the tissues is called the radiation dose, which is measured in a unit called ‘gray’(Gy). Different tissues of the body could tolerate various amounts of radiation (measured in centigrays). For example the kidneys can tolerate around 1800 cGy. The total dose of radiation is usually divided into smaller doses (called fractions) that are given daily over a specific time period.

Low-dose palliative treatments usually cause minimal or no side effects. Long-term treatment causes medium and long-term side effects, for example:

1. Fibrosis of the tissues – tissues become less elastic due to diffuse scarring process
2. Hair loss – usually seen in patients who receive radiotherapy to the brain. The radiation could spoil the matrix of the hair follicles thus producing permanent hair loss.
3. Fatigue
4. Cancer and secondary malignancies – this side effect is seen in a very small minority of patients, generally many years after they have received a course of radiation treatment.
5. Shortness of breath – radiation could decrease the levels of the substance called surfactant, which helps keep the air passages open. Low surfactant levels keep the lungs from fully expanding, thus causing shortness of breath or cough. These symptoms are sometimes treated with steroids. Lung fibrosis is also a possible side effect.
6. Digestive tract – radiation to the esophagus, stomach or intestine can cause pain, nausea, vomiting or diarrhea.

Treatment using higher doses causes acute side effects:

1. Damage of the epithelial tissue – for example skin, oral, pharyngeal and bowel mucosa, etc. The skin starts to become pink and sore several weeks into treatment. Although this desquamation is uncomfortable but it usually recovers quickly. There might be temporary soreness and ulceration in the mouth (mucositis) and throat if radiation is given to the head and neck area. Dry mouth and a loss of taste can be caused by radiation damage to the salivary glands and taste buds. Patients might need painkillers and nutritional support.
2. Swelling – as part of the general inflammation.
3. Infertility – the gonads are very sensitive to radiation. They may be unable to produce gametes following direct exposure of radiation.
4. Generalized fatigue.

There are 3 main types of radiotherapy. They are:

1. External radiation therapy – ‘external’ means that the radiation source is outside the body. It is used to treat most types of cancer including cancer of the bladder, brain, breast, cervix, larynx, lung, prostate, and etc. It also includes intraoperative radiation therapy (IORT) which is given during surgery to treat localized cancers that cannot be completely removed or that have a high risk of recurring in nearby tissues. External radiation therapy usually is given on an outpatient basis.
2. Internal radiation therapy / brachytherapy – uses radiation that is placed very close to or inside the tumor. The radiation is usually sealed in a small holder called an implant. Implants can be in the form of thin wires, plastic tubes, catheters, capsules, or seeds. They are put directly into the body. Brachytherapy is commonly used to treat localized prostate cancer, cervical cancer and cancers of the head and neck.
3. Systemic radiation / unsealed radiation therapy – uses radioactive materials which may be taken by mouth or injected into the body. For example, radioactive iodine (iodine-131) which is used to treat thyroid disease like thyrotoxicosis and malignant conditions like papillary thyroid cancer.

Conventional external beam radiotherapy is delivered via 2D beams using linear accelerator machines. Conventional refers to the way the treatment is planned or simulated. The aim of simulation is to accurately target or localize the volume which is to be treated. This technique is well established and reliable. The worry is that some high-dose treatments may be limited by the radiation toxicity capacity of healthy tissues which lay close to the target tumor volume. Some refinement had been done and with the help of computed tomorgraphy (CT), magnetic resonance imaging (MRI) and positron emission tomography (PET), or single photon emission computed tomography (SPECT), a 3-D image of a tumor can be obtained. Using the information from the image, special computer programs design radiation beams that conform to the shape of the tumor. These revolutions allows more accurate placement of radiation beams, thus causing less radiological effects on the nearby healthy tissues.

Intensity-modulated radiation therapy (IMRT) is another type of 3-D conformal radiation therapy that uses radiation beams (usually x-rays) of varying intensities to deliver different doses of radiation to small areas of tissue at the same time. The technology allows the delivery of higher doses or radiation within the tumor and lower doses to nearby healthy tissue. The equipment can be rotated around the patient so that radiation beams can be sent from the best angles. The beams conform as closely as possible to the shape of the tumor.

Certain chemicals can be used to modify a cell’s response to radiation. These are called the radiosensitizers and radioprotectors. Some anticancer drugs, such as 5-flurouracil and cisplatin, make cancer cells more sensitive to radiation therapy. Radioprotectors are used to protect normal (noncancerous) cells from the damage caused by radiation therapy. These agents promote the repair of normal cells that are exposed to radiation, for example Amifostine. It helps to reduce the dry mouth that can occur if the parotid glands receive a large dose of radiation.

Although radiation therapy is not 100% safe and effective for cancer treatment, it could be some cancer patients’ only hope for survival or longer lifespan. Radiation therapy will continue to play a major role in cancer treatment until more effective and safe drugs and treatments are found.

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