Physics

Inside-out of radiotherapy

Every cell in our body follows certain rules. The number of cells in the body increases through division. Cell division is a regular occurrence in the body. Usually a cell does not divide after dividing a certain number of times. Then the cell dies. All the cells in the body have the same rules.

But if for some reason the DNA structure of the cell becomes random, the regulation of the cell may change. Then some cells continue to divide and turn into cancer. Why and how a normal cell turns into a cancer cell is not yet known. However, it is known that the chances of getting cancer increase. Scientists have divided these factors into three main categories: physical carcinogens, chemical carcinogens, and biological carcinogens.

Physical carcinogens are mainly ionizing radiations — such as X-rays, gamma-rays, ultraviolet rays etc, that enter the body and damage DNA. Our body’s repair mechanism sometimes makes mistakes when it comes to repairing injured DNA. Then some cells can turn into cancer cells. Many chemicals can be named in the list of chemical carcinogens. Cigarette smoke, arsenic, asbestos, toxic chemicals from factories etc.

Biological carcinogens can be viruses, bacteria, unicellular parasites, etc. The human papilloma virus has been shown to cause cancer. People are still vulnerable to cancer because the exact cause of cancer is not known. The average life expectancy is increasing. At the same time, the number of cancer patients is increasing. Now the chances of getting cancer in old age are much higher. One in four women over the age of 65 is at risk for cancer. And one in three men of the same age is at risk for cancer. With the advancement of science, the treatment of cancer has also improved a lot. Yet about one crore people die of cancer every year.

Early diagnosis can greatly increase a patient’s chances of survival. We have seen in the previous chapters how X-rays, CT, MRI, mammography, nuclear medicine, etc. Play a role in diagnosing cancer. We have already discussed the role of nuclear medicine in the treatment of cancer. Physics plays a major role in radiotherapy in the treatment of cancer. About 80 percent of cancer patients receive radiotherapy at some point in their treatment.

The main goal of radiotherapy is to kill cancer cells without causing any damage to normal cells through radioactive radiation. But the cancer cells in the body somehow surround the normal cells. As a result, when cancer cells are killed, normal cells also die. If too many normal cells die, the opposite can happen. This can lead to paralysis of the limbs, or the onset of secondary cancer in response to radiation in normal cells. On the other hand, if not all of the cancer cells die and only a few remain — new cancer cell colonies can form from them. As a result, the whole treatment failed.

Treatment is based on a number of factors, including the location, stage, age, and overall health of the cancer tumor. In some cases radiotherapy may be the only treatment, in some cases radiotherapy is given after chemotherapy. In some cases, radiotherapy is given before the cancerous tumor is removed to shrink the tumor to facilitate surgery. In many cases radiotherapy is given even after cancer tumor surgery to prevent any cancer cells from surviving. In many cases, low-dose palliative radiotherapy is given to reduce the patient’s pain.

According to the method of application of radiotherapy, radiotherapy can be divided into two main types — teletherapy or external radiotherapy and brachytherapy or internal radiotherapy. If radiation is applied remotely without touching the patient’s body, it is called teletherapy or external radiotherapy. And if the source of the radiation is placed inside the patient’s tumor, it is called brachytherapy or internal radiotherapy.

If the oncologist advises to give radiotherapy, then radiotherapy is arranged for the patient. The cancer hospital first collects detailed information about the location, size and other conditions of the tumor by CT scan of the patient. Treatment is then planned so that more radiation does not go outside the tumor, and most cancer cells and least normal cells die. There is no doubt that giving too many doses of radiation at once will kill all the cancer cells quickly, but at the same time normal cells will also die. So do not overdose at once. The total dose is divided into several doses tolerable for normal cells at regular intervals (24 hours or 48 hours). Normal cells that are injured in response to radiation can heal themselves after a short break. But the ability of cancer cells to repair is less than that of normal cells. As a result, more cancer cells die than normal.

External radiotherapy is given primarily through the application of high-energy X-rays, gamma-rays, or electrons. Gamma rays are applied with a cobalt machine, X-rays and electrons are applied with a linear accelerator. Recently, proton radiotherapy is also being given in different countries like USA, Japan, Germany, Korea etc. Each of these methods has a different physics.

Gamma-ray radiotherapy

Cobalt machines have been using radiotherapy very effectively since the introduction of external radiotherapy in the 1950s. The treatment head of this machine contains the cobalt-60 isotope from which 1.25 million electron-volt energy gamma rays are emitted. This gamma ray is injected into the patient’s body according to the treatment plan. Cobalt-60 nuclear isotope. It always radiates gamma rays. This isotope is in a thick lead coating in the treatment head so that radiation cannot come out. During the course of radiotherapy, a window with a lead cover opened towards the patient. The area of ​​this window depends on the size of the tumor.

In cobalt machines the patient is given gamma rays only through rectangular or square windows, but the tumors come in different shapes. This results in the death of many more normal cells. Because the machine always produces radiation, the cobalt spreads more radiation from the machine. The half-life of the cobalt isotope is 5.3 years. The amount of radiation from the radiation source decreases all the time. As a result, the amount of radioactivity has to be calculated before each radiotherapy is given, and the duration of radiotherapy increases accordingly. The use of linear accelerators instead of cobalt-60 machines has been on the rise since the invention of the Linear Accelerator, a megavoltage X-ray machine in the 1970s.

X-ray radiotherapy

Since the introduction of X-rays in medicine, X-rays have been used in imaging as well as therapy. X-rays with a maximum power of 150 kPa are used for radiography. X-rays of this energy are also used to treat some tumors. This is especially true of skin cancer tumors — such as superficial Esc-ray therapy for the removal of tumors on the face, ears, nose, forehead, or shoulders. Since the strength of the X-ray beam is low, X-rays have to be applied for a long time.
Deep radiotherapy is given using X-ray power of 150 to 400 kW-electron volts. Such X-ray tubes run between one and a half million to four million volts between the anode and the cathode. Palliative radiotherapy is given with deep X-ray radiotherapy to reduce the patient’s pain if the cancer has spread to the bones.

Kilovoltage X-rays (superficial and deep) can be applied relatively easily. But it absorbs a lot more radiation on the patient’s skin. The patient’s skin may burn because it has to be held for a long time. This X-ray cannot reach very deep into the skin. As a result, very good results are not obtained in the treatment of tumors that are deep in the skin. This problem can be solved by inventing the method of preparing X-rays of megavoltage X or X-ray of million electron volt energy.

It is not possible to produce mega volt X-rays in the same way as ordinary X-rays. This is because the maximum amount of voltage that is applied through the X-ray tube can produce X-rays of that energy. For example, if you run 120 kilovolts, you can get X-rays with a maximum of 120 kilowatts of electron volts. In the ninth chapter we saw how X-rays are generated. To produce mega voltage X-rays in that way, X-ray tubes have to run millions of volts, which is not possible with X-ray generators. The mechanism for producing megavolt X-rays is through linear accelerators. The use of the medical linear accelerator began in 1953 in London. The following year began in the United States.

The application of high voltage during X-ray production results in a strong velocity of electrons in the filament. The faster the electron pushes the anode or target, the more powerful X-rays are generated. Instead of applying a million volts to a linear accelerator, the speed of the electron is increased in a different way. Here the acceleration of electrons is increased by creating vibrations through radio waves. Electrons emitted from the electron gun of the accelerator travel through a long tube (wave guide) towards the tungsten target on the other side. Then radio waves are applied in this tube. A radio wave of about three megahertz moves the electrons of the wave guide three million or thirty million times per second. It’s a lot like rocking. After swaying so hard that the electron pushes the tungsten target so hard that it produces X-rays of millions of electron volts. The linear accelerator head — called the treatment head — has two types of filters. As the X-rays pass through this filter, the energy of all the X-rays in the X-ray beam becomes almost the same. Multileve collimator can be installed on the treatment head. Any shape of X-ray beam can be given through this multileaf collimator. X-ray beams can be applied to the tumor according to the size and shape of the tumor.

Radiation delivery of modern radiotherapy is entirely computer controlled. The patient has to be set on the treatment couch in such a way that the X-ray beam can enter the tumor directly. The linear accelerator can rotate 360 ​​degrees around the tumor. The treatment plan calculates exactly how much radiation will be applied from which side. If the plan goes wrong, the patient may get the extra radiation he needs. This can lead to the death of many more normal cells. Which is undesirable. There are many types of radiotherapy that apply a specific dose of X-ray to specific target areas. For example, three-dimensional conformal radiation therapy (3D-CRT), intensity modulated radiotherapy (IMRT), image guided radiotherapy (IGRT), etc.

Proton radiotherapy

One of the major drawbacks of X-ray radiotherapy is that after the radiation reaches the tumor, the excess radiation damages the normal cells as it exits the tumor in the opposite direction. So some tumors carry a lot of risk when given radiotherapy. For example, when an X-ray is applied to an eye tumor or a brain tumor, the X-ray pierces the other cells beyond the tumor. There is no way to stop the spread of X-ray doses. But in the case of protons, it has been found that the proton beam can be applied to the target very precisely, all doses stop there. So the proton beam is more effective in treating some cancers than the X-ray beam. Such as in the case of radiotherapy for eye and brain tumors. If there is a tumor near the brain stem and nervous system, proton therapy is better than X-ray therapy.

Procton beams require much larger synchrotrons or cyclotrons than linear accelerators. The cost is very high. It costs at least 100 million to set up a proton machine.

Internal radiotherapy

In internal radiotherapy or brachytherapy, radioisotopes are placed in the patient’s body near the tumor or inside the tumor. These radio-isotopes emit radioactive radiation and destroy tumor cancer cells. Different types of isotopes are applied in different doses considering the overall condition of the tumor and the patient. Brachytherapy is usually applied to tumors of the uterus, uterus, prostate, etc. Brachytherapy involves radiation from the patient’s body. So as long as this radioactivity is present the patient should be kept as separate as possible.

Brachytherapy

Radiotherapy has some side effects. Normal cells also absorb some radiation during radiotherapy. It can cause dizziness, nausea, loss of appetite, rough skin, cracked skin, frequent sores, hair loss, dry mouth and throat, and dental problems. There may be problems swallowing or chewing food, there may be chest pain, there may be stomach problems, there may be diarrhea, there may be urinary problems. The patient has to endure these side effects to get rid of the cancer.

Cancer treatment is improving with the advancement of science and technology. It is hoped that in the near future the genes responsible for cancer will be identified using the human genome. Then there is no doubt that cancer treatment will save the lives of many more patients.

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