Physics for Radiotherapy

Physics for Radiotherapy

Learning objectives

On completion of this lesson, you will be able to learn -
  • radiotherapy
  • principles of Radiotherapy
  • short distance Radiotherapy
  • a short course in Radiotherapy treatment planning
  • closing thought on Radiotherapy.

What is Radiotherapy?

Radiotherapy is recognized as an important tool in the treatment of many types of cancer. The probability of having cancer during your lifetime is about 1 out of 4. Each year about 1 million people in the United States get cancer. Currently, three major methods are used alone or in combination to treat cancer. About half of all cancer patients receive radiation as part or all of their treatment. 

The success of radiotherapy depends on -

ūüĎČ the type and extent of the cancer

ūüĎȬ†the skill of the radiotherapist

ūüĎȬ†the physician who specializes in the treatment of cancer with radiation

ūüĎȬ†the kind of radiation used in the treatment

ūüĎȬ†the accuracy with which the radiation is administered to the tumor.

The last factor is the responsibility of the radiological physicist. Before 1950 radiological science was used for many diseases besides cancer, and it is still used occasionally for a nonmalignant condition that does not respond to other treatments. However, because of the possibility of radiation-induced cancer, it is generally safer to use other types of treatment when possible. There is evidence that an error of 5% to 10% in the radiation dose to the tumor can have a significant effect on the result of the therapy. Too little radiation does not kill the entire tumor, too much can produce serious complications in normal tissue.

Principle of Radiotherapy 

The basic principle of radiotherapy is to maximize damage to the tumor while minimizing damage to normal tissue. This is generally accomplished by directing a beam of radiation at the tumor from several directions so that the maximum dose occurs at the tumor. Some normal tissues are more sensitive to radiation than others, and this is taken into account when the therapist and physicist plan the treatment.

A Short Course in Radiotherapy Treatment Planning

The physicist working in radiotherapy has three important functions -

  • To determine how much radiation is being produced by a given therapy machine under standard conditions, that is, to calibrate the machine.
  • To calculate the dose to be administered to the tumor and any normal¬†tissues in the patient. This is not easy, and many radiotherapy¬†departments use computers to aid with this computation.
  • To confirm that the correct amount of radiation was really¬†administered to the patient at the proper location.¬†

The process to determine the best combination of radiation beams and their orientation is called treatment planning. It is usually done cooperatively by the radiotherapist and the radiological physicist. The dosimetrist aids with this process.

Before radiotherapy treatments started, the patient is carefully examined to determine the location and extent of the tumor. In some cases, the therapist will palpate (fell with his finger) the tumor and surrounding tissues. Since many tumors are deep inside the body it is often necessary to use diagnostic X-rays devices that have been developed to assist in these determinations.

Closing Thought on Radiotherapy

Until an effective cure for cancer is found we can expect scientists and politicians to be willing to spend time and money to carry on cancer research. Progress has been made but it has been slow. The combination of the talents of basic scientists and medical scientists has accounted for much of the progress to date. Modern radiotherapy is treating cancer. It will eventually be replaced by a ‚Äú cancer cure‚ÄĚ. We all hope that such a cure will be found, but in the meantime, the proper use of radiation therapy offers many cancer victims their best chance of survival. In some types of cancer, radiation therapy is the treatment of choice. When used in the early stage of the disease it has a cure rate (5 yr survival) of over 90%. The radiotherapist should have access to modern therapy equipment and should work in close cooperation with a radiological physicist to assure the accuracy of the administered dose. The result of a successful radiotherapy treatment can be dramatic.

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