Salient: Victoria University Students' Paper. Vol. 29, No. 6. 1966.
Radiotherapy
Radiotherapy
In the therapeutic application of X-rays and radioactive materials advantage is taken of the fact that radiation can destroy undesirable tissue. Broadly speaking, cells are; more vulnerable to radiation; in the growing stage. Since very active cell growth is a characteristic of cancerous tissue it follows that in many cases a destructive dose of radiation may be applied to the tumour without permanently harming the surrounding nealthy tissue. There are in general three methods by which radiation is applied for treatment.
A. By directing a beam of radiation into the tumour; from an external source.
B. By inserting a sealed source of radioactive material into the tumour.
C. By introducing unsealed radioactive materials into the body, either orally or, by injection.
A: Therapy using an external source
In this method of treatment a sharply defined beam of penetrating radiation is directed at the tumour. The successful application of external, beams of radiation demands a detailed knowledge of radiation physics. Over the years ingenious techniques have been developed to increase the tumour dose and to reduce the! exposure to the surrounding healthy tissue.
A method commonly used is to rotate the beam so that it is always aimed at the tumour which is to be destroyed. For the treatment of tumours located deep in tissue highly penetrating gamma radiation from radioactive cobalt-60 or X-rays generated at high; kilovoltages are used. For the treatment of skin conditions and tumours close to the surface of the body, low or medium energy X-rays, or beta or gamma emitting radioisotopes in the form of surface applicators are used. Radioactive strontium-90 is the most common beta emitter for surface application and is useful in eye applicators in which the limited penetrating power of beta radiation is an important factor in avoiding damage to the underlying radiosensitive tissues.
B: Insertion of a sea'el radioactive source into the body.
The treatment of various types of cancer by radium needles and tubes has been carried out for many years. The radium is inserted into the tumour by surgical means according to a predetermined plan to give the desired radiation dosage distribution. On completion of the treatment the radium is withdrawn. Radioactive cobalt-60 has been used as an alternative to radium. Among other similar treatment techniques is the permanent implantation of small radioactive "seeds." For these techniques radioactive materials which lose their radioactivity rapidly must be used. Radioactive gold, radioactive yttrium and gold encapsulated radon are commonly used for permanent implantation.
C: Introduction of unsealed radioactive materials into the body.
There are only two main procedures for this purpose. The better-known method alms at radioisotope localisation through metabolism since it results in selective absorption of the radioactive atoms inta the cells of the tissue or malignant growth to be treated. In this procedure radioactive iodine which concentrates in the thyroid gland is used successfully for the treatment of thyroid cancer, while radioactive phosphorus is used to suppress excess red cell formation in the blood and for certain types of chronic leukaemia and bone tumours. The other procedure achieves localisation by means of suspensions of nonsoluble particles of the radioisotope injected within a selected part of the organ to be treated. The radioisotope is used merely as an emitter of radiation and almost no account is taken of its biochemical property. Colloidal radioactive gold and colloidal radioactive yttrium are used for this form of therapy to reduce the spread of malignant tumours, particularly those in the abdomen and lung.
The procedures discussed represent some of the more important applications of Xrays and radioactive materials In medicine. In this rapid'y expanding field opportunities abound for graduates in medicine, physics, biology, or biochemistry, for the specialised study of the application of X-rays and radioactive materials which will enable them to contribute to the betterment of man's health and well-being.
Acknowledgements: The writer acknowledges with thanks the information supplied by a number of New Zealand medical physicists. The photographs were supplied through the courtesy of the Auckland Hospital Radioisotope Laboratory.
This article is published with the authority of the Director, National Radiation Laboratory. Department of Health, Christchurch.
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Console of whole-body scanner used to measure the distribution of radioisotopes within the body.