Alpha-rays are not employed in focal curietherapy, as their penetrative power is very small.
General Technical Principles.—Dominici (1907) showed that equal efficacy is obtained through a greater thickness of tissue if the beta-rays and the less penetrative part of the gamma rays are stopped by suitable filtration. This most important discovery led to the use of radium or radon foci enclosed in heavy metal cases, which filter all but the gamma-rays.
Radium is employed in the form of powdered sulphate (insol uble) ; with this salt the manufacturer completely fills cylindrical tubes. The length and diameter of these tubes are proportionate to the content desired. The powdered sulphate of radium is diluted with a suitable quantity of sulphate of barium, when a tube has to be completely filled with a small dose. Usually the tubes are made of platinum. The cavity is generally o.5 or 1 mm. in diameter and from to to 3o mm. in length, and the wall of the tube 0.5 or I mm. thick. The strength of the filter can be increased as desired by the addition of a detachable case made of gold, lead, etc. The content of the tube is calculated in terms of radium element ; for ordinary purposes it varies from 2 to 15 milli grammes. The tube is closed with a platinum stopper hermetically sealed with gold. One end of the tube is rounded ; the other has a needle-eye through which a thread can be passed if necessary.
The radioactivity of the tubes is not constant unless they are air tight and it should therefore be measured from time to time. Series of tubes are made containing the same weight of radium —i.e., having the same power—and interchangeable. These series of sources of radiation can be used to make up radiant surfaces of any size, shape and power desired.
For radium puncture (see p. 909), hollow needles with iridio platinum points are used. It is not advisable to charge these needles with more than I or 2 milligrammes of radium element. Their wall-thickness varies from 0.3 to o.5 mm.
Radon is generally contained in glass capillary tubes, and these tubes serve the same purposes as radium in tubes and needles.
When the radiation has to act through normal tissues (such as the skin or a mucous membrane) which it is important to protect, each platinum tube is enclosed in a secondary filter, in the form of a casing of some substance of low density containing no heavy atoms (aluminium, rubber, cork, wax, etc.), so that the secondary burning rays emitted by the denser metal of which the tubes are made may be absorbed without any harmful addi tional radiation.
The aim of all processes of radiumtherapy should be to admin ister a uniform proportion of radiation throughout the volume of tissue to be treated (homogeneous radiation). Equality of radia tion is hindered, however, by two phenomena—the dispersion of the rays, and their absorption.
Dispersion weakens the radiation from a punctiform focus in inverse proportion to the square of the distance. Its effects may be diminished by increasing : (a) the distance from the focus; (b) the radiant surface; (c) the number of positions of the focus (round or within the space to be radiated: "cross fire").
Absorption weakens radiation proportionately to the lowness of its penetrative power and to the thickness of the tissues. The injurious effects of unduly unequal absorption may be palliated by filtration.
The strength of the radiation is a matter of great importance. Excessive strength sets up radium necrosis, an accident of radium therapy which is often serious.
The strength of the radiation emitted can be measured with great accuracy by its constancy (radium) or its diminution ac cording to a mathematical formula (radon), and is expressed either by the weight of radium or quantity of radon used and the length of the period of application (i.e., milligramme-hours, milli curie-hours), or by the quantity of radon destroyed during appli cation (i.e., millicuries destroyed). The notation of the other circumstances of the application (filtration, distance, etc.) should be associated with that of the strength of radiation emitted.