The design of a 1500 curie Co60 teletherapy unit
Bates, Lloyd M.
The fundamental problem in the treatment of tumors in the interior of the body is to give the tumor a lethal dose in a manner in which healthy tissue is not irreparably damaged. If radiation is directed at the tumor from an external source, there are two procedures which aid in achieving this aim. The first is to use a high energy radiation. This tends to lower the absorption of radiation by tissue between the source and the tumor. Consequently a higher percentage of radiation incident on the skin of the patient reaches the tumor. The second method is to use a focal skin distance sufficiently larger to make the “inverse square” reduction of intensity between skin and tumor practically negligible. Originally, high energy X-Ray machines were used entirely for deep therapy. However, the machines that produce voltages greater than above 750 kv. Become unwieldy if not stationary because of their large size. Radium, which is a source of high energy photons, was originally used in therapy by applying it directly to the tumor where possible, or at a point near the tumor. However, on removing the Radium some distance from the patient (teletherapy) and allowing a beam of its radiation to be directed at the tumor, the two techniques mentioned above could be used. Also the radium unit would have a flexibility not possible with electrical machines. The great disadvantage of this would be the lower output of a unit using the amounts of Radium normally found at a therapy centre. If enough Radium could be accumulated to bring the output up to near that of an X-ray machine, the physical dimensions of the source would be so large that its penumbral effects would be severe. The output in roentgens of a teletherapy unit is a direct function of the activity of the source. If we define specific activity of a source as its activity per gram, then the specific activity of Radium is 1 curie per gram. Other substances having a much higher specific activity can be artificially produced by nuclear reactors having a high neutron flux. Co60 which emits photons having about 1 mev. energy, could be produced with specific activities up to 25 curies per gram. A source with a specific activity of this order, housed in a suitable unit, would then provide a flexible, high energy machine, with output great enough to be used as a teletherapy unit. In addition the source would have dimensions small enough that penumbral effects would be tolerable. Teletherapy units using Radium as their source of radistion have been in use for some time (3) (5) (8), and at least one unit using an artificially produced radioactive isotope has been constructed (4). This unit used Iridium of comparatively low specific activity as its source. The Co60 Unit, now installed in the University Hospital, Saskatoon, was designed to use 1500 curies of Co60 as its source. The following is a complete description of the unit. It includes photographs of detail drawings and verbal descriptions of all components. Blue prints of the drawings are available at the Physics Department. A summary of stray radiation measurements is given in Section XVIII. In addition, any significant calculations are shown. All drawings and figures will be found at the end of the thesis. In addition three photographs of the unit are included. Numbers in brackets refer to bibliography.