Gamma emitters

Radium was used for many years as the major source of gamma rays for brachytherapy. This is now obsolete. The major source of gamma rays is the gaseous daughter product, radon. Radium tubes and needles must be gas-tight and frequently checked for leaks. The gamma rays used are very penetrating and very thick lead shields are required to provide adequate radiation protection. Caesium-137 has no gaseous daughter products, a very useful half-life of 30 years, and a somewhat less penetrating 660 KeV gamma ray—it has largely replaced radium, especially for gynaecological work.

Iridium-192 is manufactured in the form of flexible wire and has many advantages over traditional radium or caesium needles for interstitial brachytherapy. Thin wires (0.3 mm in diameter) can be inserted into flexible nylon tubes or after-loading needles previously implanted into the tumour. Thicker wires (0.6 mm diameter), in the form of hairpins, can be inserted directly into a tumour through suitable introducers. In the USA, iridium is also available in the form of seeds sealed in thin plastic coating. Iridium produces a gamma ray of 330 KeV and lead shields 2 cm in thickness provide good protection. The only major disadvantage of iridium is the relatively short half-life (74 days), so that fresh material should be used for each implant.

Iodine-125 has a half-life of 59.6 days and is used for permanent implants of the prostate. As well as having a relatively short half-life, the gamma rays produced by this radionuclide (27-35 KeV) are of very low energy and very little radiation is emitted from a patient following the implant, allowing early discharge from hospital.

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