The potential of californium-252 in radiotherapy

Californium-252 is a man-made radionuclide (half-life 2.65 years) which emits a mixture of neutrons and γ-rays. It is used in radiotherapy as an alternative to radium and extends the potential benefits of neutrons to interstitials and intracavitary applications. γ-rays account for a variable proportion of the dose (30 to 50%), depending on the source filtration and the distance from the source. Dosimetry is complicated by this mixture and neutrons and γ-rays. However, measurements with paired ion-chambers, together with Monte-Carlo calculations, have produced dosimetric data that are adequate for clinical use. Many determinations of the oxygen enhancement ratio (OER) have been reported. At the low dose-rates characteristic of interstitial implants, the OER is about 1.5. This is essentially the figure for fast neutrons alone, since at very low dose-rates the contribution of the γ-rays to the biological effect is negligible. As the dose increases, there is a corresponding rise in the OER because the γ-ray contribution can no longer be ignored. The OER is likely to be about 1.8 if 252Cf is used in intracavitary treatments and 2.0 if used in 'acute' exposures in devices such as the Cathetron. The relative biological effectiveness (RBE) varies with dose-rate, and with the biological system used to measure it. Radiobiological experiments indicate that 6,000 rads of radium γ-rays in seven days is equivalent to 890 rads of 252Cf neutrons delivered in approximately the same overall time. This figure was suggested some years ago as an interim guide-line until sufficient clinical experience is accumulated. (author)