The calculation of absorbed doses for radiobiological studies involving the use of small irradiation fields

In radiobiological experiments involving small animals, small field sizes with large elongation ratios are often used. In the recent study by Hopewell et al (1987) the field size effect on the late tolerance of the rat spinal cord to single doses of X rays was investigated. Field sizes of 90 mm × 16 mm, 90 mm × 8 mmand 90 mm × 4 mm, with respective elongation ratios of 5.63, 11.25 and 22.5, were used. If correct account had not been taken of the effect of field size on the absorbed dose delivered to the rats, then the effects seen would have been over emphasized by as much as 10–20%. In clinical practice this would inevitably lead to increased dose reductions for the treatment of large volumes or larger increases in dose for smaller volumes with the associated rise in clinical complications. Other investigators (Berg & Lindgren, 1963; Albert et al, 1967; Knowles & Trott, 1988) have used smaller, slit-shaped fields than those used by Hopewell et al (1987), in which practical dose measurements even with lithium fluoride (LiF) rods (6 mm × 1 mm) would be difficult, if not impossible. The present report describes a method for the calculation of absorbed doses in small irradiation fields in the hope that a standard method of dosimetry for radiobiological experiments with small fields will be used by all workers.