Radium needle spacing effect on the dose distribution of planar implants

The dose distribution of planar radium implants consisting of a number of parallel needles (PN) with a crossing needle (CN) at one end was studied. The fraction of the implant area receiving 85% of the maximum dose is plotted against the distance of the CN beyond the active ends of the PN, for different PN spacings. The optimum dose distribution occurs when the CN is located at the physical ends of the implant. Reduced PN spacing increases the dose rate at 85% of maximum dose and improves the dose coverage over the whole range of CN locations plotted. (author)