Conditions for the equivalence of continuous to pulsed low dose rate brachytherapy

Low dose rate interstitial brachytherapy is extremely useful for those tumors that are accessible for an implant, while the introduction of remote afterloaders has eliminated exposure to nursing personnel. Currently, such machines require an inventory of many sources which are loaded into catheters implanted in the tumor and kept in place during treatment. A significant simplification of such machines would be possible in a pulsed mode, with a single source moving under computer control through the catheters. Assuming that the treatment time and average dose rate are kept unchanged, the question addressed is to find those combinations of radiation pulse widths and frequencies that would be functionally equivalent to a continuous irradiation. The linear-quadratic formalism was used to reanalyze published low dose-rate studies on cells of human origin to obtain 36 parameter sets [alpha, beta, T1/2], where T1/2 is the half time for sublethal damage repair. These data are consistent with those for human tumors. For each parameter set, those combinations of pulse width and frequency were calculated that would yield a functionally equivalent cell survival. For a regimen of 30 Gy in 60 hr, a pulse width of 10 min with a period between pulses of 1 hr would be appropriate for all the cell lines considered. Similar results were found for other possible time/dose combinations. For late effects, a 1-hr period between 10-min pulses might produce up to a 2% increase in late-effect probability, which is probably acceptable for the small volumes irradiated in interstitial brachytherapy.