CONSTRAINTS IN THE USE OF REPAIR HALF TIMES AND MATHEMATICAL-MODELINGFOR THE CLINICAL-APPLICATION OF HDR AND PDR TREATMENT SCHEDULES AS ANALTERNATIVE FOR LDR BRACHYTHERAPY

Using theoretical models based on radiobiological principles for the d esign of new treatment schedules for HDR and PDR brachytherapy, it is important to realise the impact of assumptions regarding the kinetics of repair. Extrapolations based on longer repair half times in a conti nuous LDR reference scheme may lead to the calculation of dangerously high doses for alternative HDR and PDR treatment schedules, We used th e clinical experience obtained with conventional ERT and LDR brachythe rapy in head and neck cancer as a clinical guideline to check the impa ct of the radiobiological parameters used. Biologically equivalent dos e (BED) values for the in clinical practice of LDR brachytherapy recom mended dose of 65-70 Gy (prescribed at a dose rate between 30-50 cGy/h ) are calculated as a function of the repair half time. These BED valu es are compared with the biological effect of a clinical reference dos e of conventional ERT with 2 Gy/day and complete repair between the fr actions. From this comparison of LDR and ERT treatment schedules, a ra nge of values for the repair half times of acute or late responding ti ssues is demarcated with a reasonable fit to the clinical data. For th e acute effects (or tumor control) the best fits are obtained for repa ir half times of about 0.5 h, while for late effects the repair half t imes are at least 1 h and can be as high as 3 h. Within these ranges o f repair half times for acute and late effects, the outcome of 'altern ative' HDR or PDR treatment schedules are discussed. It is predominant ly the late reacting normal tissue with the longer repair half time fo r which problems will be encountered and no or only marginal gain is t o be expected of decreasing the dose rate per pulse in PDR brachythera py.