ANALYSIS OF THE RADIOBIOLOGY OF YB-169 AND I-125 PERMANENT BRACHYTHERAPY IMPLANTS

Recently, Yb-169 has been considered as a potential replacement for I- 125 and Pd-103 in permanent implants. In spite of the uncertainties in the parameters necessary for an accurate radiobiological modelling, t he linear quadratic model can be useful in the comparative evaluation of the radiotherapeutic merit of similar implants. In order to find ou t if a Yb-169 permanent implant can be made biologically 'equivalent' to an I-125 implant, we studied the dependence of local control on the tumour cell radiosensitivity and on the balance between the rate of t umour cell killing and tumour cell proliferation, for rapidly and slow ly proliferating tumours. The extrapolated response dose (ERD) has bee n calculated for tumour and late reacting normal tissue for both types of implants and the possible biological restrictions due to the norma l tissue tolerance have been discussed. Our theoretical analysis is co nsistent with the clinical results published for I-125 permanent impla nts in prostate tumours and meningiomas. It predicts that Yb-169, whic h has only recently been used in human tumours, can provide comparable tumour control for permanent implants in slowly proliferating tumours with an initial dose rate of 13 cGy h(-1). Control might be extended to rapidly proliferating tumours by increasing the initial dose rate w ithin a range consistent with an acceptable level of normal tissue lat e reaction.