小線源治療の線量率と分割の生物モデルの再考に向けて

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  • RECONSIDERATION OF A BIOLOGICAL MODEL FOR DOSE-RATE AND FRACTIONATION IN BRACHYTHERAPY

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Clinical results of HDR brachytherapy show better than those of the LDR one. Most radiation biologists could not predict these results. At the beginning, they presumed that LDR brachytherapy was one of the best treatment modalities for cancers of the uterine cervix, mobile tongue and others. They criticized that HDR brachytherapy was a harmful modality from a biological standpoint. They proposed equations such as Nominal Standard Dose (NSD), Time Dose and Fractionation factor (TDF), Cumulative Radiation Effect (CRE) to be mandatory for the biological dose-rate conversion from LDR to HDR brachytherapy. However, these equations had not included the parameter of treatment volume that was one of the most valuable factors. Moreover, these equations consisted of parameters that were obtained from clinical results, not from the biological experiments. Instead of these biological equations, the new concept of the linear-quadratic (LQ) model was proposed. Effects of dose-rate for LDR brachytherapy and fractionation for HDR could be considered, however that of treatment volume could not be considered. In the classical Manchester system for brachytherapy, dose-rate was mainly affected by the treatment volume, because of the standard application of the Radium employed. The original prescribed dose in LDR brachytherapy of interstitial and intracavitary technique for cancers of the mobile tongue and uterine cervix using Paris and Manchester systems was 60 Gy over 144 to 168 hours. Although the prescribed dose was apparently changed by dose-rate, however, the variety of dose rate originally resulted from the size of the treatment volume in clinical practice. In addition, the 60Co from a high intensity source and the new 192Ir micro-source of relatively short half-life changed these rules. However, these sources enable us to use HDR brachytherapy with more accuracy. Now we need a new biological model with original treatment parameters obtained from the modern HDR brachytherapy itself.

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