High dose rate fractionated interstitial radiation as an alternative to low dose rate continuous interstitial radiation

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To investigate the high dose rate fractionated interstitial radiation (F-HDR) as an alternative to low dose rate continuous interstitial radiation (C-LDR), following studies were carried out.1) Theoretical investigation using LQ model for the F-HDR in uneven fractionation and uneven interval was carried out. Using LQ model, Nominal standard effect was calculated for F-HDR.For late effect, 70 Gy/7 days were considered as the tolerance dose in the former studies. However, 70 Gy was the standard treatment dose, and 84 Gy was considered as the tolerance does in this study. Accoding to this change, for the tumor LQ model had the same result of the clinical data. However, for the late effect, alpha/beta must be considered as more than 3 that was used in the previous srudies.2) Biological study using colony formation assay for SQ5 cell revealed the relative biological effectiveness (HDR/LDR,RBE_<H/L>) of 0.77. Spheroid cells showed the relatively radioresistance at 37ﾟC.According to the data of the SCD50 (50% spheroid cure dose) , the RBE_<H/L> of the spheroid cells was calculated as 0.87. This value was almost the same of the clinical data of the early tongue cancer (60 Gy in F-HDR vs 70 Gy in C-LDR) . In F-HDR radiation, RBE was decided only by total dose. Modification of the total dose was not needed in the different fractionations for F-HDR.3) We carried out the randomized clinical trial of the F-HDR vs C-LDR for the patients with the early tongue cancer. patients were treated with 70 Gy/1 week in C-LDR group and with 60 Gy/10 fractions/6 days in F-HDR.Local recurrence occurred in 2 of 15 patients treated with C-LDR,and in none of 13 patients treated with F-HDR.This clinical trial also revealed that 70 Gy of the F-HDR was almost the same as 60 Gy of C-LDR.The theoretical model, biological study and clinical study showed the same results. We concluded the F-HDR is an alternative to C-LDR.