Comparison of Photon Versus Carbon Irradiation in Pancreatic Cancer

Abstract

there is no other adequate dose-conversion model. The LQ model is, however, considered unreliable in converting hypofractionated doses to single doses. The purpose of this study was to evaluate the applicability of the repairable conditionally-repairable (RCR) model [S Z exp(-aD) + bDexp(-cD)] and the multi-target (MT) model {S Z exp(-D/d1)[1-(1exp(-D/d0)) ]} to high-dose-per-fraction radiation therapy in comparison with the LQ model. Materials/Methods: Data previously obtained for V79 and EMT6 single cells and V79 spheroids were used. The single cells received single doses of 2-12 Gy, or 2 or 3 fractions of 4 or 5 Gy each at 4 h intervals. V79 spheroids received single doses of 5-26 Gy or 2-5 fractions of 5-12 Gy at 2 or 4 h intervals. Single and fractionated doses to actually reduce cell survival to the same level were determined by a colony assay. Single doses used in the experiments and surviving fractions at the doses were substituted into equations of these models on a calculation software and then, each parameter coefficient was calculated by carrying out the optimum convergence to approximate the measured survival curve. Thereafter, using the RCR, MT and LQ models, each parameter coefficient and equivalent single doses for the hypofractionated doses were calculated. They were then compared with actually-determined equivalent single doses for the hypofractionated doses. Results: The a/b ratio was 5.1 Gy for V79 single cells and 0.36 Gy for EMT6. The three parameter coefficients of the RCR model (a, b and c) were 18.5, 1.6 and 0.7, respectively, for V79 and 17.7, 5.1 and 1.0, respectively, for EMT6. Those of the MT model (d1, d0 and n) were 0.17, 0.5 and 9.8, respectively, for V79 and 0.1, 0.9 and 80.6, respectively, for EMT6. The equivalent single doses for the hypofractionated doses calculated using the RCR, MT and LQ models were lower than the measured biologically-equivalent single doses by 6-22%, 6-26% and 18-30%, respectively, at 6 Gy or higher doses in the single cells and spheroids. Although all these models underestimated the effect of 2to 5-fraction irradiation at all dose levels, the deviation was more marked when the LQ model was applied to 6 Gy or higher doses. Conclusion: The RCR and MT models could be used for biological dose estimation using the software. The LQ model may only be applicable to doses of 5 Gy or less. The conversion models based on the RCR or MT models like the universal survival curve and generalized linear-quadratic models seem to be more reliable than the LQ model at 6 Gy or higher doses. The LQ model should not be used in hypofractionated stereotactic radiation therapy. Author Disclosure: H. Iwata: None. N. Matsufuji: None. T. Toshito: None. T. Akagi: None. S. Otsuka: None. Y. Shibamoto: None.