Room: AAPM ePoster Library
Purpose: evaluate the sub-lethal damage (SLD) repair effect in prolonged proton irradiation with various cell specific parameters of a/ß and T1/2 (repair half time). While the previous research using the biophysical model found the larger repair effect with higher physical dose, the dependence of the repair effect on cell specific parameters has not been evaluated systematically.
Methods: biological dose calculation was carried out using the linear energy transfer (LET)-dependent linear quadratic model combined with the theory of dual radiation action. Spread out Bragg peak (SOBP) irradiation in water phantom was simulated with wide ranges of cell specific parameter sets (a/ß=1-15 Gy, T1/2=0-90 min). In addition, clinical cases of prostate, liver and lung tumor were examined with the cell specific parameters for each tumor obtained from literature. D99 and D5?95 were calculated, assuming the dose rate structures with the total irradiation time of 0-60 min.
Results: of the cell specific parameters resulted in a large variation of the repair effect. Biological dose reduction found at the center of SOBP with 30 min of continuous irradiation was 8.8% with a/ß=1.5 Gy and T1/2=20 min and 1.2% with a/ß=10 Gy and T1/2=80 min, respectively. With T1/2=19 min, biological D99 decrease per 10 min was 2.6%, 1.2%, 3.0% for prostate, liver, lung case, respectively. The values of the D99 reduction was neither in the order of a/ß nor prescribed dose, but both of them comparably contribute to the repair effect. The variation of D5?95 was within the range of 0.5% for all the cases, therefore, dose distribution was not distorted.
Conclusion: biological dose reduction caused by the SLD repair largely depends on the cell specific parameters in addition to the physical dose. The parameters should be included carefully in the evaluation of repair effect in prolonged proton irradiation.