In vivo radiobiological characterization of proton beam at the National Cancer Center in Korea: effect of the Chk2 mutation.

PURPOSE: The relative biological effectiveness (RBE) in the presence or absence of CHK2 was estimated at the Korean National Cancer Center Proton Therapy Center (NCCPTC). METHODS AND MATERIALS: The proton beam was fixed at 210 MeV with 6-cm spread-out Bragg peaks (SOBPs) because this is expected to be the most frequently used clinical setting. X-rays were obtained using a 6-MV conventional linear accelerator. The RBE was estimated from the survival of jejunal crypt in C3H/He and Chk2(-/-) mice. RESULTS: The estimated RBEs of the NCCPTC at the middle of the SOBP were 1.10 and 1.05 in the presence and absence of CHK2, respectively. The doses that reduced the number of regenerated crypt per jejunal circumference to 20 (D(20)) in C3H/He mice were 14.8 Gy (95% confidence interval [CI], 13.7-15.9) for X-rays and 13.5 Gy (95% CI, 14.5-15.5) for protons. By contrast, the doses of D(20) in Chk2(-/-) mice were 15.7 Gy (95% CI, 15.0-16.4) and 14.9 Gy (95% CI, 14.0-15.8) for X-rays and protons, respectively. CONCLUSIONS: The RBE of the NCCPTC is clearly within the range of RBEs determined at other facilities and is consistent with the generic RBE value of 1.10 for 150- to 250-MeV beams. The mutation of Chk2 gave rise to radioresistance but exhibited similar RBE.

Relative biologic effectiveness determination in mouse intestine for scanning proton beam at Paul Scherrer Institute, Switzerland. Influence of motion.

PURPOSE: To determine the relative biologic effectiveness (RBE) of the Paul Scherrer Institute (PSI) scanning proton beam in reference conditions and to evaluate the influence of intestine motion on the proton dose homogeneity. METHODS AND MATERIALS: First, RBE was determined for crypt regeneration in mice after irradiation in a single fraction. Irradiation was performed at the middle of a 7-cm spread out Bragg peak (SOBP; reference position), as well as in the proximal part of the plateau and at the distal end of the SOBP. Control gamma-irradiation was randomized with proton irradiation and performed simultaneously. Second, motion of mouse intestine was determined by radiographs after copper wire markers had been placed on the jejunum and intestinal wall. RESULTS: Proton RBE (reference (60)Co gamma) was equal to 1.16 for irradiation at the middle of the SOBP and to 1.11 and 1.21 for irradiation in the initial plateau and end of the SOBP, respectively. The confidence intervals for these RBE values were much larger than those obtained in the other proton beams we have tested so far. They exceeded +/-0.20 (compared with the usual value of +/-0.07), which resulted from the unusually large dispersion of the individual proton data. The instantaneous positions of the mice intestines varied by +/-2 mm in the course of irradiation. CONCLUSION: The results of this study have shown that the RBE of the PSI proton beam is in total accordance with the RBE obtained at the other centers. This experiment has corroborated that proton RBE at the middle of the SOBP is slightly larger than the generic value of 1.10 and that there is a slight tendency for the RBE to increase close to the end of the SOBP. Also, excessive dispersion of individual proton data may be considered to result from intestine motion, taking into account that irradiation at the PSI is delivered dynamically by scanning the target volume with a pencil proton beam ("spot scanning"). Because 2-mm movements resulted in significant variations in local dose depositions, this should be considered for moving targets. Strategies to reduce this effect for the spot scanning technique have been developed at the PSI for radiotherapy of humans.