Abstract

Purpose: This work has two related objectives. The first is to estimate the relative biological effectiveness of two radioactive heavy ion beams based on experimental measurements, and compare these to the relative biological effectiveness of corresponding stable isotopes to determine whether they are therapeutically equivalent. The second aim is to quantitatively compare the quality of images acquired postirradiation using an inbeam wholebody positron emission tomography scanner for range verification quality assurance. Methods: The energy deposited by monoenergetic beams of (Formula presented.) C at 350 MeV/u, (Formula presented.) O at 250 MeV/u, (Formula presented.) C at 350 MeV/u, and (Formula presented.) O at 430 MeV/u was measured using a cruciform transmission ionization chamber in a water phantom at the Heavy Ion Medical Accelerator in Chiba (HIMAC), Japan. Dosemean lineal energy was measured at various depths along the path of each beam in a water phantom using a silicononinsulator mushroom microdosimeter. Using the modified microdosimetric kinetic model, the relative biological effectiveness at 10% survival fraction of the radioactive ion beams was evaluated and compared to that of the corresponding stable ions along the path of the beam. Finally, the postirradiation distributions of positron annihilations resulting from the decay of positronemitting nuclei were measured for each beam in a gelatin phantom using the inbeam wholebody positron emission tomography scanner at HIMAC. The depth of maximum positronannihilation density was compared with the depth of maximum dose deposition and the signaltobackground ratios were calculated and compared for images acquired over 5 and 20 min postirradiation of the phantom. Results: In the entrance region, the (Formula presented.) was 1.2 ± 0.1 for both (Formula presented.) C and (Formula presented.) C beams, while for (Formula presented.) O and (Formula presented.) O it was 1.4 ± 0.1 and 1.3 ± 0.1, respectively. At the Bragg peak, the (Formula presented.) was 2.7 ± 0.4 for (Formula presented.) C and 2.9 ± 0.4 for (Formula presented.) C, while for (Formula presented.) O and (Formula presented.) O it was 2.7 ± 0.4 and 2.8 ± 0.4, respectively. In the tail region, (Formula presented.) could only be evaluated for carbon; the (Formula presented.) was 1.6 ± 0.2 and 1.5 ± 0.1 for (Formula presented.) C and (Formula presented.) C, respectively. Positron emission tomography images obtained from gelatin targets irradiated by radioactive ion beams exhibit markedly improved signaltobackground ratios compared to those obtained from targets irradiated by nonradioactive ion beams, with 5fold and 11fold increases in the ratios calculated for the (Formula presented.) O and (Formula presented.) C images compared with the values obtained for (Formula presented.) O and (Formula presented.) C, respectively. The difference between the depth of maximum dose and the depth of maximum positron annihilation density is 2.4 ± 0.8 mm for (Formula presented.) C, compared to −5.6 ± 0.8 mm for (Formula presented.) C and 0.9 ± 0.8 mm for (Formula presented.) O vs −6.6 ± 0.8 mm for (Formula presented.) O. Conclusions: The (Formula presented.) values for (Formula presented.) C and (Formula presented.) O were found to be within the 95% confidence interval of the RBEs estimated for their corresponding stable isotopes across each of the regions in which it was evaluated. Furthermore, for a given dose, (Formula presented.) C and (Formula presented.) O beams produce much better quality images for range verification compared with (Formula presented.) C and (Formula presented.) O, in particular with regard to estimating the location of the Bragg peak.