Secondary particle intensity for experimental range verification in carbon ion therapy

Range uncertainty that reduces the dosimetric advantage of carbon ion therapy is a major problem limiting its treatment precision. To address this issue, high-precision monitoring of the in-beam range is essential. This paper proposes a method for range verification using a secondary particle intensity (SPI) system consisting of a cerium bromide (CeBr₃) scintillation detector and an integrated ionization chamber (IC). This method uses the secondary particle intensity produced by the primary particles per monitor unit (MU) observed by the SPI system to convert the carbon ion range. Secondary particles, including gamma rays, neutrons, and charged particles, can be detected in the CeBr₃ detector as long as their signal exceeds the threshold, and no discrimination by particle type is required. The Heavy Ion Medical Machine (HIMM) terminal, situated in Lanzhou, China, provided the ¹²C⁶⁺ beam with energies ranging from 160.86 to 211.44 MeV/u. The experiment demonstrated that the carbon ion range accuracy of the ¹²C⁶⁺ beam in a polymethyl methacrylate (PMMA) target can reach 0.92 ± 0.67 mm under a 10 ms measurement period. The secondary particle energy spectrum was recorded during both the in-beam and off-beam phases, clearly recognizing the 511 keV annihilation photopeak with a full width at half maximum (FWHM) of 31.37 keV. The photopeak of annihilation confirmed the accumulation of activated products. During the 15 min continuous irradiation process, the effect of activated product accumulation on the range verification is about 1.15 mm. This paper illustrates the feasibility of SPI method for fast measurement of carbon ion range, which could potentially reduce the effects of range uncertainty in carbon ion therapy.