Journal of the Korean Physical Society, Vol. 61, No. 2, July 2012, pp. 239∼242
Design Optimization of a 2D Prompt-gamma Measurement System for Proton Dose Verification Han Rim Lee, Jong Hoon Park and Chan Hyeong Kim∗ Department of Nuclear Engineering, Hanyang University, Seoul 133-791, Korea
Chul Hee Min Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, MA 02114, USA (Received 24 November 2011, in final form 27 March 2012) To verify in-vivo proton dose distribution, a 2-dimensional (2D) prompt-gamma measurement system, comprised of a multi-hole collimation system, a 2D array of CsI(Tl) scintillators, and a position-sensitive photomultiplier tube (PS-PMT), is under development. In the present study, to determine the optimal dimension of the measurement system, we employed a series of Monte Carlo simulations with the MCNPX code. To effectively measure the high-energy prompt gammas while minimizing background gammas, we determined the collimator hole size, collimator thickness, and scintillator length to be 0.4 × 0.4 cm2 , 15 cm, and 5 cm, respectively. Thereafter, the performance of the optimized measurement system was estimated for monoenergetic proton pencil beams. The peak locations of the prompt-gamma distributions for 80- and 150-MeV proton beams were clearly distinguished, and the correlation between the beam range and the peak location was confirmed by using the measurement system. For a 200-MeV proton beam, however, the peak location could not be determined due to the dominance of background gammas and the lateral dispersion of the proton beam at the end of the beam range. Based on these simulation results, a prototype 2D prompt-gamma measurement system currently is under construction and, upon completion, will be tested with therapeutic proton beams. PACS numbers: 87.53.Qc, 87.53.Wz Keywords: Proton therapy, Prompt-gamma, Range verification, Monte Carlo, MCNPX DOI: 10.3938/jkps.61.239
I. INTRODUCTION
it is impossible to directly measure them for verification of the proton dose or the beam range; hence, measuring the prompt gammas generated by proton-induced nuclear interactions has been suggested [9,10]. Recently, a close correlation between the longitudinal distribution of prompt gammas and the location of the distal dose fall-off was shown by using a prompt gamma scanning (PGS) system that provided one-dimensional information on the prompt-gamma distribution [11]. As an extension of that study, a 2-dimensional (2D) promptgamma measurement system consisting of a multi-hole tungsten collimator, a 2D array of CsI(Tl) scintillators, and a commercial position-sensitive photomultiplier tube (PS-PMT) currently is under development. The hope is that the developed system will provide the 2D promptgamma distribution, which could then be correlated with the proton dose distribution in the patient. In the present study, the optimal dimensions of the 2D prompt-gamma measurement system were determined by using Monte Carlo simulations with the MCNPX code [12]. To enable measurement of prompt-gamma distribution while effectively discriminating the background
A proton beam concentrates its radiation dose at the end of the beam range (i.e., at the “Bragg peak”) with a steep distal dose gradient [1,2]. Thanks to this favorable dose delivery pattern, proton therapy provides a very conformal radiation dose to the tumor volume compared with other conventional radiotherapy modalities. In proton therapy, however, the planned dose distribution in a tumor volume can easily be affected by patient-setup error, organ motion, a patient’s anatomical changes and density/composition inhomogeneities in the tissues traversed by the proton beam [3–5]. Variations in the proton dose distribution can degrade the accuracy of dose delivery to a tumor volume, possibly causing serious side effects in adjacent normal tissues. In proton therapy, therefore, proper monitoring of in-vivo proton dose distributions is essential both to successful treatment and patient safety [6–8]. Because protons are completely stopped in the patient, ∗ E-mail:
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Fig. 2. (a)Lateral distribution of prompt gammas at peak location. (b) Peak-to-background (PB) ratio as a function of the collimator hole size. Fig. 1. Schematic diagram of the 2D prompt-gamma measurement system.
gammas generated mostly by neutron capture processes, we determined the optimal dimensions of the multi-hole collimator and the scintillators. Finally, we determined the performance of the measurement system, as configured for the optimal dimensions, was estimated for 80-, 150-, and 200-MeV proton beams incident on a 20 cm × 20 cm × 40 cm water phantom in the longitudinal direction.
II. MATERIALS AND METHOD If the 2D distribution of prompt gammas is to be measured, it is important to effectively shield background gammas and measure only prompt gammas from the proton beam‘s passage. Furthermore, for measurement of high-energy prompt gammas, the CsI(Tl) scintillator must not be too small; otherwise, high-energy secondary electrons generated by the prompt gammas can escape, resulting in serious deterioration of the detector function. Thus, the dimensions of the multi-hole collimation system and the CsI(Tl) scintillator are important design parameters subject to optimization. In the present study, the collimator hole size, collimator thickness, and scintillator length (see Fig. 1) were optimized by using Monte Carlo simulations with the MCNPX code. The LA150 data libraries, which describe evaluated proton, neutron, and photonuclear cross-section data up to 150 MeV, and the ISABEL model were used to model the proton interactions in a water phantom. With the LA150 data libraries, the MCNPX code can precisely simulate proton-induced nuclear interactions and secondary-particle interactions up to 150 MeV [13,14]. For protons above 150 MeV, although the MCNPX code uses both the data library and physics models for particle transportation, it does not significantly affect the results considering that the proton energy is below 150 MeV near the Bragg-peak region and that there is little difference in background gammas generated by neutron captures. The cross-sectional area of the CsI(Tl) scintillator, one of the most important design parameters of the measure-
ment system, was varied identically with the collimator hole size (see Fig. 1); that is, it was not optimized separately. The pitch of the collimator holes was fixed at 6 mm under the assumption that the developed system will use a commercial PS-PMT (H8500C, Hamamatsu Photonics K.K., Japan), the pitch of which is 6 mm. For optimization, 1.21 × 109 150-MeV protons, corresponding to 10 spots in spot scanning treatment [15], were delivered as a pencil beam to the center of the 20 cm × 20 cm × 40 cm water phantom in the longitudinal direction, after which the prompt-gamma distribution was simulated varying the collimator hole size, collimator thickness, and scintillator length. To effectively discriminate the background gammas from the prompt gammas, we applied an optimal energy window of 4 – 10 MeV [16] to the scintillation detectors. The optimal dimensions of the measurement system were then determined by monitoring the prompt-gamma distributions in the lateral and longitudinal directions. In the case of the longitudinal direction, the peak-to-background (PB) ratio, defined as the ratio of the gamma counts at the peak to the average background gamma counts within a 2- to 4-cm range distal to the peak, was used. Based on results of the optimization studies, a 2D prompt-gamma measurement system was designed, and its performance was predicted using the MCNPX code. This involved delivery of monoenergetic proton beams of 80, 150, and 200 MeV to the center of the water phantom in the longitudinal direction. The 2D distributions of prompt gammas were simulated for the optimized measurement system.
III. RESULTS AND DISCUSSION First, the prompt-gamma distribution was simulated for various hole sizes of the multi-hole collimation system. Figure 2(a) shows the lateral distribution of the simulated prompt gammas as a function of the collimator hole size. The result indicates that a 2D promptgamma measurement system cannot accurately measure the prompt-gamma distribution if the collimator hole size is too small (i.e., less than 0.3 × 0.3 cm2 ). In this
Design Optimization of a 2D Prompt-gamma Measurement System · · · – Han Rim Lee et al.
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Fig. 3. (a) Lateral distribution of prompt gammas at peak location. (b) Peak-to-background (PB) ratio as a function of the collimator thickness.
Fig. 4. (a) Gamma counts at peak location. (b) Peak-tobackground (PB) ratio as a function of the scintillator length.
case, not only are the majority of the prompt gammas shielded by the collimator but also the energies of the prompt gammas are incompletely absorbed in the CsI(Tl) scintillators due to the escape of high-energy secondary electrons. Figure 2(b) plots the PB ratio as a function of the collimator hole size. Whereas the ratio can be seen to increases gradually with the collimator hole size, it falls rapidly once the hole size exceeds 0.4 × 0.4 cm2 . Such hole sizes excessively reduce the septum thickness for the fixed pitch (= 0.6 cm), rendering the collimation system ineffective for shielding against background gammas and prompt gammas in unwanted directions. Next, the prompt-gamma distribution was simulated for various collimator thicknesses. Figure 3(a) shows the lateral distribution of prompt gammas at the peak location as a function of the collimator thickness. The result clearly indicates that the relative contribution of the background gammas decreases with increasing collimator thickness, but that the variation becomes almost negligible if the thickness is equal to or greater than 15 cm. Figure 3(b) plots the variation of the PB ratio as a function of the collimator thickness. Whereas the ratio increases with increasing collimator thickness for thicknesses less than 15 cm, it drops sharply for thicknesses greater than 15 cm due to the fact that at collimator thicknesses greater than 15 cm, the reduction in the prompt gammas has more influence on the simulatedgamma distribution than the reduction in background gammas dose. Figure 4(a) shows the number of gamma counts at the peak location as a function of the scintillator length. The
Fig. 5. The upper plots show the prompt-gamma distributions simulated for the 2D prompt-gamma measurement system with optimal dimensions for 80-(upper), 150-(middle), and 200-(bottom) MeV proton beams. The white arrows indicate the proton-beam direction, and the white dashed lines represent the proton-beam range. The lower plots show the proton depth dose distributions (black broken line) and 1D prompt-gamma distributions (black step line) at the incident positions of the proton beams.
peak counts dramatically increase until the scintillator length reaches 5 cm, after which, however, they increase only rather slowly. Figure 4(b) indicates, similarly, that the PB ratio increases with the scintillator length until the scintillator length reaches 5 cm, and then decreases. This declination of the PB ratio is understandable, considering that after 5 cm, the increase in background gamma counts due to the increase in the scintillator length is even greater than that of the prompt-gamma counts. Based on a summary of the simulation results, the optimal dimensions of the 2D prompt-gamma measurement system were determined to be 0.4 × 0.4 cm2 , 15 cm, and 5 cm for the collimator hole size, the collimator thickness, and the scintillator length, respectively. Finally, the performance of the 2D prompt-gamma measurement system, as configured for the optimal dimensions so determined, was estimated for 80-, 150-, and 200-MeV proton beams. The distance between the water phantom and the measurement system was 10 cm, and the number of protons was, again, 1.21 × 109 for each simulation. Figure 5 plots the 2D distributions of the simulated prompt gammas for the optimized measurement system and the depth distributions of prompt gammas at the incident positions of the proton beams. The results show that the measurement system can identify the peak location of prompt gammas near the end of the proton beam range
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for the 80- and the 150-MeV proton beams, but not for the 200-MeV proton beam. In the 200-MeV case, the failure to determine the peak location was attributed to the dominance of the background gammas and to the lateral dispersion of the proton beam at the end of the beam range.
IV. CONCLUSION The present study employed a series of Monte Carlo simulations with the MCNPX code to determine the optimal design of a 2D prompt-gamma measurement system comprised of a multi-hole collimation system, a 2D array of CsI(Tl) scintillators, and a PS-PMT. The optimal dimensions of the system were determined to be 0.4 × 0.4 cm2 , 15 cm, and 5 cm for the collimator hole size, collimator thickness, and scintillator length, respectively. Thereafter, the system, so configured, was simulated, the results showing accurate measurement of the 2D prompt-gamma distribution along with effective determinations of the peak locations of prompt gammas near the end of the proton beam range for 80- and 150-MeV proton beams, but not for a 200-MeV proton beam. Based on these simulation results, a prototype 2D prompt-gamma measurement system currently is under construction and, upon completion, will be tested with therapeutic proton beams.
ACKNOWLEDGMENTS This research was supported by the National Nuclear R&D Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Educa-
tion, Science and Technology (Nos. 2010-0028913, 20100023825, and 2012-K001146).
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