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In proton therapy dosimetry, a fiber-optic radiation sensor incorporating a scintillator must undergo complicated correction processes due to the quenching effect of the scintillator. To overcome the drawbacks of the fiber-optic radiation sensor, we proposed an innovative method using the Cerenkov radiation generated in plastic optical fibers. In this study, we fabricated a fiber-optic Cerenkov radiation sensor without an organic scintillator to measure Cerenkov radiation induced by therapeutic proton beams. Bragg peaks and spread-out Bragg peaks of proton beams were measured using the fiber-optic Cerenkov radiation sensor and the results were compared with those of an ionization chamber and a fiber-optic radiation sensor incorporating an organic scintillator. From the results, we could obtain the Bragg peak and the spread-out Bragg peak of proton beams without quenching effects induced by the scintillator, and these results were in good agreement with those of the ionization chamber. We also measured the Cerenkov radiation generated from the fiber-optic Cerenkov radiation sensor as a function of the dose rate of the proton beam.
©2012 Optical Society of America
1. Introduction
Organic scintillators have been widely used as sensor probes in fiber-optic radiation sensors (FORSs) for radiotherapy dosimetry due to their water- or tissue-equivalent characteristics, small size, and independence from temperature and pressure, respectively [1–3]. Furthermore, the amount of scintillating light generated in the scintillator is proportional to dose rates at low ionizing density of radiation [4]. These favorable advantages make it possible to precisely measure absorbed doses with high spatial resolution without complicated calibration processes in therapeutic photon and electron dosimetries. However, organic scintillators have a critical problem in measuring absorbed doses for heavy charged particles such as protons and heavier ions in spite of these advantages. The scintillation molecules of organic scintillators can be temporarily damaged by high energy charged particles. This phenomenon, which is known as the quenching effect, causes non-proportionality between energy losses of charged particles and scintillation outputs [5,6]. It is known that the quenching effect depends on the ionization density and the properties of the scintillator. At low stopping power, therefore, this effect could be negligible. On the other hand, at high stopping power such as the Bragg-peaks of high energy proton beams, the quenching effect can limit the use of scintillators [7]. Although Birk’s formula can be used to correct the amount of scintillation loss induced by the quenching effect, it is difficult to correct this for the spread-out Bragg peak (SOBP) of the proton beam. The SOBP is the sum of different energies of Bragg peaks, and therefore requires correction for the energies of proton beams.
Meanwhile, plastic optical fiber (POF), a component of the FORS, has many advantages such as good flexibility, water-equivalence, easy processing, remote sensing, and no interference from electromagnetic fields, and accordingly has been used to transmit the scintillation signal to a light measuring device [8–13]. Generally, a number of processes such as fluorescence, phosphorescence, and photodarkening can occur when POF is irradiated by ionizing radiation, but these effects have been reduced with the development of radiation resistant optical fibers [14]. However, the Cerenkov radiation, which is produced by charged particles − premiere or scattered electrons whose energy is over 178 keV (Cerenkov threshold energy for the polymethyl methacrylate (PMMA)) − that pass through the POF with a velocity greater than that of light, is always regarded as a severe noise signal, because the amount of Cerenkov radiation depends on incident angles and energies of charged particles, as well as the irradiated lengths of the POF [15–17].
Cerenkov radiation, however, is one of the signals produced by interactions between charged particles and mediums, and therefore can be a significant signal in some cases. Measuring the Cerenkov radiation generated in the POF is potentially a very attractive solution to measure Bragg peaks or SOBPs of therapeutic proton beams without the quenching effect of the organic scintillator that occurs in the FORS. Cerenkov radiation is not generated from the scintillation molecules, and therefore there is no quenching effect according to the ionization density. In the case of therapeutic proton beams, whose commercial energies are below 235 MeV, Cerenkov radiation is not induced directly by incident proton beams but induced by subsequent electrons, because the Cerenkov threshold energy of the proton beam is about 328 MeV for the PMMA.
In this study, we fabricated a fiber-optic Cerenkov radiation sensor (FOCRS) for measuring the Cerenkov radiation induced by a therapeutic proton beam using POFs without any scintillator. An FORS incorporating an organic scintillator was also fabricated for comparison. The Bragg peaks and SOBPs of the proton beams were measured using the FOCRS and the results were compared with those of an ionization chamber and the FORS. Finally, we measured the Cerenkov radiation generated from the FOCRS as a function of the dose rate of the proton beam.
2. Materials and methods
We fabricated a FOCRS and FORS for measuring the Cerenkov radiation and scintillating lights induced by therapeutic proton beams, respectively. The POFs (SH4001, Mitsubishi) chosen in this study are multi-mode, step index optical fibers with a core/cladding structure. The outer diameter of the POFs is 1.0 mm and the cladding thickness is 0.01 mm. The refractive indices of the core and the cladding are 1.492 and 1.402, respectively, and the numerical aperture (NA) is 0.510. The materials of the core and the cladding are PMMA and fluorinated polymer, respectively, and the jacket is made of polyethylene (PE). Previous works reported that there is no significant degradation in the light attenuation of POF up to 10-15 kGy of irradiation [18].
The FOCRS for measuring the Cerenkov radiation consists of two POFs having different lengths. A reference POF and a 5 cmlonger POF were used to apply the subtraction method, as shown in Fig. 1 . In general, the subtraction method can be employed for measuring the difference of interest between two sensor signals [19,20]. In the experiments, the Cerenkov radiation generated from limited length (5 cm) of POF was obtained by using the subtraction method. Normally, the Cerenkov radiation generated from the optical fiber is a supersubtle light signal therefore we obtained the Cerenkov radiation from a quite large length of the POF. Although this length could cause poor spatial resolution in measuring the planar dose distribution for irradiation field, the spatial resolution for the depth does is just 1 mm according to diameter of the POF. To increase the collection efficiency of Cerenkov radiation, a reflective paint based on TiO2 was coated at the end of the POFs. As a sensing probe of the FORS, an organic scintillator (EJ232, Eljen Technology) with 1 mm thickness and 5 mm length and emitting peak wavelength at 370 nm was used. The surface of the scintillator was also coated with the TiO2 to increase the collection efficiency of scintillation output and the scintillator was glued optically to the end of the POF that was used as a light guide.
A photomultiplier tube (PMT: H7546, Hamamatsu Photonics) was used to measure Cerenkov and scintillating lights. The measurable wavelength range of the PMT is 300 ~650 nm and the peak sensitive wavelength is 420 nm. The typical dark current is about 0.2 nA at −800V supply voltage (C9525, Hamamatsu Photonics). The ionization chamber used in this experiment is a Markus® plane-parallel ionization chamber (N23343, CNMC Company), having an ion collector of 5.3 mm radius and an electrode separation of 2 mm thickness. Proton irradiation was carried out on proton therapeutic facility utilizing Ion Beam Applications (IBA) cyclotron PROTEUS 235 and energies of 173 and 180 MeV were employed. The irradiation field diameter of proton beam is 7 cm, and dose rates of proton beam used in these experiments are 1 ~6 Gy/min.
Figure 2 shows the experimental setup for measuring the Cerenkov and scintillating lights generated from the FOCRS and FORS, respectively. The sensors are located on the center of the proton beam field. The Cerenekov radiation or scintillation output is transmitted by 15 m POFs to the PMT. The amplified electric signals are then measured using a LabVIEW DAQ board. The relative depth doses of proton beams according to depths of the water phantom were measured using the FOCRS, FORS, and the Marcus chamber by vertical scanning.
3. Experimental results and discussion
Figure 3 shows the linearity between scintillation yields of the FORS and outputs of the Marcus chamber. This result was obtained by one-to-one correspondence of scintillation yields of the FORS and outputs of the Marcus chamber according to depths of water phantom when the 180 MeV proton beam irradiated on each dosimeter. In this result, if a linear fitting equation is the identity function, the gradient of the linear fitting equation should be ‘1’, ideally. However, the gradient of the fitting equation is 0.8804 which is caused by the quenching effect of the organic scintillator. The R-square value, which is often called the coefficient of determination, of the fitting line was 0.9822, and it represents the accuracy of correlation between the measured data and the fitting line. Generally, this value is varied from 0 to 1 and the higher represents that modeled fitting line is perfectly fitted with the measured data. The value of R-square can be obtained as,
where yi is the value of data set, fi is the modeled value, and ym is mean of the measured data.The measured relative depth dose for the Bragg peak of the proton beam with the FORS incorporating an organic scintillator is shown in Fig. 4(a) . From this result, we can find the maximum depth dose at the Bragg peak obtained by using the FORS is less than that derived with the Marcus chamber. The difference between relative depth doses measured using the FORS and the Marcus chamber at the Bragg peak was about 17%.
Fig. 4 Measured relative depth doses for Bragg peak and SOBP of proton beams using scintillation yields generated from the FORS (a) Bragg peak, (b) SOBP.
The intensity of scintillation generated from the organic scintillator reached a saturation value at the high stopping power. Therefore, the intensity of scintillation should be corrected with respect to the relative stopping power (outputs of the Marcus chamber) by using Birk’s formula, which is given by [21]:
where S is the scintillation efficiency, kB is a scintillator quenching factor (Birk’s constant), dL/dx is the scintillation yield per unit thickness, and dE/dx is the energy loss per unit thickness.Actually, B dE/dx is the density of damaged molecules by the particle, and k means the relative probability of exciton capture of damaged molecules. Generally, the kB is regarded as a parameter adjusting to fit the experimental data for a particular scintillator [6]. In this experiment, the quenching factor was determined to be 0.0039 mm/MeV using Eq. (2) with the experimental data. The corrected result was in good agreement with that of the Markus chamber, as shown in Fig. 4(a). The dose difference between the output of the Markus chamber and the corrected data of the FORS was about 0.89% at the Bragg peak. However, this process could be tedious, because the complicated correction process should be performed for each proton beam having different energy.
Measured relative depth dose for the SOBP of the proton beam using scintillation yields generated from the FORS can be found in Fig. 4(b). In general, the SOBP is the sum of Bragg peaks having different energies, and thus the maximum dose is maintained uniformly in a certain region. The quenching effect of an organic scintillator depends on linear energy transfer (LET) of charged particles. As a result, a SOBP obtained by using the FORS is decreasing when partial contribution to absorbed dose from higher LET protons is increasing. Figure 4(b) shows the decrease response of FORS about 10 ~20% in comparison with a Marcus chamber at depth 110-140mm in a water phantom. In this result, we could not correct the scintillation yield of FORS using the Birk’s formula because the SOBP consists of varied energies of Bragg peaks.
Figure 5 shows the linearity between Cerenkov light yields of the FOCRS and outputs of the Marcus chamber for the 173 MeV proton beam. This result was also obtained by same method as that of Fig. 3. The gradient of the fitting equation is 0.9840, indicating that the fitting equation is very close to the identity function. The R-square value of the fitting curve was 0.9938.
Measured relative depth dose for the Bragg peak of the proton beam using Cerenkov light yields generated from FOCRS is shown in Fig. 6(a) . The Bragg peak curve was obtained by measuring the Cerenkov radiation generated in the FOCRS. The result of FOCRS without any correction process was in good agreement with that of the Markus chamber. The dose difference between the result of the FOCRS and that of the Marcus chamber was about 1.76% at the Bragg peak.
Fig. 6 Measured relative depth doses for Bragg peak and SOBP of proton beams using Cerenkov light yields generated from the FOCRS (a) Bragg peak, (b) SOBP.
Figure 6(b) is demonstrating the measured relative depth dose for the SOBP of the proton beam using the FOCRS. A uniform region of the SOBP using the FOCRS was obtained from 75 to 95 mm depth of the water phantom, and the mean difference in the uniform region between the result of the FOCRS and that of the ionization chamber was about 0.7%. The result of the FOCRS for the measurement of SOBP of the proton beam coincided perfectly with that of the Markus chamber.
The relationship between the amounts of Cerenkov radiation produced according to dose rates of the proton beam can be found in Fig. 7 . This result was obtained by using the 180 MeV proton beam with a field size of 7 cm in diameter at a depth of 15 cm in the water phantom. As the dose rate increased, the amount of Cerenkov radiation linearly increased, because the intensity of the proton beam is increased linearly. The R-square value, as shown in Fig. 7, was 0.9988.
4. Conclusion
In proton therapy dosimetry, a FORS incorporating a scintillator must undergo correction processes due to the quenching effect of the scintillator. Through this study, to overcome the drawbacks of the FORS, we proposed an innovative method using the Cerenkov radiation generated in POFs. The Cerenkov radiation generated in a POF is normally regarded as a severe noise signal. However, it is also one of the signals produced by interactions between charged particles and a medium. In some cases, therefore, the Cerenkov radiation can be a significant signal.
In this study, we fabricated a FOCRS without an organic scintillator to measure Cerenkov radiation induced by therapeutic proton beams. Bragg peaks and SOBPs of proton beams were measured using the FOCRS and the results were compared with those of a Marcus chamber and a FORS. From the results, we could obtain the Bragg peak and the SOBP of proton beams without quenching effects, and these results were in good agreement with those of the Marcus chamber. We also measured the Cerenkov radiation generated from the FOCRS as a function of the dose rate of the proton beam. As the dose rate increased, the amount of Cerenkov radiation generated in the FOCRS increased linearly.
Further study will be carried out to compensate the defect of FOCRS by shortening the sensor probe for planar dose measurements with higher spatial resolution. It is anticipated that the novel and simple FOCRS for measuring Cerenkov radiation can be effectively used for measuring relative depth doses in proton or heavy particle therapy dosimetry.
Acknowledgments
This work was supported by the Radioactive Waste Management R&D Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (No. 20111720200010) and this work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 20110006337, and No. 20110028022). Also this paper resulted from the Konkuk University research support program.
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