Dose Linearity and Linear Energy Transfer Dependence of Cr-doped Al2O3 Ceramic Thermoluminescence Detector

1Medical Exposure Research Project, National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage-ku, Chiba 263-8555, Japan 2Division of Radiological Sciences, Faculty of Health Sciences, Tokyo Metropolitan University, 7-2-10 Higashi-Ogu, Arakawa-ku, Tokyo 116-8551, Japan 3Atomic Energy Research Institute, Kindai University, 3-4-1 Kowakae, Higashiosaka-shi, Osaka 577-8502, Japan 4Chiyoda Technol Co., 1-7-12 Yushima, Bunkyo-ku, Tokyo 113-8681, Japan 5Chiba Ceramic Mfg. Co., Ltd., 2-10-14 Inage-higashi, Inage-ku, Chiba 263-0031, Japan


Introduction
Successful radiotherapy requires the correct dose to be delivered; this can be achieved by a routine quality assurance (QA) program and dose verification before treatment. A radiochromic film such as EBT3 (1) (GAFCHROMIC TM , EBT3 dosimetry film, Ashland Co.) is capable of providing a precise two-dimensional (2D) dose distribution and is one of the most widely used dosimeters in conventional photon radiotherapy. (2)(3)(4)(5) However, the linear energy transfer (LET)dependent response of EBT3 and the fact that it can be used only once make it unsuitable for heavy charged particle (HCP) radiotherapy. (6,7) Consequently, ionization chambers (ICs) have been the most useable dosimeters for QA in HCP therapy. The scanning and multiarray arrangement methods are the most common ways to obtain the dose distribution using ICs, although these two methods have some drawbacks, such as requiring a large amount of time and providing a poor spatial resolution.
Recently, it has been reported that Al 2 O 3 -based ceramic plates show good thermoluminescent (TL) characteristics and are available for use in 2D dosimetry. (8) In addition, it has been reported that a nondoped Al 2 O 3 ceramic thermoluminescent detector (TLD) has a smaller LETdependent response than commercially available TLDs such as LiF:Mg,Ti and LiF:Mg,Cu,P. (9) The application of a nondoped Al 2 O 3 ceramic TLD to HCP therapy has been suggested. On the other hand, nondoped Al 2 O 3 had a low-temperature glow peak that increases the fading effect. Recent research shows that a Cr-doped alumina (Al 2 O 3 :Cr) ceramic TLD is suitable as a 2D imaging device in conventional photon therapy because it has a linear relationship between dose and response, and a small fading property, while it is easy to handle and reusable. (10) In this work, with the aim of evaluating the potential of Al 2 O 3 :Cr ceramic TLDs in HCP therapy, we investigated the dose linearity and LET dependence of an Al 2 O 3 :Cr ceramic TLD using several HCP beams.

Cr-doped alumina-based ceramic TLD
An Al 2 O 3 :Cr ceramic TLD was provided by Chiba Ceramic Mfg. Co., Ltd. Table 1 shows the composition of this Al 2 O 3 :Cr ceramic TLD. The bulk density and effective atomic number of the TL slab were 3.7 g/cm 3 and 11.13, respectively. Al 2 O 3 :Cr ceramic TLDs were fabricated with a size of 11 × 11 × 0.7 mm 3 (Fig. 1).

Irradiation
Irradiation experiments were performed at the BIO course of the heavy-ion medical accelerator (HIMAC) in Chiba at the National Institute of Radiological Sciences. The beams were laterally broadened by a wobbling and scattering system to produce a uniform field with a 10 cm diameter. (11) Table 2 lists the irradiation beams used in this study. The values of LET in water were calculated using Geant4. (12) For each irradiation, four dose levels were given as 0.5, 1.0, 2.0, and 5.0 Gy in water based on the predicted dose in HCP therapy. In addition, all the Al 2 O 3 :Cr ceramic TLDs were irradiated using a 6 MV photon beam as a reference from a Varian 21Ex linear accelerator at Tokyo Metropolitan University. The irradiations were performed with a 100 cm source-to-axis distance and a field size of 10 × 10 cm 2 , and by placing the film packages at a depth of 10 cm in water equivalent phantoms.

TL glow curve measurement
A readout system (9) developed in house was used to record TL glow curves of the Al 2 O 3 :Cr ceramic TLDs from room temperature to 400 °C. The TLD was heated by a brass plate connected to a programmable heat controller (Sakaguchi E.H VOC Corp. SCR-SHQ-A) at a heating rate of 0.2 °C/s in air. A thermal cut filter was set in the readout system to decrease the effect of thermal radiation. After passing through the thermal cut filter, the TL signal was collected with a pair of collecting lenses and then counted using a photon-counting unit (Hamamatsu H11890-110). Generally, the TLD demonstrates a fading effect in which the amount of TL decreases as a function of the time elapsed from irradiation to reading. To improve accuracy without correcting the fading effect, the timeline of the readout procedure was made consistent for each experiment. The wavelength spectrum of the TL of the Al 2 O 3 :Cr ceramic TLD was detected with a multichannel spectroscope (Hamamatsu PMA-12). Figure 2 shows the TL glow curve of the Al 2 O 3 :Cr ceramic TLD. The glow peaks were located at 310 °C. Figure 3 shows the wavelength spectrum of the TL; the wavelength of the TL peak is located at 694 nm due to the embedded Cr 3+ . (13)(14)(15)(16) As shown in Fig. 2, the TL intensity was evaluated as an integrated value over a 10 °C range centered on the main peak. The TL intensity was normalized using the following equation,

TL glow curve and dose linearity
where D is the irradiation dose, L(D) is the TL intensity when the irradiation dose is D Gy, L X-ray (1 Gy) is the TL intensity when an X-ray of 1 Gy is irradiated, and nL(D) is the normalized TL intensity. In addition to nL(D), to evaluate the reproducibility, the TL efficiency nL/D was defined as the TL intensity per irradiation dose. Figure 4 shows the dose response of this ceramic TLD to several species of charged particles. Dose linearity and repeatability were comparatively good. On the other hand, the TL efficiency nL/D changed with the species of HCP. Table 3 and Fig. 5 show the relationship between the LET in water of the irradiating particle and the TL efficiency of the Al 2 O 3 :Cr ceramic TLD for charged particles in this study. The TL efficiency increased from 0.8 and reached a peak value of 1.2 when the LET was 13 keV/mm and decreased to 0.6 when the LET approached 200 keV/mm. The TL efficiency of the nondoped Al 2 O 3 ceramic TLD showed a relatively small LET dependence over this LET range, (9) while the Al 2 O 3 :Cr ceramic TLD had a large LET dependence. Because of its large LET dependence, the Al 2 O 3 :Cr ceramic TLD is difficult to use directly in QA for HCP therapy. However, the difference in the LET dependence of TL efficiency for these two TLDs with similar physical characteristics may be useful for the estimation of LET. Over a limited LET range, the difference in the LET dependence of the two TLDs may be uniquely determined with respect to the value of LET. In Refs. 17 and 18, a dose estimation method for HCP using two types of optically stimulated luminescence (OSL) elements with different LET dependences has been reported. Using a similar estimation method and 2D imaging of ceramic TLDs, (8) it may be possible to measure the LET distribution in HCP therapy. Recently, research on HCP therapy has focused on not only the physical dose but also the biological dose that makes LET distribution information increasingly important. In HCP therapy with one nuclide beam, the range of the LET of the irradiated beam is limited. Hence, it may be possible for    In previous reports, the LET dependence of several TLDs based on Al 2 O 3 has been reported. Spurny and coworkers reported the LET dependence of Al 2 O 3 :Na TLD and Al 2 O 3 :C TLD. (19)(20)(21) In this work, Al 2 O 3 :Cr ceramic TLDs showed characteristics similar to the TL efficiency of the Al 2 O 3 :Na TLD that increased up to a range of tens of keV/mm and then decreased in the higher LET region. On the other hand, the TL efficiency of Al 2 O 3 :C TLD decreased monotonically in the range of several keV/mm to several hundred keV/mm. In Berger and Hajek's review (22) of the LET dependence of many TLDs other than Al 2 O 3 , most TLDs showed a TL efficiency peak in the LET region of tens of keV/mm or showed a monotonically decreasing TL efficiency with increasing LET. In addition, they reported that the TL response to different HCPs with the same LET is not a unique function of ionization density, but rather depends on the microscopic pattern of energy deposition. Therefore, further study of the LET dependence of the Al 2 O 3 :Cr ceramic TLD for each HCP is necessary to assess possible applications to HCP therapy.

Conclusions
In this study, we investigated the dose response in radiotherapy and the LET dependence of Al 2 O 3 :Cr ceramic TLDs using several HCP beams. The main peak of the glow curve located at 300 °C showed good linearity with the irradiation dose. Even though it seems that a large LET dependence may make the direct application of Al 2 O 3 :Cr ceramic TLDs in HCP therapy difficult, the combination of two Al 2 O 3 ceramic TLDs, one with Cr doping and one without Cr doping, may possibly be applied to the measurement of LET distribution because of their different LET dependences. In order to apply Al 2 O 3 :Cr ceramic TLDs to QA in HCP therapy, further investigation of the LET dependence of Al 2 O 3 :Cr ceramic TLDs for each HCP is necessary.