Verification of Lithium Formate Monohydrate in 3D-Printed
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Australasian Physical & Engineering Sciences in Medicine (2019) 42:811–818 https://doi.org/10.1007/s13246-019-00786-x SCIENTIFIC NOTE Verifcation of lithium formate monohydrate in 3D‑printed container for electron paramagnetic resonance dosimetry in radiotherapy Jin‑sol Shin1,2,3 · Hoon Choi4 · Hun Joo Shin1,2,3 · Shin Wook Kim1,2,3 · Hyeong Wook Park2,5,6 · Jina Kim1,2 · Aeran Kim1,2 · Jinho Hwang1,2 · Yunji Seol1,2 · Taegeon Oh1,2 · Hong Seok Jang2,7 · Byung Ock Choi2,7 · Young‑nam Kang2,7 Received: 27 December 2018 / Revised: 29 July 2019 / Accepted: 1 August 2019 / Published online: 13 August 2019 © Australasian College of Physical Scientists and Engineers in Medicine 2019 Abstract The nondestructive dosimetry achieved with electron paramagnetic resonance (EPR) dosimetry facilitates repetitive recording by the same dosimeter to increase the reliability of data. In precedent studies, solid parafn was needed as a binder material to make the lithium formate monohydrate (LFM) EPR dosimeter stable and nonfragile; however, its use complicates dosim- etry. This study proposes a newly designed pure LFM EPR dosimeter created by inserting LFM into a 3D-printed container. Dosimetric characteristics of the LFM EPR dosimeter and container, such as reproducibility, linearity, energy dependence, and angular dependence, were evaluated and verifed through a radiation therapy planning system (RTPS). The LFM EPR dosimeters were irradiated using a clinical linear accelerator. The EPR spectra of the dosimeters were acquired using a Bruker EMX EPR spectrometer. Through this study, it was confrmed that there is no tendency in the EPR response of the container based on irradiation dose or radiation energy. The results show that the LFM EPR dosimeters have a highly sensitive dose response with good linearity. The energy dependence across each photon and electron energy range seems to be negligible. Based on these results, LFM powder in a 3D-printed container is a suitable option for dosimetry of radiotherapy. Further- more, the LFM EPR dosimeter has considerable potential for in vivo dosimetry and small-feld dosimetry via additional experiments, owing to its small efective volume and highly sensitive dose response compared with a conventional dosimeter. Keywords Electron paramagnetic resonance · Dosimetry · Lithium formate · Radiotherapy · 3D printer * Young-nam Kang 5 Department of Medical Physics, Kyonggi University [email protected] of Korea, 154-42, Gwanggyosan-ro, Yeongtong-gu, Suwon-si, Gyeonggi-do, Korea 1 Department of Biomedicine & Health Sciences, The 6 Department of Radiation Oncology, Yeouido St. Mary’s Catholic University of Korea, 222 Banpo Road (Catholic Hospital, College of Medicine, The Catholic University University of Korea), Seocho-gu, Seoul, Korea of Korea, 10, 63-ro, Yeongdeungpo-gu, Seoul, 2 Advanced Institute for Radiation Fusion Medical Republic of Korea Technology, College of Medicine, The Catholic University 7 Department of Radiation Oncology, Seoul St. Mary’s of Korea, Annex B1 222 Banpo Road (Catholic University Hospital, College of Medicine, The Catholic University Seoul St. Mary’s Hospital), Seocho-gu, Seoul, Korea of Korea, 222 Banpo Road (Catholic University Seoul 3 Department of Radiation Oncology, Incheon St. Mary’s St. Mary’s Hospital), Seocho-gu, Seoul, Korea Hospital, College of Medicine, The Catholic University of Korea, 59 Dongsu-ro (665-9 Bupyeong-dong), Bupyeong-gu, Incheon, Korea 4 Department of Radiation Emergency Medical Team, Radiation Health Institute, 5F/B1F SNUH Health Care Innovation Park B/D, 172, Dolma-ro, Bundang-gu, Seongnam-si, Gyeonggi-do, Korea Vol.:(0123456789)1 3 812 Australasian Physical & Engineering Sciences in Medicine (2019) 42:811–818 Introduction maximum size of the LFM EPR dosimeter with parafn is 5 mm [20]. In addition, because the uncertainty depends Radiation dosimetry enhances the quality assurance on the homogeneity of the dosimeter batch, the combi- (QA) of equipment, techniques, and treatments used in nation of parafn afects the accuracy of EPR dosimeter radiotherapy [1–3]. Advanced radiotherapy techniques, [21]. There are also concerns that the binder itself may such as intensity-modulated radiation therapy (IMRT) generate a transient EPR signal, owing to radiation and/or and volumetric-modulated arc therapy (VMAT), require thermal aging. This adds interference or noise to the EPR high precision and accuracy of QA equipment [4]. Sev- signal in diferent dose ranges [22, 23], and the limitations eral dosimetry systems are available, each with advan- of parafn binder can afect the accuracy of the radiation tages and disadvantages compared with other systems. dosimetry. In particular, the thermoluminescence dosimeter (TLD) These limitations of parafn binder can afect the accu- is a widely used solid-state dosimeter for determination racy of the radiation dosimetry. We propose a binder-free of in vivo doses during external beam radiotherapy [5]. LFM EPR dosimeter that is made into a 3D-printed insert However, this requires complicated and prolonged anneal- to overcome the limitations of the parafn binder. The con- ing and is difcult to repeatedly readout [6, 7]. Meanwhile, tainer made from acrylic material, a widely available plastic electron paramagnetic resonance (EPR) dosimetry ofers that has been often used as a soft tissue-equivalent material a non-destructive method and simple readout without an because of its comparable attenuation coefcients in high- annealing process. Further, EPR dosimetry using alanine energy beams [24]. With a lithium formate density of 1.48 g/ 3 3 is recommended as a technique for standard dose assess- cm close to density of 1.09 g/cm of acryl, the dosimeter ment for the high doses used by the International Atomic exhibits equivalent interaction close-to-water, so it does not Energy Agency (IAEA) [8]. Alanine has been used espe- induce rapid disturbances or discontinuities when inserted cially for in vivo dose estimates in radiotherapy [9, 10]. into a tissue equivalent phantom. However, since it is dif- However, its accuracy is reduced for doses below 1 Gy, fcult to manufacture a container having a thickness of 1 mm owing to its low sensitivity [11, 12]. Various compounds according to a general acrylic processing procedure, con- have been proposed for use in radiotherapy, and lithium tainers have been manufactured using a 3D printer capable formate monohydrate (LFM) is one of the most promising of more precise and delicate manufacture with a resolution materials [13–15]. LFM is two to six times more sensitive of 100 μm. The 3D-printed container has an EPR signal as than alanine [13]. In addition, the efective atomic num- a background signal in the process of hardening by irradi- ber of LFM (Z ef = 7.3) is closer to soft tissue (Z ef = 7.0) ating with ultraviolet rays. Before manufacturing the LFM than lithium fuoride (Zef = 8.3), which is the most widely EPR dosimeter, we evaluated the dosimetric characteristics used material for thermoluminescence dosimetry [15]. The of the container based on the irradiated dose and energy density of single crystals of lithium formate monohydrate because an increase in the EPR signal with the irradiated is reported to be approximately 1.48 g/cm3 [16]. Thus, the dose or radiation energy might cause it to afect the dose probability of interaction of LFM with ionizing radiation readout. In addition, the dosimetric characteristics of the is similar to that of tissue compared with lithium fuoride. LFM EPR dosimeter, including the reproducibility, linear- Further, LFM has been evaluated to determine the fading ity, energy dependence, and angular dependence, have been signal under normal atmospheric conditions, something evaluated and verifed using a radiation therapy planning not observed for at least 28 days [13, 15, 17, 18]. system (RTPS). These experiments widen the knowledge of In precedent studies, solid parafn was needed as a LFM EPR dosimetry and facilitate further study to extend binder material to make LFM EPR dosimeter stable and dosimeter implementation and furthermore have signifcant nonfragile, but its use complicates dosimetry [19]. The potential for in vivo dosimetry and small feld dosimetry. mass attenuation coefcients of parafn difer considerably from those of lithium formate, as they are more than a fac- tor of 2 lower at photon energies approximately 5–10 keV, Materials and methods approximately equal at 50 keV and increasing to a factor of 1.1 higher at 1 MeV [19]. In addition, the use of parafn LFM EPR dosimeters with a low dielectric constant afects the size of the LFM EPR dosimeter. The sample of higher dielectric constant High-purity (98%) lithium formate monohydrate increases the non-resonance absorption of the microwave (CHLiO2·H2O) (LFM, Tokyo Chemical Industry Co., Ltd, energy, giving rise to losses in the EPR spectrometer sen- Tokyo, Japan) was used. There was no mixing with a binder sitivity [20]. Therefore, an increase in the sample’s die- to avoid any possible contamination or contribution to the lectric constant requires that its diameter decrease. The radiation-induced signal of LFM. The dosimeter container is a cylindrical pellet, as shown in Fig. 1, made by a 3D 1 3 Australasian Physical & Engineering Sciences in Medicine (2019) 42:811–818 813 EPR spectrum, and the signal amplitude for each dosimeter was defned as the average of three measurements to reduce the deviation of the acquisition data. Irradiation condition A clinical linear accelerator (LINAC) (Rapid Arc, Varian Medical