ISSSD 2020 ONLINE ______Radioluminescence response of Ge-doped cylindrical and flat silica fibers for realtime dosimetry

Adebiyi Oresegun1, Zubair H. Tarif1,5, Louay Ghassan1, Mohd Hafiz Bin Mohd Zin2, Hairul Azhar Abdul-Rashid1*, and David A. Bradley3,4

1Fibre Optics Research Centre, Faculty of Engineering, Jalan Multimedia 63100, Cyberjaya, .

2 Advanced Medical and Dental Institute, Universiti Sains Malaysia (USM) Bertam 13200, Kepala Batas Penang, Malaysia 3Centre for Radiation Sciences, Sunway University, 46150 PJ, Malaysia.

4Department of Physics, University of Surrey, Guildford, GU2 7XH, UK.

5Lumisysns Technology Sdn Bhd, Cyberjaya 63100, , Malaysia

*Corresponding author email: [email protected]

Abstract

This paper reports the radioluminescence (RL) response of Ge-doped Silica flat and cylindrical fiber exposed to photon irradiation beams (6MV and 10MV) with varying doses. The fibre under study is custom fabricated with varying germanium (Ge) doping concentrations (6 and 10 mol%) and cut into 20mm length samples. Each sample was exposed under similar parameters to observe their performance as potential dosimeters, particularly the RL linearity, dose-rate dependence, energy dependence and reproducibility. All measurements were made under the same conditions, with the same field size and same source to surface distance (SSD). The RL response for 6 mol% concentration Ge doped samples showed a higher yield compared to the 10 mol% samples. Both samples, flat and cylindrical fibers, responded linearly to the absorbed dose, with the cylindrical fiber showing a 38% higher yield compared to the flat fiber. The cylindrical fiber also exhibits dose repeatability of <1%, compared to the flat fiber at a 6MV photon energy.

Keywords: Radioluminescence, Real-time Dosimetry, Optical Fiber Scintillator, Germanium

______173 Proccedings of the ISSSD 2020 Volume 1

ISSSD 2020 ONLINE ______1. INTRODUCTION

Radiation dosimetry plays a significant role in ensuring effective therapy and safety concerns for patients and operators. Radiation detectors are thus required to measure the absorbed doses with an excellent level of precision. The necessity for precise dose measurement detectors is denoted by the past occurrence of significant radiotherapy incidents due to human errors and systematic errors (Boadu & Rehani, 2009; IAEA, 2000; Mayles, Nahum, & Rosenwald, 2007; Williams, 2007). Various dosimetry techniques are adapted to fill this role. To satisfy specific radiotherapy dosimetric demands, dosimeters of different technologies are tested for performance and precision, from pulse resolved measurements using diamond detectors (Hugtenburg, Johnston, Chalmers, & Beddoe, 2001; Velthuis et al., 2017), to quality assurance using a MOSFET (Butson et al., 2007)and to flexible film detectors (Cho et al., 2020). The ionization chamber became the standard benchmark for dosimetry measurement. However, a limitation arises during complex radiotherapy modalities such as intensity-modulated radiation therapy (IMRT) and stereotactic radiosurgery with different dose gradients and smaller fields (Leybovich, Sethi, & Dogan, 2003).

Further insight to these limitations is discussed by Laub & Wong, (2003) and Low, Parikh, Dempsey, Wahab, & Huq, (2003). Although using a smaller detector would be an appropriate fix, this would cause a reduction in both sensitivity and yield. The decrease in dosimetry volume causes a decrease in ionization deposition within the detector. Therefore, scintillation dosimeters can be an appealing way to resolve these issues (Beddar, Kinsella, Ikhlef, & Sibata, 2001).

Mones et al., (2008) reports on dosimetric properties of Ce3+ doped silica fiber when irradiated under photon beams form a cobalt-60 and 6MV linear accelerator source fitted with a stereotactic collimator. Similarly, Tanyi et al., (2010) reported the same technique and dopant that showed temperature independence within the range from 22 to 51oC during irradiation under a 15MV photon beam. Justus, Falkenstein, & Huston, (2004) used Cu- based doped silica fiber for realtime invivo radiotherapy measurement using the fiber- coupled dosimetry system. Pain et al., (2000) reports on their application of scintillating ______174 Proccedings of the ISSSD 2020 Volume 1

ISSSD 2020 ONLINE ______optical fiber to quantitatively measure the neuropharmacological radiotracer kinetics and the dose distribution in lively and unchained animals. By attaching an intracerebral probe of minute size, achieved by implanting surgically in the animals’ skull. This technique measures the neurophysiological activity in lively and unchained small animals. Currently, there are few studies into the RL response of geometrically varying differences of scintillating silica fibers. Most of these studies are carried out using cylindrical shaped silica fibers with the major varying difference in their size.

Although, a previous study carried out into the effect of Ge-doped silica under TL technique (Entezam et al., 2016; Lam et al., 2019a), highlights the effect of luminescence response in cylindrical-, flat- photonic crystal fibers (PCF). In their respective studies, a few parameters such as repeatability, angular-independence and dose-rate independence the flat-fibres offer superior performance compared to that of the cylindrical fibre. Recent studies by Rahman et al., (2018) into RL dosimetry of Ge doped silica fiber compared to

Al2O3:C, which is the closest to the structural variant with respect to the shape, the latter showing memory effect and slower temporal response. Although a high luminescence response is recorded with Al2O3:C, the limitation it shows makes it ineffective as a RL potential dosimeter.

This paper reports on the RL response of Ge-doped silica cylindrical and flat fiber exposed to photon irradiation beams (6MV and 10MV) with varying doses. The fibers under study are custom fabricated with varying germanium (Ge) doping concentrations (6-10 mol%) and cut into 20mm length samples. Each sample is exposed under similar parameters to observe their performance as potential dosimeters, particularly the RL yield linearity, least detectable dose, energy dependence, and reproducibility. All measurements were made under the same conditions, with the same field size and same source to surface distance (SSD). The RL response for lower concentration Ge doped samples showed a higher yield in comparison to the higher concentration samples. Both samples, flat and cylindrical fibers, respond linearly to the absorbed dose, with the cylindrical fiber showing a 38% higher RL yield compared to the flat fiber. The cylindrical fiber also exhibits dose repeatability of <1%, compared to the flat fiber at a 6MV photon energy. The following

______175 Proccedings of the ISSSD 2020 Volume 1

ISSSD 2020 ONLINE ______sections present the material and methods used in the study, discussion of results, and conclusion.

2. Materials and Method

This section discusses the design and fabrication of the probe which houses the scintillator, as part of the realtime dosimetry system. Followed by scintillator design and fabrication. Finally, we present the dosimetry experimental setup, parameters and procedure. 2.1 Probe design and fabrication Each probe housing the scintillator, is designed while considering the dimensions of scintillating optical fiber and the optical fiber waveguide. In selecting the probe material, the major factor considered is the effect of ionizing radiation on the probe. Preferably, the material should have minimal response to ionizing radiation, limiting the noise perturbing the RL from the scintillator. For this, an organic material like Polyoxymethylene (POM) was chosen. Besides this, the material used for the probe must not be luminescent at room temperature, in the presence of ambient light and background radiation. For the probe fabrication, a 1 meter black POM rod with an outer-diameter (OD) of 5mm was obtained, cut and smoothen at both ends into 40mm samples. Both ends of the POM material are drilled according to the scintillators and PMMA fiber diameter, respectively. The schematic below in Figure 1 shows a butt-coupling technique of the scintillating fibre and the PMMA. The probe is designed so that different scintillator optical fiber samples with similar size can be interchanged.

Figure 1 probe schematic for housing scitillating fiber which is butt-coupled to the PMMA fiber 2.2 Scintillator design and fabrication ______176 Proccedings of the ISSSD 2020 Volume 1

ISSSD 2020 ONLINE ______The scintillator material is chosen based on a few criterias. One of the criteria is high RL yield that are found in inorganic scintillating materials (Eijk, 2002; Lecoq, Annenkov, Gektin, Korzhik, & Pedrini, 2017). The other criteria is the fast temporal RL response that provides the ability to make realtime dosimetry measurements (Lecoq, Gektin, Korzhik, & Christian Pedrini, 2016). Another aspect of the scintillator design, particularly for radiotherapy in comparison to Ionizing Chambers, is the spatial resolution. As the scintillator optical fiber, which has submillimeter radius and millimeter length, has the potential for high spatial resolution with precise and accurate 3D dose measurement (Goulet, Archambault, Beaulieu, & Gingras, 2013). In this study, two sets of silica optical fibers doped with germanium are tested each of which varies in both geometry and concentration. Table 1, can illustrate these variations. The RL yields were compared with each other in relation to concentration for the scintillating fibers with similar geometry.

Table 1: Details on the types of silica based optical fibres used in study of RL properties Preform Fiber properties Dopant Dimensions Batch No Concentration (mol Fiber type Mass (g) ±0.0003 mg (±0.1μm) %) Cylindrical 611 / 97.1 10.36 Ge1 6 Flat 750 * 220 6.57 Cylindrical 555 / 51.3 8.68 Ge 2 10 Flat 750 * 200 5.42

The in-house fabricated fibers were developed from the preforms using the modified chemical vapour deposition (MCVD) method. The respective preforms of different germanium (Ge) dopants are pulled into the desired optical fibre using a drawing tower. The techniques adapted to this pulling of each preform into a fibre of varying geometry are detailed by Entezam et al., (2016) and Lam et al., (2019b). For this dosimetric experiment

study under a high-energy photon beam, the Ge doped silica (SiO2:Ge) fibre. Each fibre is cut into a 20mm length, polished on each end using a swirl eight-hand motion under a 30- 6µm grit lapping film (Thorlabs®). The respective diameter for each fibre is given in Table 1. The fibres are without any polymer coating; therefore, an induced luminescence effect from the polymer coat is absent. 2.3 Radioluminescence procedure and Setup

______177 Proccedings of the ISSSD 2020 Volume 1

ISSSD 2020 ONLINE ______Following the International Atomic Energy Agency (IAEA) TRS-398 dosimetry code of practice, the probe containing the scintillating optical fibre is placed 4cm into the standard radiation field (10 x10 cm2). The probe laid within a water phantom (bolus), which the maximum percentage depth-dose, dmax is 1.5 cm. Using an Elekta Linear accelerator as the radiation source with a 6MV high-energy x-ray, the radiation produced is bremsstrahlung in nature. Figure 1 illustrates the position of the scintillating fibre and the plastic optical fibre (PMMA) used in the propagation of the luminescence signal. Both ends of the probe from the scintillating fibre insertion to the PMMA end going into the probe is sealed to prevent errors due to ambient light. Within the probe, the butt-coupling method is applied for connecting the scintillating fibre and the PMMA(Elsey et al., 2007). While, Figure 2, illustrates the RL procedure as luminescence is propagating through the PMMA fibre 15m in length and connects the dosimetry measuring hardware system (LS-1000) to the Scintillating optical fiber. The RL response detected by the hardware dosimetric system is captured with an in-house lab-view based controller software and interface.

Figure 2 Schematic illustration of the real time radioluminescence setup procedure

3. Result and Discussion 3.1 Linearity Following the procedure stated in section xx, the field-sized is maintained at 10 x 10 cm. The scintillating fibre samples were placed within the solid-water™ phantom on the central axis 4cm with the field, at the isocentre at 1.5 cm depth, a depth at which the maximum dose is delivered to the detectors. The linearity and sensitivity measurements were performed for a range of doses typical given to the patient, which are conventionally fractionated doses of 25, 50, 100, 200 and 400 cGy for each type of fibre listed in Table 1. ______178 Proccedings of the ISSSD 2020 Volume 1

ISSSD 2020 ONLINE ______

Figure 3. dose-dependency for flat and cylindrical fibres, in comparison to the ionizing chamber

The dose response of each of the fibre is taken and compared to similar dose measurement in realtime from an ionizing chamber. Figure 3a and Figure 3b, can illustrate the dose measured by the respective dosimeter from the Linac machine.

In each Figure 3a and 3 b, for which the data reflect on the flat - and cylindrical fiber in comparison to the IC; there are two key differences, the response to the overshoot from the Linac at high dose-rates and phosphorescence decay after irradiation. As seen in the both figure 3a and b, the flat fibers tends show and incline incremental response at high-rates of 600cGy/min, with a dose of 400cGy. This phenomena is not reserved to just the high-dose rate but to lower dose rates as well, although the effect is far reduced and visible. As seen,

______179 Proccedings of the ISSSD 2020 Volume 1

ISSSD 2020 ONLINE ______the comparison of the FF with the IC, shows the inability of the FF to respond to the Linacs overshoot at high dose-rate as well as slow decay after the Linac is turned off. In the following case of the cylindrical fiber (CF), its response to the overshoot is visible as well, and it shows a consistent response with the IC at both high dose-rates of 600cGy/ min and lower dose-rate.

In the de-excitation of the dosimeters (turning off the Linac), the scintillation for both the FF and CF show a phosphorescence decay with varying periods of decay. Irrespective of the dosimeter decay spectral response, there is a consistency seen with measured dose delivered as Figure 4 illustrates the linearity response to dose for each sample fiber. It is of importance to note that the radioluminescence yield in each fiber varies with respect to either the fiber geometry or the dopant concentration. As for the sample fibers, Table 1 gives the dimensions as well as the concentration levels, making the concentration being the only differing factor. It shows that 6 mol% has a higher RL response of ~17.6% in comparison to its 10 mol% counterpart. This relation is also seen in flat fiber, as the lower concentration (6 mol%) has a high RL response of ~ 10.5%.

Figure 4 shows the dosimetric response for cylindrical fibres and flat-fibres (6 & 10% Ge-doped), irradiated with 6 MV photons for doses from 25cGy – 400 cGy.

3.2 Dose-rate dependence

______180 Proccedings of the ISSSD 2020 Volume 1

ISSSD 2020 ONLINE ______

Figure 5 Dose rate dependency response for Ge-doped SiO2 (a) cylindrical fibre with different concentration (6-10 mol %), (b) cylindrical and flat fibre, and (c) histogram representation of (b) irradiated with 6 MV photon beams delivering doses 50cGy over a dose-rate of 50 - 600 cGy. error bars are not visible due to the minute size of the data points

______181 Proccedings of the ISSSD 2020 Volume 1

ISSSD 2020 ONLINE ______The dosimetric response of an ideal dosimeter should remain constant irrespective of the dose-rate (IAEA, 2005). As with the Ge-doped silica used in this study, the dose-response was demonstrated with the varied geometry, under 6MV photon beam with a constant dose of 50cGy and a varying dose-rate. The illustration in Figure 4 a gives the RL dosimetric response of the delivered dose. While Figure 5b, c shows the total RL response at a given dose-rate.

(Lam et al., 2019a) dosimetric study of Ge-doped silica of varied geometry under thermoluminescence (TL) response gave a dose-rate independent response. This independent response was observed at a high dose above 1500cGy. However, this independent response is observed at 300cGy to 600cGy in RL studies. While, at lower dose-rate from 50cGy to 150cGy, a dosimetric dependence is observed. This phenomenon is present irrespective of the fiber geometry, although the fibers RL response remains dependent on the fiber geometry.

3.3 Repeatability The repeatability of each fiber was conducted with a fixed dose-rate of 600cGy and three specific doses were delivered (100cGy, 300cGy and 600cGy). Each dose was repeated over a 10 times to verify the repeatable capability of the doped-silica fiber.

The repeatability of the fibre types is determined using the resultant data and the coefficient of variation percentage was obtained (CV%). It is necessary to note that for each type of fibre fabricated was the same used to conduct the whole of the experiment. Although previous parameters were compared with the Ionizing Chamber, for the repeatability, we focused on comparing the various geometry and concentration of fibres. This parameter would also give a highlight into the effect of prolonged use of the fibre in realtime situations for dosimetry measurements.

Further investigation of reproducibility is carried out for 6-, 10-MV photon energy at a dose of a 100, 300 and 600cGy for the four selected fibres presented in Table 1. Additionally,

______182 Proccedings of the ISSSD 2020 Volume 1

ISSSD 2020 ONLINE ______Table 2 shows the standard deviation (SD) values of each fiber at different doses and energies, these emphasis the repeatability of results obtain form the dose delivered from the Linac, confirming the accuracy and robustness of the doped fiber as potential dosimeters.

Table 2 records the dose repeatability of the flat-, cylindrical- both having dopant concentrations of 6 mol% and 10 mol% to range between 2% to 3%, 0.7% to 0.7% respectively at the 100cGy dose. At a dose of 100cGy, flat- fibers have a highest deviation at lower doses, although at a higher dose of 600cGy there is a lower deviation between 0.19% to 0.32%.

Table 2 Dose-response repeatability of the Ge-doped silica fibres irradiated at a fixed dose-rate of 600cGy over 3 doses, 100cGY, 300cGy and 600cGy in 10×10 cm2 field for a 6MV and 10MV photon beam.

6MV 10MV Fibres Dose Photon counts Photon counts Mean SD CD% Mean SD CD% Cylindrical 100 725735 6315.88 0.87027 647161.5 4826.004 0.745719 fibre 6 mol% 300 2138009 6161.02 0.28817 1894551 9953.942 0.525399 600 4267124 3801.41 0.08909 3758221 6458.006 0.171837

Cylindrical 100 546112 4920.76 0.90105 549659 4224.256 0.768523 fibre10mol% 300 1594638 7586.4 0.47574 1600341 16687.01 1.04716 600 3181493 10202.5 0.32068 3158577 1803.122 0.057087

Flat fibre 100 404726 5003.19 1.23619 344018 8548.921 2.485021 6 mol% 300 1257801 10025.6 0.79707 1102587 13466.14 1.221322 600 2562409 11535 0.45016 2248875 4381.234 0.197396

Flat fibre 100 364743 3745.5 1.02689 618276.5 18785.71 3.038399 10 mol% 300 1140099 10642.3 0.93345 393486.5 188.7975 0.047981 600 2327107 6539.91 0.28103 2411498 7881.41 0.3268264

______183 Proccedings of the ISSSD 2020 Volume 1

ISSSD 2020 ONLINE ______3.4 Energy dependency

Figure 6 Comparison of sensitivity of 100 μm core sizes cylindrical and flat fibres (10% & 6% Ge- dopant) for different energies is shown.

The above defined sensitivity of the detectors is interpreted as the ratio of readout counts value to a given dose. The sensitivity of the fibres which were obtained for two photon energies, 6 MV and 10 MV, Two sets of fibres with different geometry are selected cylindrical and flat fiber and further irradiation was carried out. Measurement for all four fibre of each fibre type was carried out at 100 cm SSD and 10 × 10 cm2 field size. Figure 6 represents the results of sensitivity of the studied fiber which vary on the basis of geometry and concentration and they are labelled with their respective numerical values.

The cylindrical fiber shows a higher sensitivity to the flat fiber irrespective of the dopant concentration. However a significant difference is visible in the sensitivity for each cylindrical and flat fiber when focusing on their dopant concentration. As seen in Figure 6, for each type of fiber the lower concentration (6 mol%), shows a higher variation in sensitivity when comparing the response from 6 MV and 10MV photon energy. For the higher concentration (10 mol%), the cylindrical fiber shows an almost minute sensitivity for the 6M. As compared to the flat fibers of similar concentration, there is a higher

______184 Proccedings of the ISSSD 2020 Volume 1

ISSSD 2020 ONLINE ______sensitivity at the higher energy of 10MV. Although, the overall sensitivity is obtained in the 6M compared to the 10MV photon energy,

3.5 Absorbed Dose

Figure 7. RL response from Ge-doped 6 mol% silica fibre (a) flat fibers (b)cylindrical fiber versus IC response for varied delivered doses(1cGy – 400cGy) at 600 cGy/min, 6 MV Photon beam

In this section, the dose is delivered as low as 1cGy to as high as 400cGy with a fixed dose rate of 600cGy/min. it is necessary to note that under a 600cGy/min dose-rate, it takes the linac ~0.1s to deliver 1cGy dose. The aim of this study was to observe the response and recovery pattern of sample doped silica fiber for a continuous real-time measurement. Although, the minimum dose delivered by the linace is in no way conforming to the least minimum dose detectable, but just an insight to behavioural characteristics of the doped silica. Making reference to figure 3a and figure 3, the dose response at a high dose-rate showed unique characteristics in the flat fiber. This is further studied in Figure 7a, as the flat fiber is exposed to a fixed dose rate of 600cGy while varying the doses. This figure gives a rather slow response to the dose delivered. With an inclined increase in photon peaks are the dose increased. Another peculiar characteristic noticed of the flat fiber is its slow recovery, which is long phosphorescence decay. Although ample amount of time was not given to the flat fiber to completely decay, this study was as stated conducted with the thought of the rigorous measurement to be conducted with the potential dosimeter. Similar ______185 Proccedings of the ISSSD 2020 Volume 1

ISSSD 2020 ONLINE ______study using the cylindrical fiber showed a rather different result, as some of the effects which plagued the flat fiber are less prominent.

In this study, dose is delivered at 1cGy to as high as 400cGy, at a dose-rate of 600cGy/min, demonstrated in Figure 7. Under the same reference condition from previous literature, it would take 0.1s to deliver a dose of 1cGy (Almond & Biggs, 1999). A dose this low is not typically administered to a patient in an ideal radiotherapy situation. Although, it is used for the benefits of studying the minimum dose delivered that is detectable by the doped silica fiber. Figure 7b illustrates this while comparing the RL response of the doped silica fiber to the response from a clinically calibrated IC. Both dosimeters are able to respond to the dose delivered. However the calibrated uniqueness of the IC comes into play once doses increase from 5cGy to 400cGy. The count intensity of the IC is shown to be ~ 600, as compared to our doped silica fiber. This occurrence is seen as the dose increases to 400cGy.Hence, it poses the question if this is the working of the calibrated IC reading or the unique dose delivery system of the linear accelerator. As the Linac has to conform to the working principles and guidelines from American Association of Physicists in Medicine (AAPM) (Smith et al., 2017). Due to previous results from Figure 3, we do understand that the Linac tends to over shoot before stabilizing, this phenomenon is present and consistent in the IC reading shown. In the case of Ge-doped silica fiber, the RL yield is dependent on the scintillation efficiency with respect to the dopant concentration as seen in Figure 4. The doped silica fiber response rather oddly, in that doses from 5cGy to 400cGy, has a peak photo count at the lower dose and then lowers to saturation at higher dose. This creates a sort of an exponential decay like phenomena occurring with respect to the intensity (counts) for each dose. A few hypotheses to this effect come to mind; a) an optimum luminescence response is obtained at low doses of 5cGy, b).the actual reading recorded as the Linac has not stabilized. Based on the last hypothesis, a few things come to mind, as a Linac isn't meant to deliver such a low dose at that specific reference setting under typical circumstances. Also, due to the Linac being an electrically operated device, this tends to play a role in the beam start-up which affects the burst of photon packets. This is evident from the IC reading as we can observe an exponential decay till saturation at the top of doses 50- 400cGy. As stated earlier, the IC being the standard calibrated reference would

______186 Proccedings of the ISSSD 2020 Volume 1

ISSSD 2020 ONLINE ______not increase higher than a specific dose for a given dose-rate. It is necessary to note that this falls within the parameters listed in AAPM practice guidelines for a linear accelerator (Smith et al., 2017).

4. Conclusion Geometry dependence of RL dosimetry is important to ensure accuracy and potentially increase the sensitivity of such measurements. This paper reports the radioluminescence (RL) response of Ge- doped Silica flat and cylindrical fiber exposed to photon irradiation beams (6MV and 10MV) with varying doses. Both samples, flat and cylindrical fibers responded linearly to the absorbed dose, with the cylindrical fiber showing a 38% higher RL yield compared to the flat fiber. The cylindrical fiber also exhibits dose repeatability of <1%, compared to the flat fiber at a 6MV photon energy. Each fiber was able to demonstrate a favorable response to doses as low as 0.1Gy with a 37% lesser response when compared to doses ranging from 2Gy to 4Gy. Although the response of the flat-fiber shows a rather uncharacteristic response to the low doses delivered at high dose-rates, this phenomena can be attributed to the retrapping of hole- and electron pairs. The traps can be classified as stress induced defects which are created due to the unique fabrication process of the fibers. These results demonstrate a strong potential for cylindrical Ge-doped optical fiber to be used as scintillators in realtime RL dosimetry systems.

Acknowledgement We are grateful for the grant support provided in carrying out this project, namely ICF (MMUE/190082).

REFERENCES

Almond, P. R., & Biggs, P. J. (1999). AAPM ’ s TG-51 protocol for clinical reference dosimetry of high-energy photon and electron beams. (September), 1847–1870.

Beddar, A. S., Kinsella, T. J., Ikhlef, A., & Sibata, C. H. (2001). A Miniature ― Scintillator- Fiberoptic-PMT ‖ Detector System for the Dosimetry of Small Fields in Stereotactic Radiosurgery. IEEE Transaction on Nuclear Science, 48(3), 924–928. ______187 Proccedings of the ISSSD 2020 Volume 1

ISSSD 2020 ONLINE ______Boadu, M., & Rehani, M. M. (2009). Unintended exposure in radiotherapy: identification of prominent causes. Radiotherapy and Oncology, 93(3), 609–617.

Butson, M. J., Rozenfeld, A., Mathur, J. N., Carolan, M., Wong, T. P. Y., & Peter, E. (2007). A new radiotherapy surface dose detector : The MOSFET. Medical Physics, 23(5), 1996. https://doi.org/10.1118/1.597702

Cho, J. D., Son, J., Sung, J., Choi, C. H., Kim, J. S., Wu, H., … Kim, J. (2020). Flexible film dosimeter for in vivo dosimetry. Medical Physics, 47(7), 3204–3213.

Eijk, C. W. E. van. (2002). Inorganic scintillators in medical imaging. Physics in Medicine and Biology, 47(8), R85–R106. https://doi.org/10.1088/0031-9155/47/8/201

Elsey, J., McKenzie, D. R., Lambert, J., Suchowerska, N., Law, S. L., & Fleming, S. C. (2007). Optimal coupling of light from a cylindrical scintillator into an optical fiber. Applied Optics, 46(3), 397–404. https://doi.org/10.1364/AO.46.000397

Entezam, A., Khandaker, M. U., Amin, Y. M., Ung, N. M., Bradley, D. A., & Maah, J. (2016). Thermoluminescence Response of Ge-Doped Cylindrical- , Flat- and Photonic Crystal Silica-. PLOS ONE, i, 1–15. https://doi.org/10.1371/journal.pone.0153913

Goulet, M., Archambault, L., Beaulieu, L., & Gingras, L. (2013). 3D tomodosimetry using long scintillating fibers: A feasibility study. Medical Physics, 40(10), 101703.

Hugtenburg, R. P., Johnston, K., Chalmers, G. J., & Beddoe, A. H. (2001). Application of diamond detectors to the dosimetry of 45 and 100 kVp therapy beams: comparison with a parallel-plate ionization chamber and Monte Carlo. Physics in Medicine & Biology, 46(9), 2489.

IAEA. (2000). Lessons learn from accidental exposures in radiotherapy. Safety Reports Series 17.

IAEA. (2005). Radiation Oncology Physics : A Handbook for Teachers and Students (Podgorsak,). Vienna.

Justus, B. ., Falkenstein, P., & Huston, A. (2004). Gated Fiber-Optic-Coupled Detector for In Vivo Real-Time Radiation Dosimetry. Applied Optics, 43(8), 1663–1668. https://doi.org/10.1364/AO.43.001663

Lam, S. E., Bradley, D. A., Mahmud, R., Pawanchek, M., Rashid, H. A. A., & Noor, N. M. (2019a). Dosimetric characteristics of fabricated Ge-doped silica optical fibre for small-field dosimetry. Results in Physics, 12(October 2018), 816–826. https://doi.org/10.1016/j.rinp.2018.12.030

Lam, S. E., Bradley, D. A., Mahmud, R., Pawanchek, M., Rashid, H. A. A., & Noor, N. M. (2019b). Dosimetric characteristics of fabricated Ge-doped silica optical fibre for small-field dosimetry. Results in Physics, 12, 816–826. https://doi.org/10.1016/j.rinp.2018.12.030

______188 Proccedings of the ISSSD 2020 Volume 1

ISSSD 2020 ONLINE ______Laub, W. U., & Wong, T. (2003). The volume effect of detectors in the dosimetry of small fields used in IMRT. Medical Physics, 30(3), 341–347.

Lecoq, P., Annenkov, A., Gektin, A., Korzhik, M., & Pedrini, C. (2017). Scintillation Mechanisms in Inorganic Scintillators. In Inorganic Scintillators for Detector Systems (pp. 125–174). https://doi.org/10.1007/3-540-27768-4

Lecoq, P., Gektin, A., Korzhik, M., & Christian Pedrini. (2016). Inorganic Scintillators for Detector Systems (physical principles and crystal engineering., Ed.). https://doi.org/10.1007/978-3-319-45522-8

Leybovich, L. B., Sethi, A., & Dogan, N. (2003). Comparison of ionization chambers of various volumes for IMRT absolute dose verification. Medical Physics, 30(2), 119–123.

Low, D. A., Parikh, P., Dempsey, J. F., Wahab, S., & Huq, S. (2003). Ionization chamber volume averaging effects in dynamic intensity modulated radiation therapy beams. Medical Physics, 30(7), 1706–1711.

Mayles, P., Nahum, A., & Rosenwald, J. C. (2007). Handbook of radiotherapy physics: Theory and practice. Handbook of Radiotherapy Physics: Theory and Practice, pp. 1–1453. https://doi.org/10.1118/1.2969650

Mones, E., Veronese, I., Vedda, A., Loi, G., Fasoli, M., Moretti, F., … Brambilla, M. (2008). Ce- doped optical fibre as radioluminescent dosimeter in radiotherapy. Radiation Measurements, 43(2–6), 888–892. https://doi.org/10.1016/j.radmeas.2008.01.031

Pain, F., Laniece, P., Mastrippolito, R., Charon, Y., Comar, D., Leviel, V., … Valentin, L. (2000). SIC, an intracerebral radiosensitive probe for in vivo neuropharmacology investigations in small laboratory animals: theoretical considerations and practical characteristics. IEEE Transactions on Nuclear Science, 47(1), 25–32.

Rahman, A. K. M. M., Begum, M., Begum, M., Zubair, H. T., Abdul-Rashid, H. A., Yusoff, Z., & Bradley, D. A. (2018). Radioluminescence of Ge-doped silica optical fibre and Al2O3: C dosimeters. Sensors and Actuators A: Physical, 270, 72–78.

Smith, K., Balter, P., Duhon, J., White Jr, G. A., Vassy Jr, D. L., Miller, R. A., … Fairobent, L. A. (2017). AAPM Medical Physics Practice Guideline 8.a.: Linear accelerator performance tests. Journal of Applied Clinical Medical Physics, 18(4), 23–39. https://doi.org/10.1002/acm2.12080

Smith, K., Balter, P., Duhon, J., White, G. A., David, J., Jr, V., … Fairobent, L. A. (2017). AAPM Medical Physics Practice Guideline 8 . a .: Linear accelerator performance tests The American Association of Physicists in Medicine. (January), 23–39. https://doi.org/10.1002/acm2.12080

Tanyi, J. A., Krafft, S. P., Ushino, T., Huston, A. ., & Justus, B. L. (2010). Performance characteristics of a gated fiber-optic-coupled dosimeter in high-energy pulsed photon radiation dosimetry. Applied Radiation and Isotopes, 68, 364–369. https://doi.org/10.1016/j.apradiso.2009.10.042

Velthuis, J. J., Page, R. F., Purves, T. M., Beck, L., Hanifa, M. A. M., & Hugtenburg, R. P. (2017). ______189 Proccedings of the ISSSD 2020 Volume 1

ISSSD 2020 ONLINE ______Toward pulse by pulse dosimetry using an SC CVD diamond detector. IEEE Transactions on Radiation and Plasma Medical Sciences, 1(6), 527–533.

Williams, M. V. (2007). Radiotherapy near misses, incidents and errors: radiotherapy incident at Glasgow. Clinical Oncology, 19(1), 1–3.

______190 Proccedings of the ISSSD 2020 Volume 1