STUDIES OF DOSIMETRY PROTOCOLS FOR ACCELERATED PHOTONS AND ELECTRONS DELIVERED FROM MEDICAL LINEAR ACCELERATOR

A.K.M. Moinul Haque Meazea,*, Santunu Purohita, Md. Shakilur Rahmanb, Abdus Sattara, S.M. Enamul Kabirc, Md. Kawchar Ahmed Patwaryd, Kamrunnahar Kalid, Md. Jubayer Rahman Akhande aDepartment of Physics, University of Chittagong, Bangladesh bSecondary Standard Dosimetry Laboratory, Bangladesh Atomic Energy Commission, Savar, Dhaka, Bangladesh cNational Institute of Cancer Research, Dhaka, Bangladesh dDepartment of Physics, Comilla University, Bangladesh eBangladesh Military Academy, Bhatiary, Chittagong, Bangladesh *Corresponding Author: meaze@cu.ac.bd; mhqmeaze@yahoo.com Received October 22, 2020, accepted December 15, 2020

Approximately 60% of cancer patients are referred for external beam radiotherapy, for which the most commonly used equipment is a medical LINAC that produces an electron beam and photon beam [1]. The precise planning of the treatment depends on the tumor type, size, position, stage, and health condition of patients [1,2]. By considering various uncertainty components associated with beam calibration factors, a study of the uncertainty in determining of the absorbed dose to water had been carried out by C. Pablo et.al. [3] Their results showed a typical uncertainty in the determination of absorbed dose to water during beam calibration approximately 1.3% for photon beams and 1.5% for electron beams (k = 1 in both cases). M. S. Huq et. al. [4] performed a study by comparing International Atomic Energy Agency Technical Report Series No. 398 (IAEA TRS-398) and AAPM TG-51 absorbed dose to water protocols in the dosimetry of highenergy photon and electron beams. They compared the two protocols in two ways: (i) by analyzing the differences of the basic data included in the two protocols for photon and electron beam dosimetry in detail and (ii) by performing experiments in clinically accelerated photon and electron beams and determining the absorbed dose to water following the recommendations of the two protocols [4]. For electron beams, the ratios TG-51/TRS-398, of the absorbed dose to water Dw were observed to be lie between 0.994 and 1.018 depending upon the chamber and electron beam energy used, with mean values of 0.996, 1.006, and 1.017 respectively, for the cylindrical, well-guarded and not well-guarded planeparallel chambers [4]. A dosimetric study comparing NCS report-5, IAEA TRS-381, AAPM TG-51 and IAEA TRS-398 in three clinical electron beam energies was carried out by H. Palmans et. al. [5]. In their work, they compared dosimetry for three clinical electron beam energies using two NE2571-type cylindrical chambers, two Markus-type plane-parallel chambers and two NACP-02-type plane-parallel chambers [5]. Another comparison of high-energy photon and electron dosimetry for various dosimetry protocols was performed by F. Araki et. al. [6] They calculated the absorbed dose to water calculated according to the Japanese Association of Radiological Physics, IAEA TRS-277 and IAEA TRS-398 protocols, and compared it to that calculated using the TG-51 protocol. A comparison of protocols for external beam radiotherapy beam calibrations was carried out by S S Al-Ahababi et. al. [7] where they used the IAEA TRS-398, AAPM TG-51 and IPEM 2003 protocols. The comparisons were carried out by delivering electron beams of nominal energies of 6,9,12,16 and 20 MeV using Physikalisch-Techische Bundesanstalt (PTW) Markus and NACP-02 plane-parallel chambers.
Different group of dosimetrists did experiments several times to ensure lower uncertainty, best suited protocols and improvement of protocols for the commissioning of medical Linac and more precisely healthcare purposes. The aims of our work is to analyze the dosimetry applying three different most preferable protocols maintaining the QA parameters for high energy photon and electron beams delivered from the medical linear accelerator (Clinac). Different ionization chambers were used to calculate the absorbed dose to water and a comparison among chambers was investigated. For each chamber the absorbed dose to water was calculated using three different protocols. Sometimes in same reference conditions absorbed dose differs from Clinac to Clinac because of wall material of jaws. To confirm that dose variations Studies of Dosimetry Protocols for Accelerated Photons and Electrons...

EEJP. 1 (2021)
we use two different medical LINAC and same chamber response with LINAC in this research work. This study will be helpful for defining more accurate dosimetry and developing more general protocol for ensuring patient safety during treatment planning.

METHODS AND MATERIALS Absorbed dose to water calibration in 60 Co
The calibrations in terms of absorbed dose to water are available only for 60 Co gamma radiation [8]. The reference point of the chamber was at 5g/cm² water depth. The size of the radiation field (50% isodose level) at the reference plane was 10 cm×10 cm [9 -12]. The PTW Markus chamber was set up for determining the calibration factor in a water phantom, and then the Physikalisch-Techische Bundesanstalt (PTW) UNIDOSE electrometer was used to obtain the dose rate. From these dose rates the calibration factor was measured using the IAEA TRS-398 protocol. The same procedure was used to calibrate the Exradin A10 and IBA FC65-G (2009) chambers. The descriptions of different protocols are presented in Table 1.

Calibration of Ionization Chambers
The calibration factors of Markus, A10 and FC65-G are listed in Table 2. a. Beam quality.
The measurement of K Q using three different protocols are presented in Table 3.

b. Comparison among protocols.
To make a comparison among protocols, we considered three main correction factors: pressure temperature correction, ion recombination correction and polarity correction factors. The values of these parameters are listed in Table 4. A comparison of the maximum dose depths (D max ) measured with three different protocols is presented in Table 5.
Studies of Dosimetry Protocols for Accelerated Photons and Electrons... We found that the percentage of the depth dose increases with increasing of energy, and the maximum dose D max decreases. This is because the main influencing correction factor K Q decreases with increasing energy. The variation of the maximum dose depth at D max for FC65G (2005) and FC65G (2009) according to IAEA TRS 398 and AAPM TG 51 was found to be 1.18% and 1.03% in 15 and 6 MV photon energies respectively. However, in DIN 6800-2 the variation of dose at D max for FC65G (2005) and FC65G (2009) was found to be less than 0.5% in both 6 and 15 MV photon energies.

Absorbed dose to water for Electron beam a. PDD Curves.
The PDD curves were observed at energies of 4, 6, 9, 12 and 15 MeV for Clinac 2300CD, and at energies of 6, 9, 12 and 15 MeV for DHX-3186. All comparative curves for limited length are shown in Figures 1 and 2. Since the electron beam has significantly low penetration power the reference depth for an electron is close to the phantom water surface.  The dose percentage with respect to the energy and depth is presented in Table 6.