Absorbed Dose Response in Water of Kilovoltage X-rays Beams of Radiochromic Film and Thermoluminescent for Brachytherapy Dosimetry

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1 Absorbed Dose Response in Water of Kilovoltage X-rays Beams of Radiochromic Film and Thermoluminescent for Brachytherapy Dosimetry Chien-Hau Chu 1, Uei-Tyng Lin 1, Ngot-Swan Chong 2, Wen-Song Hwang 1, Hsiao-Fang Pang 1* 1 National Radiation Standard Laboratory, Health Physics Division, Institute of Nuclear Energy Research, No.1000, Wunhua Rd., Jiaan Village, Longtan Township, Taoyuan County,32546, Taiwan hfpang@iner.gov.tw 2 Far Eastern Memorial Hospital, 21, Nan-Ya S. Rd., Sec.2 Pan-Chiao, Taipei, Taiwan Abstract. Previous conventional calibration methods of dosimeters have shown the preference of using the conversion coefficient for exposure to dose-water. This paper presents an easy and precise method for calibrating dosimeters in absorbed dose to water of low-energy x rays for brachytherapy dosimetry. According to the AAPM TG-61 protocol, the in- method was recommended to determine the absorbed dose on the solid water phantom surface for low energy x-ray dosimetry. In our study, comparisons were made on the photon energy dependence of the sensitivities of Gafchromic MD-55 film and TLD 100H chip over the photon energy range from 40 kv (20 kev) to 300 kv (250 kev). Monte Carlo calculations and experiments were performed to evaluate the dosimetry differences from electron buildup and dosimetry attenuation. These results showed that this method was in good agreement with the applied absorbed dose and uncertainty of the dosimeter calibration was under 5 %. We suggest that the method should also be applied in dosimeter calibrations as it is a reliable technique. 1. Introduction The low energy x-rays absorbed dose standard to water is not popular and neither easy to establish. National Radiation Standard Laboratory in Institute of Nuclear Energy Research (INER) has set up absorbed-dose to-water standard for 60 Co gamma rays to which measurements for radiation therapy and for radiation protection have been traced. In kilovoltage x-ray energy range, we offered calibrations of dosimeters in kerma standards and obtained absorbed-dose by calculations in clinically. Typical dosimeters in brachytherapy such as themoluminescent dosimeters(tlds), diodes and films are often calibrated against low energy x-ray beam, and have been in routine use for dose measurement. Thus, it is important to be familiar with energy response of the dosimeters. In the experiment, the change of x-ray spectrum was related to the energy dependence of the dosimeter and uncertainty during calibration process even directly affected the dosimeter results. Several reports recommended that dosimeters must be calibrated against a reference standard dosimeter under the condition of use. A conversion from optical density to dose is necessary, when photon energy varies with depth, this may involve energy dependence at several points. Comparisons on the photon energy dependence the sensitivity of the three dosimeters to different energies were carried out [1,2] over the photon energy range 28 kev to 1.7 MeV. Analyses were performed for dosimeters frequently used in afterloading treatment including Gafchromic DM1260 films, Silver halids and LiF TLDs (Harshaw). Besides, the TLD-100 chip was evaluated for its energy dependence [3] over the low energy range 10 kev to 35 kev. Since it has to be considered in energy dependence calibration the effect of photon scattering caused by the irradiation field and the positioning is difficult during the dosimeters calibration process, the AAPM TG-61 protocol recommended[4] for the absorbed dose to water calibration of low energy x-rays of the ionization chamber that measures doses in radiotherapy and radiobiology. In the protocol, AAPM gave detailed parameters situations and operation procedures of calibration. It is intended in this research to provide calibration of absorbed dose to water correctly within low energy x-rays ranges based on the protocol. The aim of the paper is to perform Gafchromic MD-55 film and TLD 100H calibrations using the in method of TG-61 as a basis and replace the situation of positioning the ionization chamber on the water surface by using the surface of solid water-phantom and modify some correction factors. Experiments and Monte Carlo simulations will be employed for evaluation of influence caused by different buildup conditions, dose rates and electron contamination. 1

2 2. Methods and Materials To provide x-ray beam equivalent to various photon energies a Pantak 420 kv x-ray machine was used. Its output voltages were from 40 kv to 250 kv. Irradiation conditions were formed according to the changes of output voltages and the additional aluminum, copper, tin and lead films to produce different beam qualities and spectrum. Each set of beam qualities of x-rays were determined by the ionization chamber measuring the half-value-layer(hvl) thickness of the aluminum and copper films and irradiated in the center of a 10 cm 10 cm field at 1 m from the source. The first and second HVL can be obtained from the measurement attenuation curve. The relationship of the first HVL to the linear attenuation coefficient u is: u = ln(1/ 2) / HVL (1) The value of u can be found in the list [5] and the effective energy of x-rays can be obtained by interpolation calculation. The dosimeter was fixed on a solid water phantom 10 cm in thickness to get the full backscatter situation. Also buildup materials were added to the dosimeter surface to make the produced maximum electron energy fall within continuous-slowing-down approximation (CDSA) range. The irradiation conditions were given as Table I and the absorbed dose to water at phantom surface shall be determined according to D = MN k B u en ρ w (2) Where D is the absorbed dose to water of the phantom surface, M is the reading of the free ionization chamber, N k is kerma calibration coefficient to each beam quality, B is backscatter w conversion factor, u en is ratio for water to of the mean mass energy absorption coefficients ρ averaged over the incident photon spectrum. The effective energy range of the x-ray beam quality is from 22 kev to 142 kev, the buildup depth of polystyrene from 3 mg cm -2 to mg cm -2 The backscatter correction factor and the mass energy absorption coefficient are as given in Table II. Table I. Experimental conditions for x-rays beam Beam quality Effective energy (kev) Total wall Half value layer thickness (mg cm -2 ) Al(mm) Cu(mm) Air-kerma rate (mgy/min) M M M M M M Quantities of Gafchromic films was supplied by the Nuclear Associates, Model Number MD-55 (no ). Precautions in handing of radiochromic film, according to the recommendations given in the AAPM TG-55 protocol [6]. The double-exposure method [7] was adopted to decrease the nonhomogeneity of the results after they had been analyzed and read. Since the reaction of the dosimeter sensibility to the film surface was inconsistent, the film was given 5 Gy irradiation dose in the beginning to correct the variations of sensibility. The scanning system used for reading out the 2

3 response of the film. Optical density distributions of radiochromic film pieces were measured by a helium-neon laser (632.8) Kodak LS50 scanning densitometer. The films were irradiated up to doses of 20 Gy in 5 Gy intervals. During experiment, storage and analysis were kept in temperature of about 22 ± 2 thus reducing the effects of time and temperature dependent evolution and readout of the absorption spectra of the film. The film is only removed from a light tight envelop during irradiation and readout to reduce the effects of ambient light. The net optical density for each film was obtained 2 days in the black light sheath after irradiation. Wearing gloves was a must to avoid possible impacts on the reading from fingerprints and other stains on the film surface. Table II. Ratios of average mass energy absorption coefficients water to and backscatter factor as a function of beam quality Beam quality Backscatter factor w u en ρ M M M M M M In each TLD-100H exposure for determination of photon-energy dependence, three LiF chips were irradiated to reduce random errors. TLDs were irradiated up to doses of 50 cgy and 100 cgy. A batch of 100 LiF TLD chips, each with dimension of 4.5 mm 4.5 mm mm(tld-100h, Harshaw Co., now Englehard Corp.), was used in this work. Before each experiment, TLDs were annealed in aged aluminum tray at 240 for 10 minutes. After irradiation, the responses of the TLD chips were measured with a TLD reader (System 310, Rexon Components Inc., Beachwood, Ohio). The response of individual chip was corrected; the calibration factors were determined by previous measurements of the response of all chips to a common dose. The relative sensitivity of chips annealed together varies little between annealing cycles, and the reproducibility of all TLDs were within ±3 % for confidence level of 95 %. All chip responses compared were from the same annealing cycle. transmission percenta M50 M60 M100 M150 M200 M250 depth(mm) FIG. 1. Dose attenuation of x-rays in different beam qualities against materials When performing dosimeter calibrations, it is necessary to buildup material on the phantom surface to reach the state of electron equilibrium. In this study, Monte Carlo calculations and experiments were applied to evaluate the effect to the dosimeter irradiated on the phantom surface caused by electron buildup and dose attenuation. Since the dose rate decreases when the buildup depth increases, the relationship between the buildup depth and the irradiated dose was evaluated by experiment: each set 3

4 of x-ray beam qualities was added with a 0.1 mm polystyrene film to measure the dose attenuation under the same irradiation condition. Monte Carlo calculations were performed to calculate the absorbed dose to water measured by the dosimeter when single-energy photon beams penetrate the buildup material on the water-phantom surface. With the changes of the buildup depth, the increase of dose could be evaluated when the dosimeter was under the conditions of electron equilibrium and no electron contamination. From the results of experiments and calculations, the dose differences arose from the low-energy x-ray buildup condition could be evaluated. Figure 1 shows the experiment results of dose attenuation of x-rays in different beam qualities against polystyrene materials of different thickness. Under this irradiated method conditions, the higher photon energy, the higher transmission ability percentage. It has been observed that the greatest attenuation decrease in effective photon energy 22 kev( M50 beam quality). when the polystyrene depth was increased to 1.5 mm, the difference of dose attenuation to photons is above 5 %. However, in 142 kev(m250 beam quality), when the polystyrene was increased to 3.0 mm, the dose difference was only less than 1 %. percentage(%) M50 M60 M100 M150 M200 M FIG. 2. The electron buildup status Depth(mm) of different beam qualities using Monte Carlo calculations Monte Carlo transport calculations were used to determine the dose differences caused by electron buildup of x-rays in different beam qualities is shown in Figure 2. Under this irradiated method process, the higher photon energy, the thicker is the buildup depth needs to be. It has also been observed that the greatest buildup depth in effective photon energy 142 kev( M250 Beam quality). In M250 beam quality, the dose difference is observed to increase by about 26 % than the without electron equilibrium condition. In M50 beam quality, the difference was only about 1 %. 3. Results and discussions Fig. 3 shows the optical density of Gafchromic MD-55 film irradiated at 5 Gy, 10 Gy, 15 Gy and 20 Gy as effective photon energy of 22 kev to 142 kev.. The variations are in two parts: firstly, M250 x- ray (effective energy 142 kev) and 60 Co (effective energy 1.25 MeV) agree with each in optical density; secondly, M150 x-ray (effective energy 67 kev) and M250 x-ray (effective energy 142 kev) differ with each other in optical density about 25 % to 40 %. Fig. 3 shows that the four batches of irradiation doses of 5 Gy to 20 Gy, each batch shows the same variation trend as described above. Fig. 4, the energy-dependence of M50 x-ray (effective dose 22 kev) to M150 x-ray (effective dose 67 kev), it shows that when the photon energy is lower than 67 kev, under the same irradiation dose but different photon energy, the optical densities of Gafchromic MD-55 films are the same. It is of the same trend as the photon energy higher than 142 kev. It can be known that the energy-dependence of the Gafchromic MD-55 film varies mainly between 67 kev to 142 kev. 4 ical density (OD) Gy 10 Gy 15 Gy 20 Gy

5 optical density (OD) effective energy (kev) 5Gy 10Gy 15Gy 20Gy FIG. 4. Results for the optical density as function of effective energy (22 kev to 142 kev) for Gafchromic MD-55 film. The typea standard uncertainty on each point is typically 2.0 % Fig. 5 shows the absorbed dose responses of TLD 100 H chips at 0.5 Gy and 1 Gy as effective photon energy range of 22 kev to 142 kev. A maximum response of 1.22 is observed at around 41 kev in the range. For the Gafchromic MD-55 film and TLD 100H chips, the photon energy dependence was found to be similar to that response in the literature. response Gy 1Gy effective energy(kev) FIG. 5. Results for absorbed dose response as a function of effective energy for TLD 100H. The type A standard uncertainty on each point is typically 1 % on percentag M50 M60 M100 5

6 FIG. 6. The plateau shows up between 20 kev to 250 kev with a thickness of 0.1 mm to 0.5 mm Fig. 6 shows that the plateau appeared between 20 kev to 250 kev x-rays with buildup thickness of 0.1 mm to 0.5 mm. For the highest energy x-ray, it could reach the state of electron equilibrium when the buildup depth got to be 0.1 mm. When the buildup depth got to be 0.5 mm after it has reached the state of electron equilibrium, the dose difference was within 3 %. The uncertainties in this experimental measurement included N k from standard Laboratory (1 %), calibration processing(1 %), measurement readings (1 %), backscatter factor (3 %), mass energy absorption coefficient (3 %), Gafchromic MD-55 film reading (3 %) and TLD 100H reading (3 %), thus the estimated combined standard uncertainty of the dosimeter calibrated using this method was about 4.6 % at confidence level of 95 %. 4. Conclusion This paper has presented a calibration method for the absorbed dose to water of dosimeters in lowenergy range x-rays. Under electron equilibrium, it was feasible to accurately calibrate the energydependence of Gafchromic films and TLDs and the uncertainty evaluated was below 5 %. However, this method could provide easy and accurate calibration results at such a time that many laboratories have not been able to establish the standard of the absorbed dose to water. REFERENCES 1. W.L. McLaughlin, Chen Yun-Dong and C.G. Soares, Sensitometry of the response of a new radiochromic film dosimeter to gamma radiation and electron beams, Nucl. Instrum. Meth. Phys. Res. A 302, (1991). 2. P. J. Muench, A. S. Meigooni, R. Nath and W.L. McLaughlin, Photon energy dependence of the sensitivity of radiochromic film and comparison with silver halide film and LiF TLDs used for brachytherapy dosimetry, Med. Phys. 18, (1991). 3. H. I. Amols, L. E. Reinstrin, and J. Weinberger, Dosimetry of a radioactive coronary balloon dilitation catheter for treatment of neointimal hyperplasia, Med. Phys. 23(10), (1996). 4. C. M. Ch, C. W. Coffey, L. A. DeWerd, C. Liu, R. Nath, S. M. Seltzer, J. P. Seuntjens, AAPM protocol for kev X-ray beam dosimetry in radiotherapy and radiobiology, Med. Phys. 28(6), (2001) 5. H. E. Johns and J.R.Cunningham, physice of Radiology ( Charles C. Thomas, Springfield, IL, 1983, A-4e and A-4g, pp732 and 734 6

7 6. A. Niroomand-Rad, C. R. Blackwell, B. M. Coursey, K. P. Gall, J. M. Galvin, W. L. Mclaughlin, A. S. Meigooni, R. Nath, J. E. Rodgers, C. G. Soares, Radiochromic film dosimetry Recommendations of AAPM Radiation Therapy Committee Task Group 55, Med. Phys. 25, (1998). 7. Y. Zhu, A. S. Kirov, V. Mishra, A. S. Meigooni, J. F. Willamson, Quantitative evaluation of radiochromic film response for two-dimensional dosimetry, Med. Phys. 24(2), (1997). 7