• Progress in Medical Physics
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• v.6 no.2
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• pp.41-50
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• 1995

The absorbed dose and contaminant electron distribution of therapeutic X-ray beam (15MV photon) was studied with a half blocked beams of 30$\times$30$\textrm{cm}^2$ and field size ranging from 5$\times$5 to 30$\times$30$\textrm{cm}^2$. For a 15MV photon beam energy, the value of the depth of dose maximum, d$_{max}$, gradually decrease with increasing field size from 5$\times$5 to 30$\times$30$\textrm{cm}^2$ due to mainly by contaminant electrons which are produced in the flattening filter and scattered by collimator jaws, tray holder〔Lucite〕, blocking block and air. The results suggest that separate dosimetry data should be kept for blocked and unblocked field. The inherence of the contaminant electrons to the open field depth of maximum dose can lead to mistaken results if attenuation measurements are made at that depth. A nurmerous contaminant electrons mainly were distributed as shape of corn in the central photon beam and their path length in the water were shorter than 30mm because of the electrons energy having around 6MeV. These results clearly appears that the substraction of scattered electrons (electrons and positrons) from the total depth dose curve not only lowers the absolute dose in the bulidup region and surface dose, it also causes a shift of d$_{max}$ to a deeper depth. In the terapeutic high energy photon beam, the absorbed dose near the buildup region is the combined result of incident contaminant electrons and phantom generated electronsrons.

• Radiation Oncology Journal
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• v.21 no.4
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• pp.322-329
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• 2003

Purpose: It is difficult to exactly determine the surface dose and the dose distribution In buildup region of high energy X-rays by using the conventional ion chamber. The aim of this study Is to evaluate the accuracy of widely used dosimetry systems to measure the surface dose and the depth of maximum dose (d$_{max}$). Materials and Methods: We measured the percent depth dose (PDD) from the surface to the d$_{max}$ in either a water phantom or in a solid water phantom using TLD-100 chips, thimble type ion chamber, diode detector, diamond detector and Markus parallel plate ion chamber for 6 MV and 15 MV X-rays, 10$\times$10 cm$^{2}$, at SSD=100cm. We analysed the surface dose and the d$_{max}$. In order to verify the accuracy of the TLD data, we executed the Monte Carlo simulation for 5 MV X-ray beams. Results: The surface doses In 6 MV and IS MV X-rays were 29.31% and 23.36% ior Markus parallel plate ion chamber, 37.17$\%$ and 24.01$\%$ for TLD, 34.87$\%$ and 24.06$\%$ for diamond detector, 38.13$\%$ and 27.8$\%$ for diode detector, and 47.92$\%$ and 35.01$\%$ for thimble type ion chamber, respectively. in Monte Carlo simulation for 6 MV X-rays, the surface dose was 36.22$\%$, which Is similar to the 37.17$\%$ of the TLD measurement data. The d$_{max}$ In 6 WV and 15 MV X-rays was 14$\~$16 mm and 27$\~$29 mm, respectively. There was no significant difference in the d$_{max}$ among the detectors. Conclusion: There was a remarkable difference in the surface dose among the detectors. The Markus parallel plate chamber showed the most accurate result. The surface dose of the thimble ion chamber was 10$\%$ higher than that of other detectors. We suggest that the correction should be made when the surface dose of the thimble ion chamber Is used for the treatment planning ion the supeficial tumors. All the detectors used In our study showed no difference in the d$_{max}$.

• Journal of radiological science and technology
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• v.41 no.3
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• pp.201-207
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• 2018

The purpose of this study is to evaluate the performance of a "stealth chamber" as a novel reference chamber for measuring percentage depth dose (PDD) and profile of 6, 8 and 10 MV photon energies. The PDD curves and dose profiles with fields ranging from $3{\times}3$ to $25{\times}25cm^2$ were acquired from measurements by using the stealth chamber and CC 13 chamber as reference chamber. All measurements were performed with Varian VitalBeam linear accelerator. In order to assess the performance of stealth chamber, PDD curves and profiles measured with stealth chamber were compared with measurement data using CC13 chamber. For PPDs measured with both chambers, the dosimetric parameters such as $d_{max}$ (depth of maximum dose), $D_{50}$ (PDD at 50 mm depth), and $D_{100}$ (PDD at 100 mm depth) were analyzed. Moreover, root mean square error (RMSE) values for profiles at $d_{max}$ and 100 mm depth were evaluated. The measured PDDs and profiles between the stealth chamber and CC13 chamber as reference detector had almost comparable. For PDDs, the evaluated dosimetric parameters were observed small difference (<1%) for all energies and field sizes, except for $d_{max}$ less than 2 mm. In addition, the difference of RMSEs for profiles at $d_{max}$ and 100 mm depth was similar for both chambers. This study confirmed that the use of stealth chamber for measuring commission beam data is a feasible as reference chamber for fields ranging from $3{\times}3$ to $20{\times}20cm^2$. Furthermore, it has an advantage with respect to measurement of the small fields (less than $3{\times}3cm^2$ field) although not performed in this study.

• Journal of Radiation Protection and Research
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• v.34 no.2
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• pp.43-48
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• 2009

When the PDD (percentage depth dose) in the megavoltage beams is measured in the water phantom, the polarity and ion recombination effects of ionization chambers with depth in water are not usually taken into consideration. We try to investigate if those variations with depth should be taken into consideration or could be ignored for the thimble type semiflex ionization chamber (PTW $31010^{TM}$, SN 1551). According to the recommendation of IAEA TRS-398, the 4 representative depths of $d_s$, $d_{max}$, $d_{90}$ and $d_{50}$ were used for the electron beams. For the photon beams, the 4 depths were arbitrarily chosen for the photon beams, which were $d_s$, $d_{max}$, $d_{10}$ and $d_{20}$. For the high energy photon beam both polarity and ion recombination factors of the chamber with depth in water gives the good agreements within the maximum $\pm$0.2%, while the $C_{polS}$ with depth came within the maximum $\pm$ 0.4% and the $C_{IRS}$ within the maximum $\pm$0.6% in every electron beam used. This study shows that PDI (percentage depth ionization) could be a good approximation to PDD for the chamber used.

• Journal of the Korean Society of Radiology
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• v.16 no.6
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• pp.719-725
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• 2022

This study aimed to measure, quantitatively evaluate, and set the criteria for the minimum lead(Pb) shield thickness per level of clinically applied electron beam energy. The lead shield thickness per electron beam energy was measured using the primary field 95% reduction based on the open field at the depth of maximum dose (d_max) and depth from the surface as the reference depth of tissue dose(10 mm). The measured values were 1.906 mmPb and 1.992 mmPb at the d_max and 10 mm, respectively, regarding the lead shield thickness for 6 MeV electron beam; 2.746 mmPb and 3.743 mmPb for 9 MeV electron beam, 3.718 mmPb and 6.093 mmPb for 12 MeV electron beam, 7.300 mmPb and 15.270 mmPb for 16 MeV electron beam, and 16.825 mmPb and 25.090 mmPb for 20 MeV electron beam. Consequently, a thicker lead shield was required if the measurement was at 10 mm. The required lead shield thickness was also higher than that of the theoretical formula for electron beams of ≥ 16 MeV.

• Progress in Medical Physics
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• v.8 no.2
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• pp.3-17
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• 1997

Because a wedged beam consists of attenuated primary photons and scattered radiations from wedge, the spectrum of the wedged beam does not coincide with that of an open beam with same geometry. The aims of current report are to get exact information about whether effects of 15-60$^{\circ}$ wedge for 4 -10 MV photon beams should be considered for dose calculation or not, and to suggest a reference condition for measurement of wedge transmission factor. Percent depth dose of both open and wedged fields with angles of 15, 30, 45, 60$^{\circ}$ for beams of 4 MV(Clinac 4/100, Varian), two 6 MV(Clinac 6/100 and Clinac 2100C, Varian), 10 MV(Clinac 2100C, Varian) X-rays were measured to 30cm deep in water using ionization chambers. Hardening factors of photon beams were calculated with measured PDDs. Both field size factors and transmission factors of wedge filters were measured at d$_{max}$ in water. Beam hardening factors of wedged fields of 4 and 6 MV X-ray were larger than 1 for all wedge angles, field sizes and depths deeper than d$_{max}$ Beam hardening factors for wedge angles 15, 30, 45, 60$^{\circ}$ for 10$\times$10cm were respectively 1.010, 1.014, 1.023 and 1.034 for 4MV X-ray, 1.005, 1.008, 1.019, and 1.024 for 6MV X-ray of Clinac 6/100, 1.011, 1.021, 1.032, 1.036 for 6MV X-ray of Clinac 2100C, and 1.008, 1.012, 1.012 and 1.012 for 10MV X-ray. Beam hardening factors of 10MV X-ray were 1 within 1.2％ difference for all wedge angles, depths and field sizes. It was made clear that for 6MV X-rays, the beam hardening factor depends on treatment machine. The relationship of the factor and depth was linear. Field size factor at d$_{max}$ was independent of wedge angle except for the field of 15$\times$15cm. and maximum difference of the field size factors for the field size was 1.4％ for 4MV X-ray. When the wedge factor is determined, dependence of the factor on field size is negligible at d$_{max}$ but should be considered at deeper depth. Calculating dose distribution or MU, the beam hardening factor should be applied for 4~6MV X-ray beams, but might not be considered for 10MV beam. When wedge transmission factor was determined at d$_{max}$ or in air, field size factors for open field are also applicable to wedged fields, but otherwise, field size factor for each wedge or wedge factor depending on field size should be applied.

• Radiation Oncology Journal
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• v.12 no.2
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• pp.225-232
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• 1994

Purpose : This study was to obtain the basic dosimetric data using the 10 MV X-ray for the total body irradiation. Materials and Methods : A linear accelerator photon beam is planned to be used as a radiation source for total body irradiation (TBI) in Chonnam University Hospital. The planned distance from the target to the midplane of a patient is 360cm and the maximum geometric field size is 144cm x 144cm. Polystyrene phantom sized $30{\times}30{\times}30.2cm^3$ and consisted of several sheets with various thickness, and a parallel plate ionization chamber were used to measure surface dose and percent depth dose (PDD) at 345cm SSD, and dose profiles. To evaluate whether a beam modifier is necessary for TBI, dosimetry in build up region was made first with no modifier and next with an 1cm thick acryl plate 20cm far from the polystyrene phantom surface. For a fixed sourec-chamber distance, output factors were measured for various depth. Results : As any beam modifier was not on the way of radiation of 10MV X-ray, the $d_{max}$ and surface dose was 1.8cm and $61\%$, respectively, for 345cm SSD. When an 1cm thick acryl plate was put 20cm far from polystyrene phantom for the SSD, the $d_{max}$ and surface dose were 0.8cm and $94\%$, respectively. With acryl as a beam spoiler, the PDD at 10cm depth was $78.4\%$ and exit dose was a little higher than expected dose at interface of exit surface. For two-opposing fields for a 30cm phantom thick phantom, the surface dose and maximum dose relative to mid-depth dose in our experiments were $102.5\%$ and $106.3\%$, respectively. The off-axis distance of that point of $95\%$ of beam axis dose were 70cm on principal axis and 80cm on diagonal axis. Conclusion: 1. To increase surface dose for TBI by 10MV X-ray at 360cm SAD, 1cm thick acrylic spoiler was sufficient when distance from phantom surface to spoiler was 20cm. 2. At 345cm SSD, 10MV X-ray beam of full field produced a satisfiable dose uniformity for TBI within $7\%$ in the phantom of 30cm thickness by two-opposing irradiation technique. 3. The uniform dose distribution region was 67cm on principal axis of the beam and 80cm on diagonal axis from beam axis. 4. The output factors at mid-point of various thickness revealed linear relation with depth, and it could be applicable to practical TBI.

• Asian Pacific Journal of Cancer Prevention
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• v.17 no.1
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• pp.153-157
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• 2016

Background: In radiation therapy, estimation of surface doses is clinically important. This study aimed to obtain an analytical relationship to determine the skin surface dose, kerma and the depth of maximum dose, with energies of 6 and 18 megavoltage (MV). Materials and Methods: To obtain the dose on the surface of skin, using the relationship between dose and kerma and solving differential equations governing the two quantities, a general relationship of dose changes relative to the depth was obtained. By dosimetry all the standard square fields of $5cm{\times}5cm$ to $40cm{\times}40cm$, an equation similar to response to differential equations of the dose and kerma were fitted on the measurements for any field size and energy. Applying two conditions: a) equality of the area under dose distribution and kerma changes in versus depth in 6 and 18 MV, b) equality of the kerma and dose at $x=d_{max}$ and using these results, coefficients of the obtained analytical relationship were determined. By putting the depth of zero in the relation, amount of PDD and kerma on the surface of the skin, could be obtained. Results: Using the MATLAB software, an exponential binomial function with R-Square >0.9953 was determined for any field size and depth in two energy modes 6 and 18MV, the surface PDD and kerma was obtained and both of them increase due to the increase of the field, but they reduce due to increased energy and from the obtained relation, depth of maximum dose can be determined. Conclusions: Using this analytical formula, one can find the skin surface dose, kerma and thickness of the buildup region.

• Progress in Medical Physics
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• v.13 no.2
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• pp.62-68
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• 2002

A muiltileaf collimator (MLC) is used as a replacement for conventional blocks. The MLC, however may not be appropriate for a fine field shaping. For the fine field shaping, conventional block can be added under the MLC. But it may significantly affect on the dosimetric characteristics such as surface dose of skin, buildup region and percent depth doses. We performed the study to evaluate the surface dose and the maximum depth dose using MLC conjunction with conventional blocks for various field sizes and energies. We confirmed the surface dose was increased by using the additional conventional block under the MLC ranging from 10 to 35.6％ according to various field sizes and radiation beam energies. The surface dose was effectively reduced by application of 2 or 3 m thickness of lead plate as electron filter.

• Proceedings of the Korean Society of Medical Physics Conference
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• 2002.09a
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• pp.119-120
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• 2002

The aim is to urge the need of elaborate commissioning of 3D RTP system from the firsthand experience. A 3D RTP system requires so much data such as beam data and patient data. Most data of radiation beam are directly transferred from a 3D dose scanning system, and some other data are input by editing. In the process inputting parameters and/or data, no error should occur. For RTP system using algorithm-bas ed-on beam-modeling, careless beam-data processing could also cause the treatment error. Beam data of 3 different qualities of photon from two linear accelerators, patient data and calculated results were commissioned. For PDD, the doses by Clarkson, convolution, superposition and fast superposition methods at 10 cm for 10${\times}$10 cm field, 100 cm SSD were compared with the measured. An error in the SCD for one quality was input by the service engineer. Whole SCD defined by a physicist is SAD plus d$\sub$max/, the value was just SAD. That resulted in increase of MU by 100${\times}$((1_d$\sub$max//SAD)$^2$-1)%. For 10${\times}$10 cm open field, 1 m SSD and at 10 cm depth in uniform medium of relative electron density (RED) 1, PDDs for 4 algorithms of dose calculation, Clarkson, convolution, superposition and fast-superposition, were compared with the measured. The calculated PDD were similar to the measured. For 10${\times}$10 cm open field, 1 m SSD and at 10 cm depth with 5 cm thick inhomogeneity of RED 0.2 under 2 cm thick RED 1 medium, PDDs for 4 algorithms were compared. PDDs ranged from 72.2% to 77.0% for 4 MV X-ray and from 90.9% to 95.6% for 6 MV X-ray. PDDs were of maximum for convolution and of minimum for superposition. For 15${\times}$15 cm symmetric wedged field, wedge factor was not constant for calculation mode, even though same geometry. The reason is that their wedge factor is considering beam hardness and ray path. Their definition requires their users to change the concept of wedge factor. RTP user should elaborately review beam data and calculation algorithm in commissioning.