• Progress in Medical Physics
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• v.20 no.4
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• pp.317-323
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• 2009

The standard dosimetry systems based on an absorbed dose to water recommend to use a planeparallel chamber for the calibration of such a low-megavoltage electron beam as a nominal energy of 6 MeV. For this energy ranges of an electron beam a cylindrical chamber should not be used for the routinely regular beam calibration, but the feasibility of the temporary use of a cylindrical chamber was studied to give temporary solutions for special situations users meet. The PTW30013 chambers and the electron beam quality of $R_{50}=2.25\;g/cm^2$ were selected for this study. 10 PTW30013 chambers, a cylindrical type of chamber, were calibrated in KFDA, the secondary standards dosimetry laboratories, and given the absorbed dose-to-water calibration factors, respectively. A "temporary" $k_{Q,Q_0}$ for each chamber were calculated using the absorbed dose determined by a cross-calibrated planeparallel chamber, with the result of an average 0.9352 for 10 chambers. This value for PTW30013 chamber was used to determine an absorbed dose to water at the reference depth. The absorbed doses determined by PTW30013 chambers were in an agreement within 2% with that by ROOS chamber. In a certain situation where a cylindrical chamber be used instead of a planeparellel chamber, the value of 0.9352 might be useful to determine an absorbed dose to water in the same beam quality of electron beam as this study.

• Journal of radiological science and technology
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• v.31 no.2
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• pp.183-188
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• 2008

Aim of this study is to investigate the feasibility of 2D ion chamber array as a substitute of the water phantom system in a periodic Linac QA. For the feasibility study, a commercial ion chamber matrix was used as a substitute of the water phantom in the measurement for a routine QA beam properties. The device used in this study was the I'm RT MatriXX (Wellhofer Dosimetrie, Germany). The MatriXX consists of a 1,020 vented ion chamber array, arranged in $24{\times}24\;cm^2$ matrix. Each ion chamber has a volume of $0.08\;cm^3$, spacing of 0.762 cm. We investigated dosimetric parameters such as dose symmetry, energy ($TPR_{20,10}$), and absolute dose for comparing with the water phantom data with a Farmer-type ionization chamber (FC65G, Wellhofer Dosimetrie, Germany). For the MatriXX measurements, we used the white polystyrene phantom (${\rho}:\;1.18\;g/cm^3$) and also considered the intrinsic layer (${\rho}:\;1.06\;g/cm^3$, t: 0.36 cm) of MatriXX to be equivalent to water depth. In the preliminary study of geometrical QA using MatriXX, the rotation axis of collimator and half beam junction test were included and compared with film measurements. Regarding the dosimetrical QA, the MatriXX has shown good agreements within ${\pm}1%$ compared to the water phantom measurements. In the geometrical test, the data from MatriXX were comparable with those from the films. In conclusion, the MatriXX is a good substitute for water phantom system and film measurements. In addition, the results indicate that the MatriXX as a cost-effective novel QA tool to reduce time and personnel power.

• Progress in Medical Physics
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• v.8 no.2
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• pp.87-102
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• 1997

The absolute absorbed dose can be determined according to the measurement conditions; measurement material, detector, energy and calibration protocols. The purpose of this study is to compare the absolute absorbed dose due to the differences of measurement condition and calibration protocols for photon beams. Dosimetric measurements were performed with a farmer type PTW and NEL ionization chambers in water, solid water, and polystyrene phantoms using 6MV photon beams from Siemens linear accelerator. Measurements were made along the central axis of 10cm $\times$ 10cm field size for constant target to surface distance of 100cm for water, solid water and polystyrene phantom. Theoretical absorbed dose intercomparisons between TG21 and IAEA protocol were performed for various measurement combinations of phantom, ion chamber, and electrometer. There were no significant differences of absorbed dose value between TG21 and IAEA protocol. The differences between two protocols are within 1％ while the average value of IAEA protocol was 0.5％ smaller than TG21 protocol. For the purpose of comparison, all the relative absorbed dose were nomalized to NEL ion chamber with Keithley electrometer and water phantom, The average differences are within 1％, but individual discrepancies are in the range of - 2.5％ to 1.2％ depending upon the choice of measurement combination. The largest discrepancy of - 2.5％ was observed when NEL ion chamber with Keithley electrometer is used in solid water phantom. The main cause for this discrepancy is due to the use of same parameters of stopping power, absorption coeficient, etc. as used in water phantom. It should be mentioned that the solid water phantom is not recommended for absolute dose calibration as the alternative of water, since absorbed dose show some dependency on phantom material other than water. In conclusion, the trend of variation was not much dependent on calibration protocol. However, it shows that absorbed dose could be affected by phantom material other than water.

• KOREAN JOURNAL OF CROP SCIENCE
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• v.64 no.4
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• pp.422-431
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• 2019

Severe droughts in spring have occurred frequently in Korea in recent years, exerting a critical impact on corn yield. Therefore, it is necessary to find physiological and/or molecular indicators of the response to drought stress in maize plants. In this study, we investigated the effects of water-deficit stress on two Korean elite F₁ maize hybrids, Ilmichal and Gwangpyeongok, by withholding water for 10 days at tassel initiation. The water deficit drastically reduced the relative leaf water content, leaf number, leaf area, and stem length, leading to dry matter reduction. Moreover, it reduced the SPAD values and stomatal conductance of leaves in drought-stressed plants of both hybrids. Importantly, the number of leaves and SPAD value were non-destructive and easy to investigate in response to water-deficit stress, suggesting that they may be useful indicators for screening drought-tolerant genetic resources. We detected more than 100 spots that were differentially accumulated under drought stress. Of these spots, a total of 21 protein spots (≥1.5-fold) from drought-exposed maize leaves were successfully analyzed by MALDI-TOF-TOF mass spectrometry. Functional annotation using Gene Ontology analysis revealed that most of the identified proteins were involved in carbohydrate metabolism, stress response fatty acid catabolism, photosynthesis, energy metabolism, and transport. The protein expression levels were increased in both Ilmichal and Gwangpyeongok, except for triosephosphate isomerase, fructose-bisphosphate aldolase, and an uncharacterized protein. The lactoylglutathione lyase delta (3,5)-delta (2,4)-dienoyl-CoA isomerase was overexpressed in Gwangpyeongok only. The results obtained from this study suggest that the drought-specific genes may be useful as molecular markers for screening drought-tolerant maize genotypes.