• The Journal of Korean Society for Radiation Therapy
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• v.18 no.1
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• pp.21-28
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• 2006

Purpose: The purpose of this study is to find a optimal beam spoiler condition on the dose distribution near the surface, when treating a squamous cell carcinoma of the head and neck and a lymphatic region with 10 MV photon beam. The use of a optimal spoiler allows elivering high dose to a superficial tumor volume, while maintaining the skin-sparing effect in the area between the surface to the depth of 0.4 cm. Materials and Methods: The lucite beam spoiler, which were a tissue equivalent, were made and placed between the surface and the photon collimators of linear accelerator. The surface-dose, the dose at the depth of 0.4 cm, and the maximum dose at the dmax were measured with a parallel-plate ionization chamber for $5{\times}5cm\;to\;30{\times}30cm^2$ field sizes using lucite spoilers with different thicknesses at varying skin-to-spoiler separation (SSS). In the same condition, the dose was measured with bolus and compared with beam spoiler. Results: The spoiler increased the surface and build-up dose and shifted the depth of maximum dose toward the surface. With a 10 MV x-ray beam and a optimal beam spoiler when treating a patient, a similer build-up dose with a 6 MV photon beam could be achieved, while maintaining a certain amount of skin spring. But it was provided higher surface dose under SSS of less than 5 cm, the spoiler thickness of more than 1.8 cm or more, and larger field size than $20{\times}20cm^2$ provided higher surface dose like bolus and obliterated the spin-sparing effect. the effects of the beam spoiler on beam profile was reduced with increasing depths. Conclusion: The lucite spoiler allowed using of a 10 MV photon beam for the radiation treatment of head and neck caner by yielding secondary scattered electron on the surface. The dose at superficial depth was increased and the depth of maximum dose was moved to near the skin surface. Spoiling the 10 MV x-ray beam resulted in treatment plans that maintained dose homogeneity without the consequence of increased skin reaction or treat volume underdose for regions near the skin surface. In this, the optimal spoiler thickeness of 1.2 cm and 1.8 cm were found at SSS of 7 cm for $10{\times}10cm^2$ field. The surface doses were measured 60% and 64% respectively. In addition, It showed so optimal that 94% and 94% at the depth of 0.4 cm and dmax respectively.

• The Journal of Korean Society for Radiation Therapy
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• v.8 no.1
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• pp.75-81
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• 1996

When high energy photon beam is incident upon an air cavity interface the effect of ionization build-up observed . This phenomenon is resulting from the surface layers of the lesions are significant deficiency of electrons reaching the layers because of the replacement, of solid scattering material by the air cavity, that is lack of electronic equilibrium. Measurement have been made in an acrylic phantom with a parallel plate chamber and high energy photon beams, CO-60, 4MV, 6MV and 10MV X-rays have been investigated. The result of our study show that a significant effect was measured and was determined to be very dependent on field size, air cavity dimension and photon energy. The reductions were much larger for 10MV beam, underdosage at the interface was 12, 12.2, 16.9 and $20.6\%$ for the CO-60, 4MV, 6MV and 10MV, respectively. It was found that this non-equilibrium effect at the interface is more severe for the higher energy beams than that of lower energy beams and the larger cavity dimensions the larger beam reductions occur. This problem is of clinical concern when lesions such as carcinoma beyond air cavities are irradiated, such as larynx, glottic and the patients with maxillectomy and ethmoidectomy and so forth.

• Progress in Medical Physics
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• v.10 no.1
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• pp.55-62
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• 1999

In an effort to study the characteristics of x-rays utilized in radiation therapy, we calculated the energy distribution and the mean energy of x-rays generated from a tungsten target bombarded by 6, 10, and 15 MeV electron beams, using a Monte Carlo technique. The average photon energies calculated as a function of the beam radius lied in 1.4 ∼ 1.6, 2.1 ∼ 2.5 and 2.8 ∼ 3.3 MeV ranges for 4, 10, and 15 MV electron beams, respectively, which turned out to have no strong dependence on the radius. Using the energy distributions of 6,10, and 15 MV x-rays obtained for the target distance of 100 cm, percentage depth doses were determined using Monte Carlo calculations. For the case 10 MV, a comparison was made between our calculation and measurement performed by others. The calculated percentage depth dose appeared somewhat smaller than the measured one except in the surface region. We conclude that this is due to the fact that the beam hardening effect resulting from the flattening filter was not properly allowed for in our Monte Carlo calculations.

• Asian Pacific Journal of Cancer Prevention
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• v.17 no.4
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• pp.1685-1689
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• 2016

Physical wedges still can be used as missing tissue compensators or filters to alter the shape of isodose curves in a target volume to reach an optimal radiotherapy plan without creating a hotspot. The aim of this study was to investigate the dosimetric properties of physical wedges filters such as off-axis photon fluence, photon spectrum, output factor and half value layer. The photon beam quality of a 6 MV Primus Siemens modified by 150 and 450 physical wedges was studied with BEAMnrc Monte Carlo (MC) code. The calculated present depth dose and dose profile curves for open and wedged photon beam were in good agreement with the measurements. Increase of wedge angle increased the beam hardening and this effect was more pronounced at the heal region. Using such an accurate MC model to determine of wedge factors and implementation of it as a calculation algorithm in the future treatment planning systems is recommended.

• Journal of the Korean Society of Radiology
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• v.7 no.1
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• pp.93-98
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• 2013

The purpose of this study is to investigate the calibration of an optically stimulated luminescent nanoDot dosimeter(OSLnD) to 6 MV photon beam. Dose ranges of the calibration of linear and non-linear from the analysis of dose response of the OSLnD were decided. To evaluate the accuracy of calibration equation and the calibration, the sets of the calibration and quality control dosimeter were used to make. The calibrations were performed by the linear and the non-linear in the dose range of 0~300 cGy and 20~1300 cGy, respectively. The errors of the calibration were acquired less than 0.1% respectively from the measurement of the quality control dosimeters for the calibration of linear and the non-linear. This study provides the calibration equation of the OSLnD to the 6 MV photon beam.

• Journal of the Korean Magnetics Society
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• v.27 no.1
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• pp.18-22
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• 2017

The purpose of this study was to present an improved method of dose modulation over the increase of build-up for existing 6 MV photon beam. Two neodymium permanent magnets with a strength of 0.5 T (Tesla) were applied with a magnetic field perpendicular to the photon beam. The effects of dose on build-up region with or without the magnetic field were measured according to the magnet-to-magnet distance (MMS) and the magnet-to-surface distance (MSD). For MMS = 6 cm and MSD = 2.5 cm, $D_{0mm}$, $D_{2mm}$, $D_{5mm}$, and $D_{10mm}$ showed improved doses of 6.8 %, 14.6 %, 6.9 %, and 2.1 %, respectively, as compared with 6 MV open beam. In this study, the device with low strength magnetic field can be applied directly to the outside of the human body when the target volume located close to the skin is delivered with radiation. It is expected that the method of build-up modulation using a low strength magnetic field will be feasible in the clinical applications.

• Proceedings of the Korean Society of Medical Physics Conference
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• 2003.09a
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• pp.56-56
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• 2003

For the treatment of superficial tumors like squamous cell carcinoma of the head and neck, 6 MV photon beam is not appropriate and a spoiler is widely used to increase dose in the buildup region, while preserving the skin sparing effect. However, commercially available treatment planning systems assume a normal unspoiled beam, thereby cannot predict the buildup dose with spoiler accurately. We aimed to implement a Monte Carlo (MC) based planning system to apply it to the radiation treatment of head and neck. Lucite with thickness of 10-mm was used for the beam spoiler with Siemens Primus 6 MV photon beam. BEAM/DOSXYZ MC system was employed to model the linac and the spoiler. To verify the calculation accuracy of MC simulations, the percent depth doses (PDDs) and profiles with and without spoiler were measured using a parallel-plate chamber. For the MC based planning, we adopted a hybrid interface system between Pinnacle (Philips, USA) and BEAM/DOSXYZ to support treatment parameters of Siemens linac and the spoiler. The measurements of PDDs and profiles agreed with the corresponding MC simulations within 2% (lSD), which demonstrate the reliability of our MC simulations. The spoiler generated electrons make a contribution to the absorbed dose up to depth of 2cm, which shows that the dominant source of increased dose from spoiler system is the contaminating electrons created by the spoiler. The whole procedures necessary for MC based treatment planning were performed seamlessly between Pinnacle and BEAM/DOSXYZ system. This ability helps to increase the clinical efficiency of the spoiler technique. In conclusion, we implemented a MC based treatment planning system for a 6 MV photon beam with a spoiler. We demonstrate sophisticated MC technique makes it possible to predict dose distributions around buildup region accurately.

• Progress in Medical Physics
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• v.23 no.4
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• pp.317-325
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• 2012

The Varian PORTALVISION (Varian Medical Systems, US) shows significant overresponses as the off-center distance increases compared to the predicted dose. In order to correct the dose discrepancy, the off-axis correction is applied to VARIAN iX linear accelerators. The portal dose for $38{\times}28cm^2$ open field is acquired for 6 MV, 15 MV photon beams and also are predicted by PDIP algorithm under the same condition of the portal dose acquisition. The off-axis correction is applied by modifying the $40{\times}40cm^2$ diagonal beam profile data which is used for the beam profile calibration. The ratios between predicted dose and measured dose is modeled as a function of off-axis distance with the $4^{th}$ polynomial and is applied to the $40{\times}40cm^2$ diagonal beam profile data as the weight to correct measured dose by EPID detector. The discrepancy between measured dose and predicted dose is reduced from $4.17{\pm}2.76$ CU to $0.18{\pm}0.8$ CU for 6 MV photon beam and from $3.23{\pm}2.59$ CU to $0.04{\pm}0.85$ CU for 15 MV photon beam. The passing rate of gamma analysis for the pyramid fluence patten with the 4%, 4 mm criteria is improved from 98.7% to 99.1% for 6 MV photon beam, from 99.8% to 99.9% for 15 MV photon beam. IMRT QA is also performed for randomly selected Head and Neck and Prostate IMRT plans after applying the off-axis correction. The gamma passing rare is improved by 3% on average, for Head and Neck cases: $94.7{\pm}3.2%$ to $98.2{\pm}1.4%$, for Prostate cases: $95.5{\pm}2.6%$, $98.4{\pm}1.8%$. The gamma analysis criteria is 3%, 3 mm with 10% threshold. It is considered that the off-axis correction might be an effective and easily adaptable means for correcting the discrepancy between measured dose and predicted dose for IMRT QA using EPID in clinic.

• Proceedings of the KOSOMBE Conference
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• v.1992 no.05
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• pp.148-151
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• 1992

There is a definite requirement to continuously monitor the operating characteristics of radiation therapy machines. It is advisable to monitor the symmetry, flatness, and energy stability of x-ray beams. The semiconductor system was developed using commercially available rectifier diode for th assessment of quality assurance In radiation therapy, which is capable of the above measurements. The beam characteristics of 6MV, 10MV and 21MV photon of Microtron electron accelerator were measured using seven-diodes as detectors and the results were compared with that of using a film results dosimetry with a X-Y plotter. The seven-diode detetor is versatile enough to be used for checking beam profile, flatness, symmetry and energy.

• Journal of Radiation Protection and Research
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• v.13 no.1
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• pp.21-30
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• 1988

The 6 MV photon beam of a linear accelerator (Mevatron 67) was studied for electron contamination. The surface dose, attributable almost entirely to contamination electrons, has a linear dependence on field width for square fields and an inverse square dependence on distance from the bottom of the fixed head assembly. Build-up and surface dose measurements were taken with and without an acrylic blocking tray in place. Further measurements were made with a copper filter designed to reduce secondary electrons emitted by photon interactions with the acrylic tray. The results are discussed in relation to skin sparing effect for radiation therapy patients. To achieve the maximum skin sparing effect, the selection of the optimum SSD and TSD is needed.