Characteristics of Detectors for Measurements of Photon Depth Doses in Build-Up Region / 의학물리

To determine the appropriate method out of various available methods to measure build-up doses, the measurements and comparisons of depth doses of build-up region including the surface dose were executed using the Attix parallel-plate ionization chamber, the Markus chamber, a cylindrical ionization chamber, and a diode detector. Based on the measurements using the Attix chamber, discrepancies of the Markus chamber were within 2% for the open field and increased up to 3.9% in the case of photon beam containing the contaminant electrons. The measurements of an cylindrical ionization chamber and a diode detector accord with those of the Attix chamber within 1.5% and 1.0% after those detectors were completely immersed in the water phantom. The results suggest that the parallel-plate chamber is the best choice to measure depth doses in the build-up region containing the surface, however, using cylindrical ionization chamber or diode detector would be a reasonable choice if no special care is necessary for the exact surface dose.

Reduction of Electron ContaminationUsing a Filter for 6MV Photon Beam / 대한치료방사선과학회지

PURPOSE: Secondary electrons generated by interaction between primary X-ray beam and block tray in megavoltage irradiation, result in excess soft radiation dose to the surface layer. To reduce the surface dose from the electron contamination, electron filters were attached under the tray when a customized block was used. MATERIALS AND METHODS: Cu, Al or Cu/Al combined plate with different thickness was used as a filter and the surface dose reduction was measured for each case. The measurement to find optimal filter was performed with 10cm x 10cm field size and 78.5cm source to surface distance. The measurement points are positioned with 2mm intervals from surface to maximum build-up point. To acquire the effect of field size dependence on optimal electron filter, the measurement was performed from 4cm x 4cm to 25cm x 25cm field sizes. RESULTS: The surface dose was slowly increased by increasing irradiation field but rapidly increased beyond 15cm x 15cm field size. Al plate was found to be inadequate filter because of the failure to have surface dose kept lowering than the dose of deep area. Cu 0.5mm plate and Cu/Al= 0.28mm/1.5mm combined plate were found to be optimal filters. By using these 2 filters, the absorbed dose to the surface layer was effectively reduced by 5.5%, 11.3%, and 22.3% for the field size 4cm x 4cm, 10cm x 10cm, and 25cm x 25cm, respectively. CONCLUSION: The surface dose attributable to electron contamination had a dependence on field size. The electron contamination was increased when tray was used. Specially the electron contamination in the surface layer was greater when the larger field was used. 0.5mm Cu plate and Cu/Al=0.28mm/1.5mm combined plates were selected as optimal electron filters. When the optimal electron filter was attached under the tray, excessive surface dose was decreased effectively. The effect of these electron filters was better when a larger field was used.

Study on the Reduction of Electron Contamination with A Cobalt-60 Gamma Ray / 대한치료방사선과학회지

Electron contamination due to the interaction between radiation beam and material was analyzed for the factors such as source-skin distance(SSD), field size, tray characteristics and position of filter, which can affect the surface dose in Cobalt teletherapy. Surface dose in open beam was more influenced by SSD with increasing field size. Relative surface charge(RSC) increased with the use of tray(solid, circular hole, slotted), compared with open beam, which is thought to be doe to increased electron contamination of the tray. To reduce the surface dose, 0.4mm thick Lipowitz metal filter was used. Compared with open beam, RSC decreased by 8.8%, 11.3%, 13.3%, 16.6%, 19.3% and 21.7% for the field size of 5x5, 10x10, 15x15, 20x20, 25x25 and 30x30 cm2, respectively. On the contrary, use of Lipowitz metal filter increased RSC at 60cm or less SSD. Suface dose was effectively reduced with Lpowitz metal filter placed right below solid tray in Cobalt teletherapy.

Contamination of an Alcyon Co-60 gamma rays by Electrons / 대한치료방사선과학회지

The Alcyon Co-60 gamma rays 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. And to achieve the maximum skin sparing effect, the selection of the optimum SSD and TSD is needed.