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ABSTRACT: The purpose of this study was to find out the effect of various physical parameters on the skin and build-up doses of 15-MV photon beams. The effects of field dimensions, acrylic shadow tray, focus to-skin distance (FSD) on surface and buildup dose were determined for open, motorized 60° wedge (MW) and blocked fields. A 'Markus' plane parallel plate chamber was used for these measurements in an Elekta (6-15MV) linear accelerator. The surface dose for MW fields was lower than the dose for an open field, but the trend reversed for large fields and higher degree wedges. With the use of an acrylic shadow tray, the surface dose increased for all field sizes, but the increase was dominant for large fields. The surface dose for blocked fields was lower than the dose for open fields. The percentage depth dose of 10 × 10 cm(2) field at surface (PDD(0)) for open beam were 13.89%, 11.71%, and 10.74% at 80 cm, 100 cm, and 120 cm FSD, respectively. The blocking tray increased PDD(0) of 10 × 10 cm(2) field to 26.29%, 14.01%, and 11.53%, while the motorized 60° wedge decreased PDD(0) to 11.32%, 9.7%, and 8.9 % at these FSDs. The maximum PDD difference seen at surface (i.e., skin) for 5 × 5 cm(2), 15 × 15 cm(2), and 30 × 30 cm(2) are 0.5%, 4.6%, and 5.6% for open field and 0.9%, 4.7%, and 7.2% for motorized 60° wedge field, when FSDs varied from 80 cm to 120 cm. The maximum PDD difference seen at surface for 5 × 5 cm(2), 15 × 15 cm(2), and 30 × 30 cm(2) fields are 5.6%, 22.8%, and 29.6%, respectively, for a 1.0-cm perspex-blocking tray as the FSD is changed. The maximum PDD difference was seen at the surface (i.e., skin) and this decreased with increasing depth.
Journal of Medical Physics 10/2010; 35(4):202-6.
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ABSTRACT: The purpose of this study was to learn the skin dose estimation for various beam modifiers at various source-to-surface distances (SSDs) for a 6 MV photon. Surface and buildup region doses were measured with an acrylic slab phantom and Markus 0.055 cc parallel plate (PP) ionization chamber. Measurements were carried out for open fields, motorized wedge fields, acrylic block tray fields ranging from 3 x 3 cm(2) to 30 x 30 cm(2). Twenty-five percent of the field was blocked with a cerrobend block and a Multileaf collimator (MLC). The effect of the blocks on the skin dose was measured for a 20 x 20 cm(2) field size, at 80 cm, 100 cm and 120 cm SSD. During the use of isocentric treatments, whereby the tumor is positioned at 100 cm from the source, depending on the depth of the tumor and size of the patient, the SSD can vary from 80 cm to 100 cm. To achieve a larger field size, the SSD can also be extended up to 120 cm at times. The skin dose increased as field size increased. The skin dose for the open 10 x10 cm(2) field was 15.5%, 14.8% and 15.5% at 80 cm, 100 cm and 120 cm SSDs, respectively. The skin dose due to a motorized 60 degrees wedge for the 10 x 10 cm(2) field was 9.9%, 9.5%, and 9.5% at 80 cm, 100 cm and 120 cm SSDs. The skin dose due to acrylic block tray, of thickness 1.0 cm for a 10 x 10 cm(2) field was 27.0%, 17.2% and 16.1% at 80, 100 and 120 cm SSD respectively. Due to the use of an acrylic block tray, the surface dose was increased for all field sizes at the above three SSDs and the percentage skin dose was more dominant at the lower SSD and larger field size. The skin dose for a 30 x 30 cm(2) field size at 80 cm SSD was 38.3% and it was 70.4% for the open and acrylic block tray fields, respectively. The skin doses for motorized wedge fields were lower than for open fields. The effect of SSDs on the surface dose for motorized 60 degrees wedge fields was not significant for a small field size (difference was less than 1% up to a 15 x 15 cm(2) field size), but for a larger field (field size more than 15 x 15 cm(2)), the difference in a percentage skin dose was significant. The skin dose for the open field was more than that for the MLC blocked field and lower than that for the acrylic blocked tray field. The block was 25% of the 20 x 20 cm(2) open field. Skin doses were increased as the SSD decreased and were dominant for larger field sizes. The surface dose was weakly dependent on the MLC block.
Journal of Medical Physics 04/2009; 34(2):87-92.
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ABSTRACT: In some cases of Intensity-modulated radiotherapy (IMRT) point dose measurement, there exists significant deviation between calculated and measured dose at isocenter, sometimes greater than ±3%. This may be because IMRT fields generate complex profiles at the reference point. The deviation arises due to lack of lateral electronic equilibrium for small fields, and other factors such as leakage and scatter contribution. Measurements were done using 0.125-cc ion chamber and Universal IMRT phantom (both from PTW-Freiburg). The aim is to find a suitable point of measurement for the chamber to avoid discrepancy between calculated and measured dose. Various beam profiles were generated in the plane of the chamber for each field by implementing patient plan on the IMRT phantom. The profiles show that for the fields which are showing deviation, the ion chamber lies in the steep-gradient region. To rectify the problem, the TPS (Treatment Planning System) calculated dose is found out at various points in the measurement plane of the chamber at isocenter. The necessary displacement to the chamber, as noted from the TPS, was given to obtain the optimum result. Twenty cases were studied for optimization, whose percentage deviation was more than ±3%. The results were well within tolerance criteria of ±3% after optimization. The mean percentage deviation value for the 20 cases studied, with standard deviation of 2.33 under 95% confidence interval, was found out to be 2.10% ± 1.14. Those cases that have significant variation even after optimization are further studied with film dosimetry.
Journal of Medical Physics 10/2007; 32(4):156-60.
