Attalla EM, El-sayed AA, Hessein H, Abouelenein HS, Ashour EF
Introduction: The purpose of this study was to investigate surface doses of 6 and 10 MV energies of linear accelerator for different clinical setup parameters including (field size, gantry angle, SSD (source skin distance), PW (physical wedge), acrylic block tray and bolus), Comparison between measured surface dose by P.P (parallel plate) and calculated surface dose by TPS (Treatment Planning system). Comparison between surface doses measured by TLD (Thermo luminescence dosimeter) and P.P ion chamber, Comparison between the surface dose of 3DCRT and IMRT techniques.
Materials and Methods: Surface dose measurements were carried out using a (PTW) Markus parallel-plate ion chamber in a plastic water plastic water phantom for various setup parameters using Primus Siemens (6, 10 MV) linear accelerator. For the normalization depth, i.e., the depth of maximum dose 1.5, 2.5 cm were chosen for 6, 10 MV photon beams, respectively.
Results and Discussion: The measured skin dose values for 10 MV were lower than those of 6 MV, the skin dose increased as field size increased. The measured surface dose by P.P for 6 and 10 MV are 16%, 25%, for 10 × 10 cm2 square field size, within the first 2 millimetres of the build-up region, at field size 10 × 10 cm2 the PDD for a 6 and 10 MV photon beam increases from 24% to 62%, 16% to 44% respectively. With increasing the gantry angles (0 to 30) produces a minimal effect of dose, (40-70) gantry angles produce a significant increase. When studying the surface dose with different SSD found that the percentage of surface dose is nearly stable, the absolute surface dose (cGy) increased with decreasing SSD. The absolute skin dose for wedge fields were lower than for open fields, for field size 10 × 10 cm2, while the values of surface dose of wedge 30 are 20% and 14% for 6 and 10 MV respectively. The skin dose for a wedge field increased as field size increased. Bolus 1 cm material increased the surface dose for 10 × 10 cm2 from 24% to 96%, and from 16% to 87%, for 6 and 10 MV respectively. With the use of an acrylic block tray, the surface dose increased to the open fields for all field sizes higher than 10 cm2, but the increase was dominant for large fields, for field size 20 × 20 cm2 the surface dose increased from 29% to 41%, from 22% to 34% for 6 and 10 MV respectively.
Conclusion: Agreement between skin doses calculated by multidata TPS and those measured by P.P ion chamber in water plastic water phantom was better than 20% for 96% of measurement points and this is indicate that the dose calculation in the build-up region using multidata TPS is good. For irregular tumor shape and closed OAR the IMRT is a good solution to cover the tumor and spare the critical organs without increasing the skin dose.
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