Abstract
The aim of the study was to investigate surface and buildup region doses for 6 MV and 15 MV photon beams using a Markus parallel-plate ionization chamber, GafChromic EBT3 film, and MOSFET detector for different field sizes and beam angles. The measurements were made in a water equivalent solid phantom at the surface and in the buildup region of the 6 MV and 15 MV photon beams at 100 cm source-detector distance for 5 × 5, 10 × 10, and 20 × 20 cm2field sizes and 0°, 30°, 60°, and 80° beam angles. The surface doses using 6 MV photon beams for 10 × 10 cm2field size were found to be 20.3%, 18.8%, and 25.5% for Markus chamber, EBT3 film, and MOSFET detector, respectively. The surface doses using 15 MV photon beams for 10 × 10 cm2field size were found to be 14.9%, 13.4%, and 16.4% for Markus chamber, EBT3 film, and MOSFET detector, respectively. The surface dose increased with field size for all dosimeters. As the angle of the incident radiation beam became more oblique, the surface dose increased. The effective measurement depths of dosimeters vary; thus, the results of the measurements could be different. This issue can lead to mistakes at surface and buildup dosimetry and must be taken into account.
Highlights
The deposited dose at the boundary between phantom and air is defined as the surface dose
For 6 MV and 15 MV, the percentage doses at the surface and buildup regions for different field sizes measured using the Markus parallel-plate ionization chamber, EBT3 film, and Metal oxide semiconductor field effect transistor (MOSFET) dosimeter are given in Tables 1, 2, and 3
percentage depth dose (PDD) values were investigated at the depth of 0.07 mm, which was suggested for surface dose measurements by International Commission on Radiation Units and Measurements (ICRU) Report 39 [16]
Summary
The deposited dose at the boundary between phantom and air is defined as the surface dose. Surface dose in radiation therapy is important in cases where the patient skin is doselimiting tissue or part of the target volume in the treatment area. A therapeutic photon beam has electron contamination in the first few millimeters of skin caused by photon interactions in air or interactions with collimator and such scattering materials in the path of the beam. The surface dose depends on field size, source to skin distance (SSD), beam angle, beam energy, and beam modifiers such as blocks and multileaf collimator (MLC) systems. Accurate knowledge of surface dose is important, but the measurement of the dose at such shallow depths is a challenging issue. Due to each dosimetric tool having its own specific physical property, the results of surface dose measurements may vary
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