Abstract
The purpose of this study was to compare the Pencil Beam (PB) with Monte Carlo (MC) calculated dosimetric results using phantoms for air cavity region. Measurements in Tough water phantom with air gaps were used to verify the calculated dose. The plane-parallel ionization chamber was moved from 2 mm to 20 mm behind air gap. Calculations were performed for various air gaps (1.0, 2.0, 3.0 and 4.0 cm) and field sizes (4.2 × 4.2, 6.0 × 6.0 and 9.8 × 9.8 cm2). The lateral missing tissue measurement was performed using the radiochromic RT-QA film. Dose difference between PB and chamber measurement near an air gap was greater for smaller field size, larger air gap thickness, and shallower depth behind air gap. As the distance from the phantom edge became shorter, the dose differences of the PB calculation and film measurement became larger. MC calculations were found within 3% agreement to the measured dose distributions. Our results demonstrate an excellent agreement between ionization chamber and radiochromic RT-QA film measurements and MC calculations.
Highlights
The effect of heterogeneous corrections is an important issue that has increasingly drawn the attention of the medical physics community for last several years
Significant dose difference can be observed between Pencil Beam (PB) and Monte Carlo (MC) calculations in re-build up region
A direct relationship can be observed between calculated dose and field size with fixed air gap
Summary
The effect of heterogeneous corrections is an important issue that has increasingly drawn the attention of the medical physics community for last several years. In the report of Task Group No 65 of the Radiation Therapy Committee of the American Association of Physicists in Medicine, inhomogeneity correction algorithms were categorized according to the level of anatomy sampled for scatter calculation and the inclusion or exclusion of electron transport [1]. Hurkmans et al reported the limitations of dose calculations in the case of head & neck tumor using the PB algorithm [2]. MC algorithm was able to predict the dose distributions with a higher accuracy [3]. Many researchers have investigated the effect of air cavities on the dose distribution and dose reduction near air cavity, depending on geometry, beam energy, and field size using various MC codes in water equivalent phantoms [4,5,6,7]
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