Abstract
The lateral buildup ratio (LBR) used to estimate the depth dose distribution of electron beams for an irregular cutout field was obtained for a 4‐MeV energy beam from a Varian 21 EX linear accelerator. The depth‐dose curves for a group of circular cutout fields starting from a 2‐cm diameter were measured. Electron diodes were used in a large water tank to measure the LBR values for 6, 9, 12, and 16 MeV electron beam energies and a 10×10cm2 applicator. The results agreed with the published data. When the same equipment, setup, and technique were used to determine the LBR values for the 4‐MeV energy beam, the values were only reasonable, being within the clinical treatment range (i.e., LBR <1) for the smallest 6×6cm2 applicator. The calculated LBR values were clinically unacceptable for the circular cutout fields with a diameter larger than 2 cm with the 10×10cm2 applicator. The difficulty in the LBR measurement may be due to the significant contribution of scattered electrons from the beam defining system. This study also focused on how well the sigma values for the 4‐MeV beam can predict depth‐dose curves for other field sizes and whether the values are applicator‐dependent.PACS numbers: 87.53.Fs; 87.53.Hv; 87.66.Jj
Highlights
It is well known that the elementary pencil beam algorithms[1,2,3,4,5] have been used to calculate the dose distribution for electron beam treatment planning
The ratio is related to the lateral pencil beam spread function, σ(z, E), where z is the depth, and E is the energy of a circular electron beam of radius r.(11,12) By using the σr(z, E) values derived from the measured lateral buildup ratio (LBR) for a small circular reference field, sector-type integration can be used to calculate the average LBR at any depth of an irregular field.[10,12,13,14]
The above five electron beam energies are more frequently used by most centers for clinical treatment, 4 MeV is an available option.[16]. This energy, with its relatively shorter practical range and depth of maximum dose, is useful for treating superficial lesions when the superficial X-ray treatment unit is not available.[17,18,19,20] 4-MeV beam data such as depth-dose and beam profiles are more unstable, due to difficult beam tuning, and are difficult to measure accurately compared to higher energies.[21,22,23] The aim of this study is to investigate whether the LBR approach is beneficial to the 4-MeV electron beam through measurements
Summary
It is well known that the elementary pencil beam algorithms[1,2,3,4,5] have been used to calculate the dose distribution for electron beam treatment planning. The dose per monitor unit of irregularly shaped cutout fields has individually been measured in clinical practice These patient-specific measurements were timeconsuming, so different models based on the pencil beam approach were proposed to calculate the output factor in irregularly shaped electron fields.[6,7,8,9] Khan et al[10,11,12] introduced a semiempirical model based on the lateral spread of the pencil beam and proposed a function called the lateral buildup ratio (LBR). The ratio is related to the lateral pencil beam spread function, σ(z, E), where z is the depth, and E is the energy of a circular electron beam of radius r.(11,12) By using the σr(z, E) values derived from the measured LBRs for a small circular reference field (usually with a 2cm diameter), sector-type integration can be used to calculate the average LBR at any depth of an irregular field.[10,12,13,14]
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