Abstract
In this study, dose distributions and outputs of circular fields with dimensions of 5 cm and smaller, for 6 and 9 MeV nominal energies from the Siemens ONCOR Linac, were measured and compared with data from a treatment planning system using the pencil beam algorithm in electron beam calculations. All dose distribution measurements were performed using the GafChromic EBT film; these measurements were compared with data that were obtained from the Computerized Medical Systems (CMS) XiO treatment planning system (TPS). Output measurements were performed using GafChromic EBT film, an Advanced Markus ion chamber, and thermoluminescent dosimetry (TLD). Although it is used in many clinics, there is not a substantial amount of detailed information in the literature about use of the pencil beam algorithm to model electron beams. Output factors were consistent; differences from the values obtained from the TPS were at maximum. When the dose distributions from the TPS were compared with the measurements from the ion chamber and GafChromic EBT films, it was observed that the results were consistent with 2 cm diameter fields and larger, but the outputs for 1 cm diameter fields and smaller were not consistent.
Highlights
In recent years, electron beams, rather than low X-ray beams, have been used for the treatment of superficial lesions, because of the different dosage characteristics of the two sources
Dose distribution measurements were made for these circular fields, 1–5 cm in diameter, using a MP3-M automatic water phantom and a Semiflex (0.125 cc) ion chamber (PTWNew York Corp., Hicksville, NY), with a source-to-surface distance (SSD) of 100 cm. 1, 2, 3, and 4 cm cut-outs were placed into the 5 cm diameter standard circular cone
For the 6 MeV electron beam, the dmax value obtained from the Semiflex ion chamber (0.125 cc) is 12 mm in the 10 × 10 cm2 square field size
Summary
Electron beams, rather than low X-ray beams, have been used for the treatment of superficial lesions, because of the different dosage characteristics of the two sources. Electron beams have uniform dose distribution at the surface and a rapid decrease in the dose below a specific depth [1]. Electron beam therapy is an indispensable practice in radiotherapy, and most commercial treatment planning systems use electron beam programs alongside photon planning programs. Electron beam dose calculations were originally based on empirical functions that utilised ray line geometries [2, 3]. More advanced pencil beam algorithms are based on multiple scattering theories [4, 5]. The major limitation of both empirical methods and pencil beam algorithms is their inability to predict depth dose distributions and accurately monitor units for small field sizes. A Monte Carlo-based dose calculation algorithm has been investigated by several groups.
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