A comparison of TPS and different measurement techniques in small-field electron beams

Abstract

In recent years, small-field electron beams have been used for the treatment of superficial lesions, which requires small circular fields. However, when using very small electron fields, some significant dosimetric problems may occur. In this study, dose distributions and outputs of circular fields with dimensions of 5cm and smaller, for nominal energies of 6, 9, and 15MeV 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), using the gamma-index method in the PTW VeriSoft software program. Output measurements were performed using the Gafchromic EBT film, an Advanced Markus ion chamber, and thermoluminescent dosimetry (TLD). Although the pencil-beam algorithm is used to model electron beams in many clinics, there is no substantial amount of detailed information in the literature about its use. As the field size decreased, the point of maximum dose moved closer to the surface. Output factors were consistent; differences from the values obtained from the TPS were, at maximum, 42% for 6 and 15MeV and 32% for 9MeV. When the dose distributions from the TPS were compared with the measurements from the Gafchromic EBT films, it was observed that the results were consistent for 2-cm diameter and larger fields, but the outputs for fields of 1-cm diameter and smaller were not consistent. In CMS XiO TPS, calculated using the pencil-beam algorithm, the dose distributions of electron treatment fields that were created with circular cutout of a 1-cm diameter were not appropriate for patient treatment and the pencil-beam algorithm is not convenient for monitor unit (MU) calculations in electron dosimetry.

Keywords: EBT film; Electron dosimetry; Pencil-beam algorithm; Small fields.