Dosimetry of irregular field sizes in electron beam therapy

Electron beams are used to treat superficial lesions in radiation oncology by taking advantage of the sharp dose fall-off and the limited range of the particles in tissue. The irregular shape of individual tumors, however, often requires custom made shielding in order to geometrically shape the radiation field to the target, while minimizing the dose to surrounding tissues. In many institutions, low melting alloy or lead cutouts are used for electron beam shaping. In this work, electron dosimetry beam parameters such as percentage depth dose (PDD), outputs, and beam profiles, were measured with ten different electron beams from two linear accelerators. The dependence of beam characteristics on field size and shape, particularly for small cutouts, was investigated. In addition, this project examined different methods for measuring electron PDDs, including film densitometry, ion chambers, and diode dosimetry.The work presented here demonstrates that the depth dose effect is significant when one of the field dimensions of the cutout is less than R p, the practical range of electrons. For these cutouts, it was observed that both PDD and outputs vary significantly due to the lack of lateral electronic equilibrium. As the cutout becomes smaller, the depth of dose maximum (dmax) shifts towards the surface, while the output at dmax decreases. Therefore, it is crucial that PDDs and outputs are either measured or calculated for small field electron cutouts used in the clinical setting