We describe the use of polystyrene wedges to match adjacent electron beams with improved dose uniformity. These wedges were designed to increase the penumbra width at the field junction from about 1.5 to about 3.5 cm, to achieve dose uniformity. Measurements using thermoluminescent dosimeters (TLD) and therapy localization film showed that the use of polystyrene wedges (penumbra generators) produced only a small increase (less than 3%) in the surface dose and a small increase (less than 1%) in the x-ray contamination. Without wedges at the field junction, lateral mismatching of beam edges by 2 or 3 mm may introduce high dose variations (120% or more or 50% or less). Similar 2-3 mm set-up errors did not cause more than +/- 5% dose variations when plastic wedges were used to match the fields. These wedges are particularly useful when matching fields of different beam energies or matching fields on curved surfaces, such as the chest wall.
Purpose: Kilo‐voltage (kV) photons and low megavoltage (MeV) electrons are the most common options for treating small superficial lesions, but they present complex dosimetry. Using a tertiary lead shield may protect the surrounding critical structures. Our goal was to quantitatively evaluate the dosimetric impact resulting from applying tertiary shields on superficial lesions. Method: We directly compared the beam characteristics of 80 kV (0.8 mm Al) photon setup abutting the water phantom surface and 6 MeV electron setup at 100 cm SSD. Profiles and depth doses were acquired using a 3D scanning water tank and an ion chamber (active volume 0.01 cm^3). Beam profiles were scanned at Dmax. Three lead sheets (2 mm thickness) with 2.7, 2.2, and 1.6, cm diameter circular cutouts were fabricated and placed at the water surface for both photon and electron fields. Results: The penumbra (80% – 20%) of the open 4×4 cm^2 electron insert was 10.7 mm, compared to an average of 7.2 mm with the tertiary cutouts. The penumbra of the open kV photon beam was 2.8 mm compared to an average of 1.8 mm with the tertiary cutouts. For field widths 2.7, 2.2, and 1.6 cm, the flatness of the electron beams was 16%, 17.3%, and 21%, respectively, and for the kV photon beams was 1.4%, 2.3%, 3.3%, respectively. The electron depth dose (PDD) shifted shallower and the photon PDD shifted deeper as the field size became smaller. Conclusion: The penumbra of small electron fields can be improved by adding tertiary lead shields. Both modalities are clinically feasible; however, kV photons still offer sharper penumbra and better flatness than that of 6 MeV electrons with tertiary shielding. Thus, kV photons may still be a superior option for small superficial lesions.
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