Purpose: Skin collimation for electron therapy is known to minimize penumbra and avoid isodose-line constriction for small-field sizes. This study reports the dosimetric characteristics of two skin collimation materials for electron treatments, using state-of-the-art detectors suitable for small-field dosimetry.
Methods: Exradin W2 (1X3) scintillator, EBT3 film, and a 25x25 cm solid water phantom were used to measure the dosimetric properties of Matrix Thermo-Shield (30% Bismuth blend) and Leaded Vinyl (70-90% lead) using 6MeV and 9MeV electrons. A 2.5 cm diameter circular portal was used when measuring output factors, profiles, depth doses, and penumbra, while attenuation and backscatter properties were measured without the circular portal opening. EBT3 films were stacked in-between solid-water slabs at depths of 0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0 cm, and the One-Scan dosimetry protocol was followed for film calibration/scanning/analysis. W2 was positioned at the depth of maximum dose and its results were compared with the film measurements.
Results: The penumbra (20-80%) reduction for 6MeV and 9MeV were 0.13 cm and 0.38 cm for Thermo-Shield, and 0.18 cm and 0.24 cm for Leaded Vinyl. Significant backscatter was observed and 2-3mm wax was needed to reduce backscatter to <10% for both materials. On the portal measurements, the buildup-region (0-3.0 cm) doses increased with the presence of the shielding materials for both energies up to 5%. The results of W2 for output factors, attenuation, and depth doses agreed with EBT3 measurements to within 3%.
Conclusion: Dosimetric characteristics of the two skin collimation materials for 6MeV and 9MeV were presented. A properly designed skin collimation can replace custom electron cutouts and reduce the penumbra for the small-field electron treatments using low energies. Patient-specific measurement is recommended for the clinical use of skin collimation materials. Sufficient wax-coating is needed if the two materials are used for internal shielding.
Funding Support, Disclosures, and Conflict of Interest: One author has a research grant from Ashland Inc., the manufacturer of GafChromic film.
Not Applicable / None Entered.