Room: AAPM ePoster Library
Purpose: Small-field surface-collimated electrons can be employed to treat superficial skin lesions, but the dosimetry is challenging. This work evaluates the ability of the RayStation (v.8A RaySearch Laboratories, Stockholm, Sweden) Monte Carlo algorithm to model the dosimetry in such a situation.
Methods: Cerrobend blocks were made with circular apertures of nominal diameters 10 mm, 15 mm, 20 mm, and 30 mm, and thickness equivalent to 4 mm of lead foil. 5 mm and 10 mm thick bolus was cut to fit each of the blocks. 1.5 mm of beeswax was taped to the underside of each block with the center cut out to match its corresponding opening. The bottom surface of the beeswax was placed on top of solid water that was set to 100 cm SSD. This experimental setup was created in RayStation’s Monte Carlo module using 5 million histories/cm² and a 1.0 mm dose grid. GAFCHROMIC EBT3 film was selected to measure the output, profiles, and PDDs of the small field 6 MeV electron beam. Coronal and sagittal orientations of film were used with coronal orientations placed at the calculated d(max) for each setup. The output factors were calculated relative to the manufacturer-provided open 14x14 cm² applicator. Further, the depths of R90 and R80, and widths of the 90% and 80% isodose lines were measured for each combination of cutout and bolus.
Results: Calculated and measured output factors agreed within 6%. R90 and R80 depths were within 1.0 mm of RayStation. Measured widths of the 80% and 90% isodose lines differed from RayStation by a maximum of 2.2 mm.
Conclusion: The Monte Carlo dose algorithm in RayStation can be utilized to determine the output of small electron fields for superficial lesions. RayStation generally appears to under-predict the width 80% and 90% isodose lines and R90 and R80 depths.