Purpose: To determine the effects that different orientations of a 1.5 mm brass bolus have on the resultant surface dose when irradiated with 6 and 18MV photon beams and find its water equivalent bolus thickness to characterize it for clinical use.
Methods: Brass bolus material consists of an array of flat brass discs linked loosely together on one side by small links of brass chain. While this construction allows the bolus to conform well to the surface of a patient, it also introduces variability in setup orientation. These orientations include: “discs” side facing either up or against the patient, and allowing the gaps between the discs to be either loose or tight. A 1.5mm brass bolus was placed over top of radiosensitive film and backscatter material in multiple orientations and irradiated to 400MU by both 6 and 18MV photon beams. Each strip of film was then scanned into the RIT software and the dose distributions were analyzed. A set of CT scans were also performed on the brass bolus and compared in Raystation to water boluses of known thicknesses.
Results: Initial results reveal the expected peak-and-trough pattern at the surface. A difference in peak-to-trough dose of up to 21.7% of the maximum surface dose was recorded for the 6MV photon beam and up to 25.8% for the 18MV beam. These maximum differences occurred with the bolus oriented in a deliberate “loose” configuration, with the discs facing toward the film. No notable difference in surface dose distribution was observed between the tight gap setups for either energy. Evaluation in Raystation proved this 1.5mm brass bolus to be roughly equivalent to a 3mm water bolus.
Conclusion: Orientation of brass bolus has some significant effect on surface dose distribution with the peak-and-trough pattern more pronounced at 18MV than 6MV.