Evaluation of a Brass Collimator for Superficial Irradiation Using High-Energy Electrons: A Case Study
Abstract
Summary
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Purpose
Electron therapy allows for superficial dose deposition but suffers from bulging dose clouds deeper within the patient. This work presents a case where a patient was treated for a superficial skin tumor in the scalp where the target was adjacent to previous radiation treatment. To increase dose falloff, a brass collimator was developed and placed on the patient’s surface. This work presents in-vivo dosimetry results and a phantom study to characterize the effects of the brass collimator.
Methods
A treatment plan was generated with a dose of 60 Gy in 30 fractions to the PTV. The plan utilized a 9 MeV beam, 105 cm SSD, and a 15x15 cm2 electron aperture with custom electron block. In addition, a custom bolus was modeled and designed by .decimal. The bolus included 0.7 cm of brass collimator and 0.5 cm of tissue equivalent material. The bolus was placed on the patient’s surface within the treatment field. During the first treatment, EBT4 film was placed on the patient’s surface to measure dose at the treatment field, including along the field edge and under the brass collimator. Furthermore, a phantom study was performed using the brass collimator and films to investigate the effects of the brass collimator on dose falloff superficially and at a depth. All films were scanned and analyzed 24 hours after irradiation.
Results
Clinical in-vivo dosimetry results showed good agreement, within 2 cGy, with the treatment planning system within the treatment field and underneath the brass collimator. Dose falloff in Eclipse matched film measurements from the field edge. The phantom study demonstrated that the brass collimator improved dose falloff superficially and at 1 cm depth.
Conclusion
This work showed the efficacy of using a brass collimator placed on the patient’s skin for cases that require improved dose falloff.