Purpose: Range uncertainties have been identified as a major source of error in the delivery of proton radiation therapy. In recent years, tolerances for ranges in clinical proton beams have been proposed and are typically measured using either a scanning chamber in water or a dedicated multi-layer ion chamber. For small fields with range on the order of 3 cm, such as the ones used to treat ocular melanoma, these methods are not practical. To address this unmet need we designed and tested a phantom that can be used to quickly measure the range in these challenging conditions.
Methods: 3D printed plastics were first evaluated to determine their water equivalent thickness (WET) in a 67.5 MeV proton beam. Once the WET was known, two phantoms were designed. The main component of the phantoms is water with the secondary component being the printed plastic. Both phantoms had a base of 2.5 cm, with either a cone of height 0.7 cm and base diameter of 2.3 cm, or a reverse cone of the same height and diameter. The phantoms were irradiated with a pristine 67.5 MeV proton beam with gafchromic film placed behind the phantom. The resulting 2 irradiations resulted in either a cold circle in the film center (cone) or a cold annulus (reverse cone). The diameter of the circles was related to the range of the measured proton beam.
Results: The range measurements were less than 1 mm different from the measured range performed using a water column.
Conclusion: The method presented here is robust and accurate and will be used in an updated Quality Assurance program for small fields with range less than 3 cm. Additional measurements will be performed to determine the sensitivity of this phantom to detect differences of +/- 1 mm in range.