Technical Note: Use of commercial multilayer Faraday cup for offline daily beam range verification at the McLaren Proton Therapy Center

Abstract

Purpose: Daily verification of the proton beam range in proton radiation therapy is a vital part of the quality assurance (QA) program. The objective of this work is to study the use of a multilayer Faraday cup (MLFC) to perform a quick and precise daily range verification of proton beams produced by a synchrotron.

Methods: Proton beam depth dose measurements were performed at room iso-center in water using PTW water tank and Bragg Peak ion chamber. The IBA Giraffe, calibrated against the water tank data, was used to measure the water equivalent thickness (WET) of the sample copper plates. The WET measurements provided the range calibration factors for the MLFC. To establish a baseline for in room measurements, range measurements for energies from 70 to 250 MeV in steps of 10 MeV were performed using the Pyramid MLFC at room iso-center. For the daily range verification measurements, the MLFC is permanently placed at the end of the beam line, inside the accelerator vault. The daily range constancy is performed for five representative beam energies; namely 70, 100, 150, 200, and 250 MeV. Data collected over a period of more than 100 days are analyzed and presented.

Results: The measured WET values of the copper plates increased with increasing energy. The centroid channel number in the MLFC where the protons stop, was converted to depth in water and compared to the depth of the distal 80% (d80) obtained from the water tank measurements. The depths agreed to within 2 mm, with the maximum deviation of 1.97 mm observed for 250 MeV beam. The daily variation in the ranges measured by the MLFC was within ±0.5 mm. The total time to verify five proton beam ranges varies between 4 and 5 min.

Conclusion: Based on the result of our measurements, the MLFC can be used for a daily range constancy check with submillimeter accuracy. It is a quick and simple method to perform range constancy verification on a daily basis.

Keywords: IBA Giraffe; multilayer Faraday cup; pencil beam scanning; proton therapy; range QA; range verification.