An Integrated 2-dimensional Dosimeter and Electronic Portal Imaging Device for In Vivo Dosimetry: A Feasibility Study

Abstract

Purpose/Objective(s): As radiation therapy becomes more conformal and the delivery technology becomes more complex, in-vivo dosimetry is becoming more necessary but at the same time more technically challenging. This work investigates the feasibility of a novel integrated in-vivo imaging and dosimetry device to meet the current and future requirements of radiation therapy treatment verification whereby imaging and dosimetry could be achieved concurrently. Materials/Methods: The dose response of a commercial 2-D ion-chamber array, in two configurations was investigated; (1) the experimental configuration with the ion-chamber array positioned directly under an electronic portal imaging device (EPID), (2) the standard dosimetry configuration with the ion-chamber array and 5cm solid water build-up. All measurements were conducted with the ion chamber array at isocentric distance and no additional objects in the beam. The measured dose was compared with the calculated dose from a treatment planning system (TPS). For the experimental configuration (1) the ion chamber array was positioned on the treatment couch with the EPID sitting directly on top of it. An additional 3.2 cm solid water was added to fill the air gap between the array and EPID. All the experiments were conducted at zero gantry angle. The central axis dose per Monitor unit and dose profiles for a 10x10 cm open field was measured in both configurations. The water equivalent depth of the experimental configuration (1) was determined by comparison with the TPS calculated dose. This depth was subsequently used for TPS dose calculations for further open beams, IMRT test patterns, and clinical IMRT beams. The field size factor and off-axis dose response measured with configuration (1) for open fields was compared with TPS data. The gamma analysis was performed with 2% and 3 mm criteria. Agreement between ion-chamber array and TPS was compared for both configurations. Results: The water equivalent depth of the ion chamber array in configuration (1) was 8.2 cm. The maximum dose difference between measured dose profiles with configuration (1) and TPS was 3.5% for 20x20 cm field size. The field size output factor with configuration (1) agreed within 2% with TPS. For a given set of gamma criteria the percentage of gamma pass agreed to with 0.5% for all the measurements. Conclusion: An ion chamber array integrated with an EPID maintains accurate water equivalent dose response. This configuration has the potential to provide a new technique of accurate 2D in vivo dosimetry. Further work is underway to assess the impact of the ion chamber array on imaging, and the performance of the device with an object in the beam to more closely simulate the clinical setting. Author Disclosure: S. Deshpande: None. L. Holloway: None. P. Metcalfe: None. P. Vial: None.