An interview with Michael Butkus, Chief Resident of Medical Physics from Yale University School of Medicine & Yale New Haven Hospital.
COULD YOU GIVE A BRIEF DESCRIPTION OF THE PHANTOM AND DETECTORS?
The phantom is a PMMA or PlasticWater® cylinder with a 40cm length, 22cm diameter. Leveling legs and external markings aid in alignment. Within the phantom, there are two orthogonal 20x20cm2 detection planes. In total, the planes contain 1069 p-type diodes. In the central 6x6cm2 region of each detector plane, the resolution is 0.5cm. Outside of this region, the resolution is 1.0cm.
WHAT ARE THE MAJOR DIFFERENCES BETWEEN THE DELTA4 AND PHANTOM+ MODELS?
The most notable difference between the two phantoms is the lack of external wires in the Phantom+ model. The lack of external wires greatly decreases setup time and streamlines the measurement process. The older model required external connections to power the device, transfer data, and sync to the accelerator pulse. Power is now supplied by a rechargeable battery, with an approximant 4hr life. Data is transferred wirelessly. And the sync is now under automatic control. The orientation of the arrays has also changed. Previously they were offset from the cardinal angles by 40-50°, forming an X shape. Now they are aligned in the cardinal directions, forming a plus shape. Hence the name Phantom+. Additionally, the diodes are more stable and do not require relative recalibrations over their lifetime.
WHAT BENEFIT HAS THE PHANTOM+ BROUGHT TO YOUR INSTITUTION?
The Phantom+ has provided us with accurate 3D dose measurements in stable diodes for routine patient-specific QA. The temporal sampling of data allows for segment by segment dose reconstruction and the intuitive software allows for immediate analysis of results. The system has great utility for some of our less common techniques, such as FFF and extra-long fields, techniques that have proven burdensome to accurately measure with other devices. The new Phantom+ model is a much cleaner system that has helped streamline our QA process by eliminating external wires and reducing the need for recalibrations.