Purpose: Previous studies demonstrate large (up to 4 %) variations in response using commercial ionization chambers in solid phantoms. This work investigates an optimized chamber design for accurate output measurements in solid phantoms in MRgRT radiotherapy beams with a set-up most commonly used in conventional clinical workflow.
Methods: We previously demonstrated an optimized chamber design that uses a thick (1.1 mm) brass wall to eliminate variations in response due to air gaps between the chamber and the solid phantom used for output measurements. Here, we construct and test a prototype chamber in a 7 MV Elekta Unity MR-linac with a 1.5 T magnetic field. The radiation field is collimated to 10x10 cm² at the surface of a plastic phantom with the chamber at a depth of 10 cm. The chamber is rotated about its axis through 360 degrees in the phantom with measurements made at each cardinal angle. This reveals any variation in chamber response by varying the thickness of the air gap between the chamber and the phantom. The chamber axis is perpendicular to the linac beam and tested both parallel and perpendicular to the magnetic field.
Results: The prototype chamber exhibits less than 0.3 % variation in response when rotated about its axis in the plastic phantom, independent of whether its axis is parallel or perpendicular to the magnetic field. This is much smaller than the previously reported variations for thin-walled commercial Farmer-type chambers of similar designs.
Conclusion: A prototype chamber has been constructed and validated for accurate output checks in a beam from an MR-linac. The chamber design enables the use of conventional equipment and procedures in MRgRT systems, minimizing disruption to current clinical workflow and allowing the use of solid phantoms unlike all other non-conventional solutions.