Zebra

Among the two nozzles, a measurement was made in a multi-purpose nozzle with a wobbling beam of 190 MeV with a 5-cm ridge filter. Three categories of wobbling beam, according to field size, are available: small, medium, and large wobbling radius. The middle wobbling radius was selected for a 15-cm block diameter.
Reference data were obtained to verify the MC simulations and TPS calculations by measuring the rod-free area in the center of the Gammex phantom, where the proton beam passed through the 5-cm depth of the solid water material. A ZEBRA (IBA-Dosimetry, Schwarzenbruck, Germany) was used to measure the central axis of the beam. The PDDs of the eight different materials were measured using an OCTAVIUS Detector 729 Chamber Array (PTW, Freiburg, Germany) because the dose measurement passing through the inhomogeneity plug was off-axis and not feasible with ZEBRA. Hence, the two-dimensional absolute dose was measured by changing the thickness of the solid water phantom downstream of the Gammex phantom, with 1 mm intervals. After all measurements were completed, we constructed the PDD with a point dose at 1 mm depth increments along the central position of each rod.

TG224 recommends four dosimetry tests for the monthly QA of US proton delivery.¹ Tolerance for dose output, field flatness, and field symmetry is set at ±2% relative to the baseline, whereas tolerance for the distal range is ±1 mm.¹ The monthly QA program at our center includes all four TG224 recommended parameters. Dose output is measured in water by placing a parallel‐plate (PPC05) ionization chamber (IBA Dosimetry, Schwarzenbruck, Germany) at the center of spread‐out Bragg peak (SOBP) for a proton beam that has a range (R) of 16 cm and modulation (M) of 10 cm (R16M10). The center of SOBP coincided with the isocenter. The snout position for dose output measurements was kept at 18 cm.
Field flatness and symmetry were acquired using IC Profiler (Sun Nuclear, Melbourne, FL, USA) in conjunction with the solid water for four different beams: R10M6, R16M10, R22M8, and R28M14. The detector plane was placed at the isocenter. For range measurements, the authors utilized a Zebra — a multilayer ionization chamber (MLIC) (IBA Dosimetry, Schwarzenbruck, Germany). The ranges were measured for R10, R16, R22, and R28. For both the range and profiles (flatness and symmetry) measurements, the snout was placed at 30 cm from the isocenter. For all dosimetry measurements, the aperture of 10‐cm‐circular diameter was utilized.