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No abstract

The measurements of the three-dimensional (3D) dose distribution of proton beams in water are critical for proton therapy for quality assessment (QA). Although ionization chambers are commonly used for this purpose, such measurements take a long time to calculate precise 3D dose distribution.To solve this problem, we measured 3D dose distributions using a glass plate. We placed a 1-mm thick float glass plate on the upper inside of a black box with a water tank set above the float glass plate outside the black box and irradiated the proton beam to the water tank from the upper side. The attenuated proton beam by water in the tank was detected by the float glass plate and a scintillation image was formed in the plate. The image was reflected by a first-surface mirror set below the float glass plate and detected by a cooled charge-coupled device (CCD) camera from the side. We changed the water depths in the tank and measured the scintillation images at each depth. Then we calculated the 3D scintillation images from the measured images by stacking them in the depth direction. Measurements were made for 71.2-and 100-MeV proton pencil beams and a spread-out Bragg peak (SOBP) using the imaging system. From the images, we successfully formed 3D scintillation images without quenching. The depth profiles measured from the scintillation images showed almost identical distribution with those measured by the ionization chamber within a maximum difference less than 5%. The lateral profiles were also almost identical within width differences less than 2 mm. We conclude that our proposed method is promising for the 3D dose distribution measurements of proton beams.

Luminescence was found during pencil-beam proton irradiation to water phantom and range could be estimated from the luminescence images. However, it is not yet clear whether the luminescence imaging is applied to the uniform fields made of spot-scanning proton-beam irradiations. For this purpose, imaging was conducted for the uniform fields having spread out Bragg peak (SOBP) made by spot scanning proton beams. We designed six types of the uniform fields with different ranges, SOBP widths and irradiation fields. One of the designed fields was irradiated to water phantom and a cooled charge coupled device camera was used to measure the luminescence image during irradiations. We estimated the ranges, field widths, and luminescence intensities from the luminescence images and compared those with the dose distribution calculated by a treatment planning system. For all types of uniform fields, we could obtain clear images of the luminescence showing the SOBPs. The ranges and field widths evaluated from the luminescence were consistent with those of the dose distribution calculated by a treatment planning system within the differences of  -4 mm and  -11 mm, respectively. Luminescence intensities were almost proportional to the SOBP widths perpendicular to the beam direction. The luminescence imaging could be applied to uniform fields made of spot scanning proton beam irradiations. Ranges and widths of the uniform fields with SOBP could be estimated from the images. The luminescence imaging is promising for the range and field width estimations in proton therapy.

The accurate measurement of the 3D dose distribution of carbon-ion beams is essential for safe carbon-ion therapy. Although ionization chambers scanned in a water tank or air are conventionally used for this purpose, these measurement methods are time-consuming. We thus developed a rapid 3D dose-measurement tool that employs a silver-activated zinc sulfide (ZnS) scintillator with lower linear energy transfer (LET) dependence than gadolinium-based (Gd) scintillators; this tool enables the measurement of carbon-ion beams with small corrections. A ZnS scintillator sheet was placed vertical to the beam axis and installed in a shaded box. Scintillation images produced by incident carbon-ions were reflected with a mirror and captured with a charge-coupled device (CCD) camera. A 290 MeV/nucleon mono-energetic beam and spread-out Bragg peak (SOBP) carbon-ion passive beams were delivered at the Gunma University Heavy Ion Medical Center. A water tank was installed above the scintillator with the water level remotely adjusted to the measurement depth. Images were recorded at various water depths and stacked in the depth direction to create 3D scintillation images. Depth and lateral profiles were analyzed from the images. The ZnS-scintillator-measured depth profile agreed with the depth dose measured using an ionization chamber, outperforming the conventional Gd-based scintillator. Measurements were realized with smaller corrections for a carbon-ion beam with a higher LET than a proton. Lateral profiles at the entrance and the Bragg peak depths could be measured with this tool. The proposed method would make it possible to rapidly perform 3D dose-distribution measurements of carbon-ion beams with smaller quenching corrections.