In this study, we developed an isocenter verification device for use in proton therapy. Radiation and mechanical isocenters were verified for treatment equipment including room lasers, a digital radiography system and the beam axis of a rotational gantry. The special feature of this device is its ability to correlate the position of the three isocenters in one measurement and thus improve accuracy compared to the conventional method using three separate devices. The reproducibility of the method and the fluctuation of the position of the beam axis isocenter were both investigated using this device for almost a year. Monthly measurements of the isocenter position were acquired for two gantries and it was found that the fluctuation was ±0.10 mm for the up-to-down direction and ±0.16 mm for the right-to-left direction in Gantry 1 and was ±0.14 mm for the up-to-down direction and ±0.18 mm for the right-to-left direction in Gantry 2. We could be measured with a repeatability of ±0.18 mm or less by using developed device for the relative positional relationship between each isocenters. Because we can confirm results in approximately 30 minutes, we can perform a quality control after a clinical practice.

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In this report, history of the dose standard system, calibration of ionization chamber and standard dosimetry protocol to assure dose traceability was interpreted. The water absorbed dose standard is newly established in Japan and calibration service and dosimetry protocol based on the water absorbed dose has been provided. A lot of uncertainties exist in current complicated radiation therapy. However, uncertainty of dose delivery will be reduced and further progress of quality assurance is expected by shift to the absorbed dose calibration system in radiation therapy field.

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The calibration service in terms of absorbed dose to water started from 2011 after establishment of the national primary standard using a graphite calorimeter at the national metrology institute of Japan (NMIJ) and JCSS accreditation of the association for nuclear technology in medicine (ANTM). Accordingly, a new dosimetry protocol was introduced as JSMP12, in which details of the national standard were also described. This report presents a short review of the standard, a key comparison result, and a comparison result of calibration coefficients by JSMP01 and JSMP12.

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This report describes the update of the beam quality conversion factor K_Q,Q0 of the standard dosimetry protocol in Japan. The K_Q,Q0 corrects for the difference between the response of an ionization chamber in the reference beam quality Q₀ used for calibrating the chamber and in the actual user beam quality Q. All changes of K_Q,Q0 were caused by the perturbation correction factors which were recalculated by Monte Carlo simulation. With a calculation process, unsolved problems in this update are also discussed here.

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