高エネルギー光子線の水吸収線量の計測のため，「Addendum to the AAPM's TG-51 protocol（AAPM TG-51プロトコルの補足）」を発表する．このAddendumは基本的にTG-51の手順を継承する．ただし，モンテカルロ計算で得た光子線のための新しいkQ と，プロトコルの実施において正確さと整合性を向上するためのさまざまな勧告を提供する．また，基準点の水吸収線量がもつ不確かさバジェットの成分を明らかにし，それぞれの大きさについて議論した．最後に，実験的に決定したND,wとの整合性について述べた．ユーザがTG-51に精通している場合，このAddendumの実施は簡単である．ユーザによっては新しい勧告により手順の変更を要する場合もあるが，通常，この勧告がもたらす変化は軽微である．このAddendumの実施に伴う医学物理士の労力は僅かであり，年に1度のリニアックの校正の一部で実施できるだろう．
In Japan and North America, different dosimetry protocols have been implemented to determine the absorbed dose to water: JSMP Standard Dosimetry 12 and AAPM TG-51 addendum. In this study, Japanese and Canadian reference dosimetries for high energy photon beams were compared theoretically, and then they were verified experimentally. We estimated the theoretical differences of the ion recombination correction factors, the leakage correction factors, the radial dose distribution correction factors, the calibration factors, the beam quality correction factors and the absorbed dose to water. When an influence of the radial dose distribution is negligible, the ratios of Canadian to Japanese absorbed dose in reference dosimetries ranged from 0.995 to 1.007 for all the reference-class-Farmer-type ionization chambers. This discrepancy was mainly caused by the wall correction factor included in the beam quality correction factor. Subsequently, to verify the theoretical approaches, we calibrated the same ionization chamber in 60Co gamma ray of Japanese primary and secondary standard dosimetry laboratories (PSDL and SSDL) and measured the absorbed dose of a clinical linear accelerator. It followed that the ratios of Canadian to Japanese absorbed dose in reference dosimetries increased up to 1.015 for PTW 30013 reference-class-Farmer-type ionization chamber. This increase was mainly caused by a discrepancy in the calibration factors (ND,w) observed between Japanese PSDL and SSDL. In conclusion, in order to improve the international consistency of the absorbed dose to water determined by JSMP Standard Dosimetry 12, we should reevaluate the accuracy of the wall correction factors and implement a periodic comparative test of the ND,w between Japanese PSDL and SSDL.
A calibration method for CT-number to stopping-power-ratio conversion was recently proposed as a revision of the Japanese de facto standard method that has been used at particle therapy centers in Japan for over a decade. The revised method deals with 11 representative tissues of specific elemental composition and density, based on a latest compilation of standard tissue data. We report here how the revision was actually implemented into clinical practice. We applied the revised method to 7 CT-scanning conditions currently in use for treatment planning. For each condition, we derived CT numbers and stopping-power ratios of the representative tissues to constitute a polyline conversion function. We analyzed the change of target water-equivalent depth by the revision for 38 beams in treatment plans for 13 randomly sampled patients. The revision caused a mean change of +0.3 mm with a standard deviation of 0.4 mm. The maximum change was +1.2 mm or +0.5% of the depth, which may be clinically insignificant. The transition to the revised method was straightforward and would slightly improve the accuracy of the beam range in particle therapy.