Rossomme S, Palmans H, Thomas R, Lee N, Bailey M, Shipley D, Al-Sulaiti L, Cirrone P, Romano F, Kacperek A, Bertrand D, Vynckier S
Molecular Imaging and Experimental Radiotherapy Department, Catholic University of Louvain, Brussels, Belgium.
Division of Acoustics and Ionising Radiation, National Physical Laboratory, Teddington, UK.
Med Phys. 2012 Jun;39(6Part12):3736-3737. doi: 10.1118/1.4735204.
The IAEA TRS-398 code of practice can be applied for the measurement of absorbed dose to water under reference conditions with an ionization chamber. For protons, the combined relative standard uncertainty on those measurements is less than 2% while for light-ion beams, it is considerably larger, i.e. 3.2%, mainly due to the higher uncertainty contributions for the water to air stopping power ration and the W air-value on the beam quality correction factors kQ,Q . To decrease this uncertainty, a quantification of kQ,Q is proposed using a primary standard level graphite calorimeter. This work includes numerical and experimental determinations of dose conversion factors to derive dose to water from graphite calorimetry. It also reports on the first experimental data obtained with the graphite calorimeter in proton, alpha and carbon ion beams.
Firstly, the dose conversion has been calculated with by Geant4 Monte-Carlo simulations through the determination of the water to graphite stopping power ratio and the fluence correction factor. The latter factor was also derived by comparison of measured ionization curves in graphite and water. Secondly, kQ,Q was obtained by comparison of the dose response of ionization chambers with that of the calorimeter.
Stopping power ratios are found to vary by no more than 0.35% up to the Bragg peak, while fluence correction factors are shown to increase slightly above unity close to the Bragg peak. The comparison of the calorimeter with ionization chambers is currently under analysis. For the modulated proton beam, preliminary results on W air confirm the value recommended in TRS-398. Data in both the non-modulated proton and light-ion beams indicate higher values but further investigation of heat loss corrections is needed.
The application of graphite calorimetry to proton, alpha and carbon ion beams has been demonstrated successfully. Other experimental campaigns will be held in 2012. This work is supported by the BioWin program of the Wallon Government.
国际原子能机构（IAEA）的TRS - 398实践准则可用于在参考条件下使用电离室测量水的吸收剂量。对于质子，这些测量的合成相对标准不确定度小于2%，而对于轻离子束，该不确定度则大得多，即3.2%，这主要是由于水对空气阻止本领比以及用于束流质量校正因子kQ,Q的W空气值的不确定度贡献较高。为降低此不确定度，建议使用一级标准水平的石墨量热计对kQ,Q进行量化。这项工作包括剂量转换因子的数值和实验测定，以便从石墨量热法得出水的剂量。它还报告了使用石墨量热计在质子、α粒子和碳离子束中获得的首批实验数据。
首先，通过Geant4蒙特卡罗模拟，通过确定水对石墨的阻止本领比和注量校正因子来计算剂量转换。后者也通过比较石墨和水中测量的电离曲线得出。其次，通过比较电离室与量热计的剂量响应来获得kQ,Q。
发现阻止本领比在布拉格峰之前变化不超过0.35%，而注量校正因子在接近布拉格峰时略高于1。量热计与电离室的比较目前正在分析中。对于调制质子束，W空气的初步结果证实了TRS - 398中推荐的值。非调制质子束和轻离子束的数据表明值更高，但需要对热损失校正进行进一步研究。
已成功证明石墨量热法在质子、α粒子和碳离子束中的应用。2012年将开展其他实验活动。这项工作得到了瓦隆政府BioWin计划的支持。
