Sakama Makoto, Kanai Tatsuaki, Fukumura Akifumi, Abe Kyoko
Research Center for Charged Particle Therapy, National Institute of Radiological Sciences, 4-9-1, Anagawa, Inage-ku, Chiba 263-8555, Japan.
Phys Med Biol. 2009 Mar 7;54(5):1111-30. doi: 10.1088/0031-9155/54/5/002. Epub 2009 Jan 27.
Despite recent progress in carbon therapy, accurate values for physical data such as the w value in air or stopping power ratios for ionization chamber dosimetry have not been obtained. The absorbed dose to graphite obtained with the graphite calorimeter was compared with that obtained using the ionization chambers following the IAEA protocol in order to evaluate the w values in air for mono-energetic carbon beams of 135, 290, 400 and 430 MeV/n. Two cylindrical chambers (PTW type 30001 and PTW type 30011, Farmer) and two plane-parallel chambers (PTW type 23343, Markus and PTW type 34001, Roos) calibrated by the absorbed dose to graphite and exposure to the (60)Co photon beam were used. The comparisons to our calorimeter measurements revealed that, using the ionization chambers, the absorbed dose to graphite comes out low by 2-6% in this experimental energy range and with these chamber types and calibration methods. In the therapeutic energy range, the w values in air for carbon beams indicated a slight energy dependence; we, however, assumed these values to be constant for practical use because of the large uncertainty and unknown perturbation factors of the ionization chambers. The w values in air of the carbon beams were evaluated to be 35.72 J C(-1) +/- 1.5% in the energy range used in this study. This value is 3.5% larger than that recommended by the IAEA TRS 398 for heavy-ion beams. Using this evaluated result, the absorbed dose to water in the carbon beams would be increased by the same amount.
尽管碳疗法最近取得了进展，但尚未获得诸如空气中的w值或电离室剂量测定的阻止本领比等物理数据的准确值。为了评估135、290、400和430 MeV/n的单能碳束在空气中的w值，将用石墨量热计获得的石墨吸收剂量与按照国际原子能机构协议使用电离室获得的吸收剂量进行了比较。使用了两个经石墨吸收剂量和（60）Co光子束照射校准的圆柱形电离室（PTW 30001型和PTW 30011型，Farmer型）以及两个平行板电离室（PTW 23343型，Markus型和PTW 34001型，Roos型）。与我们量热计测量结果的比较表明，在此实验能量范围内，使用这些类型的电离室和校准方法，石墨的吸收剂量低了2%-6%。在治疗能量范围内，碳束在空气中的w值显示出轻微的能量依赖性；然而，由于电离室的不确定性大且扰动因素未知，我们在实际应用中假定这些值是恒定的。在本研究使用的能量范围内，碳束在空气中的w值评估为35.72 J C(-1) +/- 1.5%。该值比国际原子能机构TRS 398为重离子束推荐的值大3.5%。使用该评估结果，碳束中对水的吸收剂量将增加相同的量。
