Acoustics and Ionising Radiation Division, National Physical Laboratory, TW11 0LW Teddington, UK.
Phys Med Biol. 2013 May 21;58(10):3481-99. doi: 10.1088/0031-9155/58/10/3481. Epub 2013 Apr 30.
The conversion of absorbed dose-to-graphite in a graphite phantom to absorbed dose-to-water in a water phantom is performed by water to graphite stopping power ratios. If, however, the charged particle fluence is not equal at equivalent depths in graphite and water, a fluence correction factor, kfl, is required as well. This is particularly relevant to the derivation of absorbed dose-to-water, the quantity of interest in radiotherapy, from a measurement of absorbed dose-to-graphite obtained with a graphite calorimeter. In this work, fluence correction factors for the conversion from dose-to-graphite in a graphite phantom to dose-to-water in a water phantom for 60 MeV mono-energetic protons were calculated using an analytical model and five different Monte Carlo codes (Geant4, FLUKA, MCNPX, SHIELD-HIT and McPTRAN.MEDIA). In general the fluence correction factors are found to be close to unity and the analytical and Monte Carlo codes give consistent values when considering the differences in secondary particle transport. When considering only protons the fluence correction factors are unity at the surface and increase with depth by 0.5% to 1.5% depending on the code. When the fluence of all charged particles is considered, the fluence correction factor is about 0.5% lower than unity at shallow depths predominantly due to the contributions from alpha particles and increases to values above unity near the Bragg peak. Fluence correction factors directly derived from the fluence distributions differential in energy at equivalent depths in water and graphite can be described by kfl = 0.9964 + 0.0024·zw-eq with a relative standard uncertainty of 0.2%. Fluence correction factors derived from a ratio of calculated doses at equivalent depths in water and graphite can be described by kfl = 0.9947 + 0.0024·zw-eq with a relative standard uncertainty of 0.3%. These results are of direct relevance to graphite calorimetry in low-energy protons but given that the fluence correction factor is almost solely influenced by non-elastic nuclear interactions the results are also relevant for plastic phantoms that consist of carbon, oxygen and hydrogen atoms as well as for soft tissues.
将石墨模体中的吸收剂量转换为水模体中的吸收剂量是通过水与石墨的阻止本领比来实现的。然而，如果在石墨和水中的等效深度处带电粒子注量不相等，则还需要注量修正因子 kfl。这在从石墨量热计测量的石墨中吸收剂量转换为水模体中的吸收剂量，即放射治疗中感兴趣的量时尤其相关。在这项工作中，使用解析模型和五种不同的蒙特卡罗代码（Geant4、FLUKA、MCNPX、SHIELD-HIT 和 McPTRAN.MEDIA）计算了从石墨模体中的剂量转换为水模体中的剂量的 60 MeV 单能质子的注量修正因子。一般来说，注量修正因子接近 1，并且当考虑二次粒子输运的差异时，解析模型和蒙特卡罗代码给出了一致的值。仅考虑质子时，在表面处注量修正因子为 1，随着深度的增加，修正因子增加 0.5%到 1.5%，具体取决于代码。当考虑所有带电粒子的注量时，由于来自 α 粒子的贡献，在浅层深度处注量修正因子比 1 低约 0.5%，并在布喇格峰附近增加到高于 1 的值。可以通过 kfl = 0.9964 + 0.0024·zw-eq 来描述直接从水和石墨中等效深度处的能量微分注量分布中导出的注量修正因子，其中相对标准不确定度为 0.2%。可以通过 kfl = 0.9947 + 0.0024·zw-eq 来描述从水和石墨中等效深度处的计算剂量比中导出的注量修正因子，其中相对标准不确定度为 0.3%。这些结果直接与低能质子中的石墨量热计有关，但由于注量修正因子主要受非弹性核相互作用的影响，因此这些结果也与包含碳、氧和氢原子以及软组织的塑料体模有关。
