Siebers J V, Keall P J, Nahum A E, Mohan R
Department of Radiation Oncology, Medical College of Virginia Hospitals, Virginia Commonwealth University, Richmond, USA.
Phys Med Biol. 2000 Apr;45(4):983-95. doi: 10.1088/0031-9155/45/4/313.
Current clinical experience in radiation therapy is based upon dose computations that report the absorbed dose to water, even though the patient is not made of water but of many different types of tissue. While Monte Carlo dose calculation algorithms have the potential for higher dose accuracy, they usually transport particles in and compute the absorbed dose to the patient media such as soft tissue, lung or bone. Therefore, for dose calculation algorithm comparisons, or to report dose to water or tissue contained within a bone matrix for example, a method to convert dose to the medium to dose to water is required. This conversion has been developed here by applying Bragg-Gray cavity theory. The dose ratio for 6 and 18 MV photon beams was determined by computing the average stopping power ratio for the primary electron spectrum in the transport media. For soft tissue, the difference between dose to medium and dose to water is approximately 1.0%, while for cortical bone the dose difference exceeds 10%. The variation in the dose ratio as a function of depth and position in the field indicates that for photon beams a single correction factor can be used for each particular material throughout the field for a given photon beam energy. The only exception to this would be for the clinically non-relevant dose to air. Pre-computed energy spectra for 60Co to 24 MV are used to compute the dose ratios for these photon beams and to determine an effective energy for evaluation of the dose ratio.
当前放射治疗的临床经验是基于将吸收剂量报告给水体的剂量计算，尽管患者并非由水构成，而是由多种不同类型的组织组成。虽然蒙特卡罗剂量计算算法有提高剂量准确性的潜力，但它们通常在患者介质（如软组织、肺或骨）中传输粒子并计算吸收剂量。因此，为了比较剂量计算算法，或者例如报告骨基质内的水体或组织的剂量，需要一种将介质剂量转换为水体剂量的方法。这里通过应用布拉格 - 格雷腔理论开发了这种转换方法。通过计算传输介质中初级电子能谱的平均阻止本领比来确定6和18兆伏光子束的剂量比。对于软组织，介质剂量与水体剂量之间的差异约为1.0%，而对于皮质骨，剂量差异超过10%。剂量比随深度和射野内位置的变化表明，对于光子束，在给定光子束能量下，整个射野内每种特定材料都可以使用单个校正因子。唯一的例外是临床上与空气无关的剂量。使用60钴至24兆伏的预计算能谱来计算这些光子束的剂量比，并确定用于评估剂量比的有效能量。
