Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, MA 02114, USA.
Phys Med Biol. 2010 Dec 21;55(24):7557-71. doi: 10.1088/0031-9155/55/24/011. Epub 2010 Nov 19.
The advantages of a finite range of proton beams can only be partly exploited in radiation therapy unless the range can be predicted in patient anatomy with <2 mm accuracy (for non-moving targets). Monte Carlo dose calculation aims at 1-2 mm accuracy in dose prediction, and proton-induced PET imaging aims at ∼2 mm accuracy in range verification. The latter is done using Monte Carlo predicted PET images. Monte Carlo methods are based on CT images to describe patient anatomy. The dose calculation algorithm and the CT resolution/artifacts might affect dose calculation accuracy. Additionally, when using Monte Carlo for PET range verification, the biological decay model and the cross sections for positron emitter production affect predicted PET images. The goal of this work is to study the effect of uncertainties in the CT conversion on the proton beam range predicted by Monte Carlo dose calculations and proton-induced PET signals. Conversion schemes to assign density and elemental composition based on a CT image of the patient define a unique Hounsfield unit (HU) to tissue parameters relationship. Uncertainties are introduced because there is no unique relationship between HU and tissue parameters. In this work, different conversion schemes based on a stoichiometric calibration method as well as different numbers of tissue bins were considered in three head and neck patients. For Monte Carlo dose calculation, the results show close to zero (<0.5 mm) differences in range using different conversion schemes. Further, a reduction of the number of bins used to define individual tissues down to 13 did not affect the accuracy. In the case of simulated PET images we found a more pronounced sensitivity on the CT conversion scheme with a mean fall-off position variation of about 1 mm. We conclude that proton dose distributions based on Monte Carlo calculation are only slightly affected by the uncertainty on density and elemental composition introduced by unique assignment to each HU if a stoichiometric calibration is used. Calculated PET images used for range verification are more sensitive to conversion uncertainties causing an intrinsic limitation due to CT conversion alone of at least 1 mm.
质子射束有限射程的优势只有在将射程能以 <2 毫米的精度（对于非移动目标）预测到患者解剖结构中时，才能在放射治疗中得到部分利用。蒙特卡罗剂量计算旨在以 1-2 毫米的精度预测剂量，而质子诱发的 PET 成像则旨在以大约 2 毫米的精度验证射程。后者是使用蒙特卡罗预测的 PET 图像完成的。蒙特卡罗方法基于 CT 图像来描述患者解剖结构。剂量计算算法和 CT 分辨率/伪影可能会影响剂量计算的准确性。此外，在使用蒙特卡罗进行 PET 射程验证时，生物衰减模型和正电子发射体产生的横截面会影响预测的 PET 图像。本工作的目的是研究 CT 转换中的不确定性对蒙特卡罗剂量计算和质子诱发的 PET 信号预测的质子束射程的影响。基于患者 CT 图像分配密度和元素组成的转换方案为组织参数定义了唯一的亨氏单位 (HU) 到组织参数的关系。由于 HU 与组织参数之间没有唯一的关系，因此会引入不确定性。在这项工作中，在三个头颈部患者中考虑了基于化学计量校准方法的不同转换方案以及不同数量的组织-bin。对于蒙特卡罗剂量计算，结果表明使用不同转换方案时，射程差异接近零 (<0.5 毫米)。此外，将用于定义单个组织的-bin 数量减少到 13 以下不会影响准确性。在模拟的 PET 图像的情况下，我们发现对 CT 转换方案的敏感性更大，平均衰减位置变化约为 1 毫米。我们得出结论，如果使用化学计量校准，蒙特卡罗计算得出的质子剂量分布仅受将每个 HU 唯一分配给密度和元素组成所引入的不确定性的轻微影响。用于验证射程的计算 PET 图像对转换不确定性更为敏感，由于 CT 转换本身至少存在 1 毫米的固有限制，因此会导致其固有限制。
