Akagi T, Kanematsu N, Takatani Y, Sakamoto H, Hishikawa Y, Abe M
Department of Accelerator Managing, Hyogo Ion Beam Medical Center, 1-2-1 Kouto, Shingu-cho, Tatsuno City, Hyogo, Japan.
Phys Med Biol. 2006 Apr 7;51(7):1919-28. doi: 10.1088/0031-9155/51/7/020. Epub 2006 Mar 21.
In the absence of a predictor of beam output in proton therapy using a broad beam, the beam output is obtained for individual treatments by calibrating the beam monitors. The calibration is carried out under conditions similar to the treatment conditions but with a phantom instead of the patient. However, the dose in the phantom a priori differs from that in the patient. In order to deliver the accurate dose, a correction factor has been introduced to correct the difference. This correction factor is referred to as a scatter factor in an analogy with photon therapy, and is defined as the ratio of the dose at the prescription point in the patient to the dose at the calibration point in the phantom. Under the calibration conditions at Hyogo Ion Beam Medical Center (HIBMC), the range compensator and the collimator, which are usually required in proton therapy with a broad beam, are not used. Therefore the scatter factor includes the effects of the devices as well as the difference between the dose in the patient and that in the phantom. We have developed an estimator using a dose calculation based on the pencil beam algorithm and implemented it in a treatment planning system (TPS) for clinical use. This estimator estimates the scatter factor by calculating the ratio of the doses under the same conditions in the TPS. In order to evaluate the performance of the estimator, demonstrations were carried out for cases with measurable outcomes using a gantry nozzle at HIBMC. We observed 2-3% differences between the measurements and the estimations. These differences were considered to result from the limitations of the dose calculation algorithm in modelling the beam and the patient.
在使用宽束质子治疗时，如果没有束流输出预测器，那么通过校准束流监测器来获得每次治疗的束流输出。校准是在与治疗条件相似的情况下进行的，但使用模体代替患者。然而，模体中的剂量与患者体内的剂量先验不同。为了给予准确的剂量，引入了一个校正因子来校正这种差异。这个校正因子类似于光子治疗中的散射因子，定义为患者处方点处的剂量与模体校准点处剂量的比值。在兵库离子束医学中心（HIBMC）的校准条件下，宽束质子治疗中通常需要的射程补偿器和准直器并不使用。因此，散射因子既包含了这些装置的影响，也包含了患者体内剂量与模体中剂量的差异。我们基于笔形束算法开发了一种剂量计算估计器，并将其应用于临床治疗计划系统（TPS）中。该估计器通过计算TPS中相同条件下的剂量比值来估计散射因子。为了评估该估计器的性能，在HIBMC使用龙门喷嘴对具有可测量结果的病例进行了演示。我们观察到测量值与估计值之间存在2 - 3%的差异。这些差异被认为是由于剂量计算算法在模拟束流和患者方面存在局限性所致。
