建立質子治療的電腦斷層校正曲線以及分析比較不同掃描參數對質子治療的影響

Build-up of CT Calibration Curve and Comprehensive Analysis of CT Parameters for Proton Therapy

蔡宗孝(Tsung-Shiau Tsai) ； 吳東和(Tung-Ho Wu) ； 吳卉蓁(Hui-Chen Wu)

《中華放射線技術學雜誌》 45卷2期 (2021/06) Pp. 82-91

摘要

電腦斷層影像除了能夠提供準確的解剖影像之外，也能夠藉由Hounsfield unit （HU）提供給治療計畫不同物質的衰減係數。質子治療具有布拉格尖峰（Bragg peak）的特性，能夠在質子的射程末端具有陡峭的劑量梯度。為了要精準計算質子射束的劑量，質子射束所經過的各個物質的阻擋本領為必要的資訊，治療計畫系統的演算法能夠將電腦斷層所提供的某一物質的HU值轉換成在水中的相對阻擋本領（relative linear stopping power），因此需要建立精確的電腦斷層HU值相對應的相對阻擋本領。質子治療的射程不確定性（range uncertainty）的主要原因是來自於HU值轉換成相對阻擋本領的過程，另外包含每日機器穩定度、病人個體差異、定位器具的影響、校正曲線等，總誤差約在3.5 %以內。電腦斷層的掃描條件也很重要，不同的掃描條件如：FOV、掃描能量、掃描程序以及不同的假體體積都會對HU值以及相對阻擋本領都有影響。掃描能量的範圍如果是使用較低的能量如80 kVp或90 kVp時，在高原子序的物質中會造成過多的光電效應影響，因此我們選擇120 kVp作為掃描的能量，而管電流會使用300 mAs、切片厚度（slice thickness）為1.25 mm、螺距（pitch）為1.375、視野（FOV）為50 cm而使用的掃描假體為身體假體（body phantom）。在保持標準的照射條件設定下，能夠將不確定性降到最低，我們也發現電腦斷層掃描的能量是影響質子計算射程最大的因素，80 KVp與120 KVp相比可能造成在每公分骨骼有2.5 mm的差異，140 KVp與120 KVp相比可能造成在每公分骨骼有0.68 mm的差異，在軟組織中不同能量則無明顯差異，其他參數也在不同組織中有微小差異。

關鍵字

質子治療 ； 電腦斷層校正曲線 ； 相對阻擋本領比值 ； 射程不確定性

並列摘要

Computed tomography (CT) not only can provide accurate anatomical images, but also can provide attenuate coefficients for different material in treatment plan system by the Hounsfield unit (HU). Proton therapy has the characteristics of Bragg peak that is steep dose gradient at the end of proton range. The stopping power of different materials through where the proton beam passes is need to be determined to accurately calculate proton dose distribution. The algorithm of treatment planning system can convert HU value of material into relative linear stopping power of water. Therefore, it is very important to establish accurate relationship between CT HU value and relative stopping power. The range uncertainty of proton therapy is mainly come from the process of converting the HU value into relative stopping power. In addition, the total error could be controlled within 3.5%, including daily machine stability, individual patient difference, the influence of immobilization device, calibration curve, etc. The scanning parameters of CT are also important. Different scanning parameters such as FOV, scanning energy, scanning protocol and different phantom volume will have impact on the HU value and relative stopping power. If the range of scanning energy is lower, such as 80kVp or 90kVp, it will cause too much photoelectric effect in high atomic number materials. Therefore, we choose 120kVp as the scanning energy. Besides, the tube current is 300 mAs, slice thickness is 1.25 mm, pitch ratio is 1.375, field of view (FOV) is 50 cm and using body phantom as standard scanning phantom. If we follow the standard scanning parameters, the uncertainty in CT scanner can be minimized. We also found that the CT scanning energy is the biggest impact factor affecting the proton range calculation. Comparing to the standard scanning energy 120 kVp, 80 kVp may cause a difference of 2.5mm per cm in bone, 140 kVp may cause a difference of 0.68mm per cm in bone. There is no obvious difference in different energy in soft tissue. And other parameter also have slight difference in different tissues.

並列關鍵字

Proton therapy ； CT calibration curve ； Relative stopping power ratio ； Range uncertainty