胸腔放射治療中正交仟電子伏特照野影像之新攝影技術開發

Development of a novel imaging technique for orthogonal kilovoltage portal images of thoracic radiotherapy

王士彥(Shih-Yen Wang)

指導教授 ： 董尚倫

中山醫學大學/醫學科技學院/生物醫學科學學系碩士班/碩士(2014年)

https://doi.org/10.6834/CSMU.2014.00208

摘要

前言
仟電子伏特(kV)能量正交照野攝影是現行胸腔影像導引放射治療之中常被用來驗證病患擺位的技術，近年來，評估正交照野攝影的影像品質與攝影劑量已經成為重要的議題。目前胸腔正交照野攝影是以前後位(AP)投射攝影為主，而改用後前位(PA)投射正交照野攝影有助於抑低攝影劑量，因此本研究的目的是發展胸腔放射治療中kV能量正交照野影像之新攝影技術。
材料與方法
本研究使用Varian 21ix直線加速器搭載的OBI系統攝影，使用男性擬人假體模擬臨床病患，並使用壓克力(PMMA)假體模擬胸腔的脊椎，以臨床胸腔曝露參數對上述兩種假體進行AP與PA投射攝影。在影像品質評估方面，擬人假體影像是以脊椎與肺部量測的平均像素值與雜訊值計算對比雜訊比(contrast to noise ratio, CNR)；PMMA假體是先建立與擬人假體脊椎影像有相同平均像素值的等效厚度，將CDRAD假體放置於PMMA的底部以模擬真實脊椎的位置並進行攝影，再以CDRAD analyser程式分析CDRAD影像，以量化影像品質分數(inverse of image quality figure, IQFinv)。在攝影劑量評估方面，本研究以蒙地卡羅軟體PCXMC來計算攝影之器官與有效劑量，最後以攝影劑量、CNR與IQFinv評估AP與PA投射正交照野攝影之差異。
結果
胸腔脊椎的等效厚度是15 cm，比整體胸腔等效厚度厚25%。在影像品質評估方面，AP投射正交照野攝影之影像CNR較PA投射正交照野攝影高11%，當CDRAD放置於14 cm PMMA假體底部攝影時，AP與PA的IQFinv分別是2.19與2.22 (1/mm2)。在劑量評估方面，AP與PA投射正交照野攝影的入射空氣克馬比值是0.87±0.04 (μGy/μGy)，使用PA投射正交照野攝影對於乳房、胸腺與心臟可以減少92、94與78%的器官劑量，肺部與食道的器官劑量則維持不變，且PA投射正交照野攝影的有效劑量可以減少44±4%。
結論
PA投射正交照野攝影可以大幅地減少攝影劑量，同時影像品質只會略微下降，因此PA投射攝影適合用於胸腔影像導引放射治療kV能量正交照野攝影。

關鍵字

影像導引放射治療 ； 正交照野攝影 ； 等效假體 ； 影像品質 ； 攝影劑量

並列摘要

Introduction
The orthogonal kilovoltage (kV) X-ray portal imaging is widely used in image-guided radiation therapy (IGRT) for the patient setup verification. In recent years, assessments of the radiation dose and image quality of kV portal imaging have become increasingly important. In thoracic IGRT, the anterior-posterior (AP) projection kV portal imaging is frequently used. It is helpful for reducing the patient doses by using the posterior-anterior (PA) projection kV portal imaging. The purpose of this study is the development of a novel imaging technique for orthogonal kV portal images of thoracic radiotherapy.
Materials and Method
A linac-based (Varian 21ix) on-board image system was used. To simulate kV portal images of patient, a RANDO phantom was imaged using clinical chest exposure settings. PMMA slabs were used to simulate bone tissues. AP and PA kV portal images were acquired. The mean and standard deviation of pixel values of vertebral body and lung tissue were measured for each image and then the contrast-to-noise ratio (CNR) was calculated. The equivalent thickness of PMMA that would require the same pixel values as the RANDO phantom was estimated. Then, the CDRAD phantom was placed in the bottom of the PMMA slabs to simulate the real position of vertebra. The CDRAD images were analyzed by using the CDRAD analyzer program. The inverse image quality figure (IQFinv) of each CDRAD image was calculated. The PC-based Monte Carlo program (PCXMC) was used for calculating the organ doses and effective dose of each exposure. The doses, CNRs, and IQFinvs of AP and PA kV portal images were compared.

Results
The equivalent thickness of vertebra was 15 cm, and was thicker than that of chest about 25%. For image quality assessments, the CNR of AP projection (bone to lung tissue) was higher than that of PA projection about 11%. When the CDRAD was positioned in the bottom of the 14 cm PMMA slabs, the IQFinv values of AP and PA projection images were 2.19 and 2.22 (1/mm2), respectively. For dosimetry assessments, the ratio of entrance air KERMA between AP and PA projection was 0.87±0.04 (μGy/μGy). Applying PA projection kV portal imaging, the organ doses of breast, thymus and heart decreased about 92, 94, and 78%. The doses of lung and esophagus form AP and PA projection were comparable. The effective doses decreased 44±4% when PA projection were applied.
Conclusion
Using the PA projection kV portal imaging, the doses to RANDO phantom can be significantly reduced and the image quality was slightly decreased. The PA projection kV portal imaging may be suitable for orthogonal kV portal images of thoracic IGRT.

並列關鍵字

IGRT ； kV portal image ； equivalent thickness ； image quality ； imaging dose

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