多葉式準直儀在強度調控放射治療的臨床運用與特性

Clinical Applications and Dosimetric Characteristics of Multi-leaf Collimator in Intensity Modulated Radiation Therapy

林桂華(Kuei-Hua Lin) ； 黃勝賢(Sheng-Sang Huang) ； 劉幕台(Mu-Tai Liu) ； 林招膨(Jao-Perng Lin) ； 蔡松樹(Song-Su Tsai)

《The Changhua Journal of Medicine》 8卷1期 (2003/01) Pp. 13-20

摘要

背景及目的：針對靜態式強度調控放射治療技術(step-and-shoot IMRT)，探討多葉式準直儀的物理特性及相關品質驗證工作，建立品質保證(quality assurance)的程序，以確保臨床劑量給予之準確性。
方法：在Siemens PRIMUS高能直線加速器下，以CEA底片進行多葉式準直儀的品質保證測試，利用底片數位掃瞄儀(Vidar VXR-12 Film Digitizer)計讀底片，取得多葉式準直儀的物理特性，如再現性、平行對照半遮照野吻合度、葉片的穿透因子和半影區域等。
結果：實際與圖示葉片位置的差異性和平行對照半遮照野吻台度測量結果皆在法規容許範圍內。葉片本身的穿透輻射，在6MV光子能量下最高約2.8%，平均滲漏輻射最大約2.0%,15MV光子下的平均滲漏輻射則約為3.2%，皆在正常的範圍內。在照野10×10平方公分，測量深度為d(下標 max)時，當照野中心點在0cm時，P(下標 90-10)=0.89cm而P(下標 80-20)＝0.34 cm；在+10cm的情況下，P(下標 90-10)＝0.84 cm而P(下標 80-20)＝0.47 cm；在-10 cm時P(下標 90-10)=0.74 cm而P(下標 80-20)＝0.42 cm。
結論：強度調控放射治療整體計畫的完成，涉及了許多分段照野的照射和不同強度的給予，多葉式準直儀的應用扮演極重要的角色，因此多葉式準直儀在臨床的設置及維修保養工作上更相形重要，唯有在良好的品保下，方能真正呈現強度調控放射治療的優越治療效益。

關鍵字

多葉式準直儀 ； 強度調控放射治療 ； 品質保證 ； 葉片穿透因子

並列摘要

Background and purpose: The step-and-shoot method of producing intensity modulation with static multileaf collimator (MLC) segments provides many advantages. To enable use of an MLC in the clinic, sufficient technical procedures must be available to create MLC leaf settings and to deliver an accurate dose of radiation through the MLC-shaped field.
Material and methods: Dosimetric data for clinical use of the MLC were measured on a Siemens PRIMUS accelerator. Dosimetric characteristics included the variation in actual and displayed leaf positions, the parallel opposed half-blocked field matching, the MLC star shot, the leaf transmission factor and the penumbra width. Standard radiographic films were used for measurements. An automatic film digitizer was used to analyze the films.
Results: Variations in actual and displayed leaf positions and in parallel opposed half-blocked field matching showed deviation of less than 1 mm. For 6 MV photons, approximately 2.0% of photons incident on the MLC were transmitted, and an additional 2.8% leakage occurred between leaves. The average leakage between leaves was about 3.2% for 15 MV photons. The end-leaf penumbra values of the 6 MV energy were little changed from the -10cm position to the +10 cm position for d(subscript max) depth.
Conclusion: The most signification impact of the MLC is the ability to increase the number of daily treatment fields, thereby reducing the dose to normal tissue, which is vital for dose escalation. For optimal radiation therapy, efforts should be concentrated on reducing daily setup variations. Through dosimetric analysis, the accuracy of field delivery increases with MLC.

並列關鍵字

Multileaf collimator ； intensity-modulated radiation therapy ； quality assurance ； leaf transmission factor