Characterization of Delivery Systems for Intensity Modulated Radiation Therapy Using a Step-and-Shoot Approach

靜態式強度調控放射治療之輸出系統特性研究

李文星(Moon-Sing Lee) ； 廖婉婷(Wan-Ting Liao) ； 羅素花(Su-Hua Lo) ； 陳鳳鴒(Feng-Ling Chen) ； 王思文(Szu-Wen Wong) ； 夏錫生(Chsia-Hsi Sheng) ； 許文林(Wen-Lin Hsu)

《慈濟醫學雜誌》 16卷6期 (2004/12) Pp. 377-388

https://doi.org/10.6440/TZUCMJ.200412.0377

摘要

目的：在靜態式強度調控放射治療中，許多短時間的小面積照野之組成著對劑量輸出的精密度及準確性造成潛在性的影響。我們研究的目的為：(1)評估應用於強度調控放射治療時，每監測單位輸出劑量的穩定性，射束的平坦度、對稱性及相對輸出因子。(2)了解多葉式準直儀系統幾何及劑量上的特性。材料與方法：我們使用游離腔和電量計測量1到100監測單位射下，每監測單位所輸出的劑量。在正方形照野邊長分別為1、2、5及10公分的情況下，使用pinpoint游離腔測量相對輸出因子。針對射束平坦度、對稱性及多葉式準直儀特性的研究則使用Ｘ光底片測量。量測包括半影寬度的測定、葉片的穿透率、tongue-and-groove效應，照射後的X光底片使用雷射密度計量掃描儀來分析。結果：對於1x1平方公分的照野，2毫米的誤差可造成測量到的輸出變化可超過10%。大於5監測單位的照射，每監測單位所輸出的劑量相對於100監測單位其變化在±2%以內。因為tongue-and-groove效應造成劑量減少的現象在最大劑量深度(31.6%)比在10公分(29.3%)深度更為嚴重。而在最大劑量深度其80-20%半影寬度(5.1-7.8mm)比在10公分深度(7.8-9.6mm)小。結論：我們建議每個小照野最好大於5監測單位，照射才會達到穩定度，而多葉式準直儀特性的研究在臨床應用是必要的。我們發現在這次研究中，本科的加速器性能表現對於靜態式強度調控放射治療技術是相當適合的。

關鍵字

強度調控放射治療 ； 靜態式 ； 多葉式準直儀 ； 監測單位

並列摘要

Objective: In the step-and-shoot intensity modulated radiation therapy (IMRT), the superposition of many small area of distribution of short duration could potentially have a significant impact on the accuracy and precision of dose delivery. The purposes of our study were to evaluate the stability of dose delivery per monitor unit (MU) and the flatness and symmetry of the beams and relative output factor when applied to IMRT, and to realize the geometric and dosimetric characteristics of the multileaf collimator (MLC) system. Materials and Methods: The ion chambers and electrometer were used to measure the dose delivered per MU for 1 to 100 MU exposures. Relative output factors were measured using a 0.015 cm^3 pinpoint ion chamber for a series of square field with side lengths of 1, 2, 5, and 10 cm. The radiographic films were used for measuring the beam flatness, the beam symmetry, and the study of the MLC characteristics. Measurements included a determination of the penumbra width, leaf transmission, and tongue-and- groove effect. Films were analyzed using a scanner laser densitometer. Results: For 1 x 1 cm^2 field, a 2 mm deviation can lead to measured output that vary by as much as 10%. The dose delivered per MU was within ±2% for exposures more than 5 MU relative to a 100 MU exposure. The dose reduction of the tongue-and-groove effect at the depth of dmax (31.6%) was more serious than at the depth of 10 cm (29.3%). Measurements of 80%-20% penumbra widths at the depth of drnax (5.1-7.8 mm) were smaller than those at the 10 cm depth (7.8-9.6 mm). Conclusions: The exposure for each segment is recommended to be more than 5 MU to reach stability, and the characterization of the MLC system is necessary for clinical practice. We found that the performance of our accelerator is well suited to the delivery of step-and-shoot IMRT.

並列關鍵字

intensity-modulated radiation therapy ； step-and-shoot ； multileaf collimator ； monitor unit