虛擬式楔型濾器穿透因子與放射治療距離、照野、深度及監控單位的相關性研究

Virtual Wedge Factors Dependence with SSD, Field Size, Depth, and Monitor Unit

劉幕台(Mu-Tai Liu) ； 林招膨(Jao-Perng Lin) ； 張雅婷(Ya-Ting Chang) ； 張東浩(Tung-Hao Chang)

《The Changhua Journal of Medicine》 6卷2期 (2001/04) Pp. 91-98

摘要

目的：本文目的是探討虛擬式楔型濾器在能量6MV與15MV, 15、30、45、60度四種角度之穿透因子，分別針對不同治療距離(source to surface distance, SSD)、不同照野、不同深度與不同監控單位(monitor unit, Mu)值下觀察穿透因子的變化。
材料與方法：此治療系統為西門子PRIMUS3008直線加速器，使用Wellhofer IC15 (0.13cc)防水游離腔，固定於wellhofer：水假體中，以CU500E 6177電子計讀器接收計讀讀數。分四方面探討穿透因子的影響：探討不同SSD，以SSD90與100公分：不同照野，分2×2、5×5、10×10、20×20、25×25cm^2（或6MV為23×23cm^2，與15MV為24×24cm^2，因受最大照野限制）五種照野；不同深度，分d(subscript max)、5與10公分三種；不同MU值，分50 MU與100 MU兩種；以上分別量測全開照野與四種不同角度（15、30、45、60度）之虛擬式楔型濾器輸出值，並求出其穿透因子。
結果：虛擬式楔型濾器6 MV與15MV之穿透因子對不同SSD的差異性在l%與3%以內；對於大部份照野皆與1.00相差2%以內；與深度（深度小於10公分）無關；除15度角度變化較大外，其餘角度與不同MU值的差異性在2%以內。
結論：雖然虛擬式楔型濾器穿透因子設計為1.00，但實際上並不完全為此數值，故仍須對穿透因子偏差較大的因素作適當的修正，期達到臨床劑量給予的精確。

關鍵字

虛擬式楔型濾器 ； 穿透因子

並列摘要

The aim of this paper is to evaluate Siemens Virtual Wedge Factors on different source-to-surface distances, field sizes, depths, and monitor unit for four wedge angles (150-600) of 6MV and 15MV.
The wedge factors have been determined for Siemen PRIMUS3008 linear accelerators with 6- and 15-MV containing 150-600 virtual wedge system. Wedge factors were measured in Wellhofer water phantom using a 0.13 cc Wellhofer ICl5 ionization chamber and an electrometer (CU500E 6177). Measurements were made with ionization chamber in water with source-skin distance of 90 and 100cm. Field sizes varied from 2×2cm^2 up to maximum allowable size. Measurements were performed at maximum dose, 5- and 10-cm depths. The monitor unit dependence of virtual wedge factor was examined for 50 and 100 MU.
Our measurements reported below show that 1) the discrepancies of 6- and 15-MV wedge factor on SSD 90 and 100cm are 1% and 3%; 2) those are within 2% for most field sizes; 3) those are independent of depth down to 10cm; 4) the discrepancies of those are within 2% for 50 and 100 MU, except virtual wedge 15 degrees of 15MV.
The virtual wedge factor for Siemens virtual wedge is designed to be approximately 1.00, but actually not 1.00. Therefore, in clinical practice, we still have to consider some factors in dose calculations.

並列關鍵字

virtual wedge ； wedge factor