利用外加延伸式鉛屏蔽塊降低輻射照野外劑量之研究

Study of Extended-Blocks to Decrease the Doses Out Side of Treatment Fields

李燕如(Yen-Ru Li) ； 蕭安成(An-Cheng Shiau1) ； 趙敏(Max M. Chao) ； 周志成(Chih-Cheng Chou) ； 林芳仁(Fang-Jen Lin)

《放射治療與腫瘤學》 10卷3期 (2003/09) Pp. 167-173

https://doi.org/10.6316/TRO/200310(3)167

摘要

目的：本研究目的在利用自行研發的鉛合金結構設計，以一系列實驗來評估其對治療照野外輻射劑量（Peripheral Dose，PD）屏蔽的效果如何，並探討輻射照野外劑量與深度、距照野邊緣距離及能量的關係。
材料與方法：利用不同長度之粗螺絲釘製作長寬分別為12、8 cm，厚度（t）分別為4、7.5及15 ㎝，且托盤與鉛合金塊表面距離（Block Surface-Tray Distance，BSTD）為15、20、25 ㎝之各組鉛合金塊。整個研究使用SIEMENS MEVATRON KDS-2直線加速器產生6和10 MV二種能量的光子射束，並針對等效照野為10 x 10 ㎝2的不對稱照野做探討。利用0.6 c.c. Farmer型式游離腔、Victoreen 500電量計和Acrylic固態假體量測水等效深度為1.5、2.5、5 ㎝，能量為6和10 MV時，各組鉛合金塊對降低PD的效果為何。此外，為評估不同深度及距照野邊緣遠近對PD之影響，則利用NWP水假體系統（Nucletron Water-Phantom System）搭配0.12 c.c. RK游離腔做測量，在6和10 MV時，距照野邊緣1、2、3、4、5及7 ㎝，且深度從0~20 ㎝，架上長寬高分別為12、8、7.5 ㎝，而BSTD＝25 ㎝之鉛合金塊再測量一次，即可得PD經屏蔽後的結果。
結果與討論：經實驗得知鉛合金塊之BSTD＝25 ㎝時其對PD的屏蔽效果最佳，以6 MV X-射線、深度5 cm、照野外距邊緣5 cm的測量結果而言，在無屏蔽的情況下，此照野外劑量與照野幾何中心軸dmax處的劑量之比值為1.74%；加上鉛合金塊後比值降為0.86%。同樣BSTD＝25 ㎝，則以t＝15 ㎝那組鉛合金塊對PD的衰減效果最好，以上述相同的點做測量其PD為0.45%。不論是6或10 MV，當量測位置接近照野邊緣，且位於較表淺的地方，PD漸增，並會隨深度增加逐漸下降到該能量之光子射束其dmax左右達到最小值。之後，隨著深度的增加PD會再次輕微升高；若距離照野邊緣愈來愈遠時，PD就隨深度變化的情形將不明顯；而與照野邊緣距離之關係是隨之增加而逐漸下降。
結論：由實驗結果發現，此種特製鉛合金塊對照野外劑量的降低確實有明顯貢獻，且其有使用簡便、製作容易、成本低之優點，有助於臨床上的使用。但其缺點為僅限用於小範圍之照野，因此，將其改良的更好、應用更廣，是我們日後努力的目標。

關鍵字

治療照野外的輻射劑量 ； 鉛合金塊 ； 不對稱照野

並列摘要

Purpose : The main purpose of this study is to estimate the attenuate effect for peripheral dose (PD) with a special designed block overhung between blocking tray and phantom surface, and find out of PD on depth , distance from field edge, and energy.
Materials and methods : Blocks with different thickness, 4, 7.5 and 15 cm, and equal length and width were made by putting different length of screws into the blocks. The distance from block surface to tray (BSTD) is 10, 15 and 25 cm for the blocks. High energy photon beams of 6 and 10 MV generated by SIEMENS KDS-2 Linac were used in this study. Asymmetric field with equivalent field size of 10 ?10 cm2 was used for all the measurements. Peripheral doses were measured by using a Farmer type chamber and Acrylic solid phantom. Ionization charges were collected by a Victoreen 500 electrometer. PD for different depth, BSTD, and block thickness were evaluated. Nucletron water phantom system with 0.12 c.c. R K chamber was used for measuring the variations of PD with distances from field edge and depth. PD with and without block were measured also.
Result and Discussion : We find that block with BSTD equal to 25 cm decrease the PD most. When distance from field edge is 5 cm and depth at 5 cm, the PD relative to Dmax (dose at dmax) at central axis is 1.74% without block, and is 0.86% with block for 6 MV. The block with thickness of 15 cm has the best attenuation effect, and reduce the PD to 0.45% relative to Dmax . The PD decreases as the distance from field edge increases. Close to the field edge, the PD drops initially with depth, the minimum apperance at the depth that deeper than the dmax (depth of maximum dose) of the central ray 0.5 cm. Behind the dmax, the PD increases gradually with depth and then become nearly constant farther. The qualitative behavior of the PD is similar for 6 and 10 MV.
Conclussion : From the results, we know that the blocks have obvious contribution for reducing the PD, and that advantage is used conviently, made easily, low cost, but shortcoming is just suitable for small treatment field. To improve the equipment for better is our object in the future.

並列關鍵字

Peripheral dose ； Block ； Asymmetry field