目的：本研究以CERROBEND 鉛合金，針對CL-2100CD直線加速器開壁式各限光筒尺寸， 製成邊長以 1cm 為級數變化之正方形庶擋塊，測量在射源至皮膚表面為 100 cm 時，各縮小照野之百分深度劑量曲線，以觀察並探討縮小照野面積的百分深度劑量曲線變化之情形及臨床應注意的事項。 材料與方法：以 CERROBEND 鉛合金，依各限光筒尺寸製作邊長以1cm 為級數之正方形中空遮擋塊，依次置於 Varian CL-2100CD 直線加速器電子限光筒底部， 以 Nucletron 三度空間水假體連接 Scanditronix p 型半導體偵測器，測量各擋塊照野電子射束能量分別為 6,9,12,16.20 MeV 射束之百分深度劑量曲線，觀察各能量各照野之最高劑量點 (D)，90%，80% 劑量點之深度，及電子束射程 (Rp) 之變化情形。 結果：最高劑量點 (D)，90%，80% 劑量點深度之變化情形為：當照野面積縮小到 1 × 1 cm ?揹氶A所有能量射束之最高劑量點皆位於 0.5 ～ 0.8cm 之間，不論限光筒大小為何，90% 劑量點深度的變化情形為：6MeV 從照野 3 × 3cm ?插AMeV 從 1.5 × 4.5cm ?插A12MeV 從 6 × 6cm?插A16MeV 從 8 × 8 cm ?插A20MeV 則從 10.5 ×10.5 cm ?揖H下開始降低。 結論：臨床上以不規則形 CERROBEND 鉛合金執行小照野電子射束治療時，其最高劑量點， 90%， 80% 劑量點之深度將會變小，因此，臨床上在決定射束能量時，若照野面積小於各能量、各劑量的降低點時，應先注意治療擋塊之大小，查得其 90%，80% 劑量點深度，再決定能量。

　　　　　Purpose:The percentage depth dose changes of high energy electron beamsgenerated from Varian CL2100C/D linear accelerator collerated with various sizesof cutout blocks were measured at 100cm source-to-sruface distance. The clinicalsignificance was discussed. Materials and Methods:CERROBEND cutout blocks weremade for open-wall MLC type applicators. The step size of the square cutout is 1cm. The percentage depth dose curve were scanned with Nucletron 3-D waterphantom system connected with Scanditronix p-type semiconductor chamber. Theelectron energies were ranged from 6,9,12,16 and 20 MeV. The depths of D, D, D?? and R ?d were measured to observed its variation with cutout size.Results:The depths of D were located at 0.5 to 0.8 cm at the cutout size equalto 1 × 1 cm ?? for all energies. The decrement points of the four parametersfor each energy were the same in all cone sizes. The decrement cutout sizes were3 × 3 cm ?? for 6 MeV, 4.5 × 4.5 cm ?? for 9 MeV, 6 × 6 cm ?? for 12 MeV, 8× 8 cm ?? for 16 MeV and 10.5 × 10.5 cm ?? for 20 MeV. Conclusions:The depthof D, 90% and 80% depth dose will shift to surface direction when the irradiatearea was decreased. It is mentioned clinically that the 90% depth should beexamined before energy selection for tumor treatment.