本篇研究主要是探討不同照野大小的高能光子射束穿過不同厚度的熱塑性面模(Thermoplastic mask)後，其皮膚表面劑量的變化情形，進而評估治療中可能造成的皮膚反應。本實驗使用直線加速器爲Elekta Synergy，所使用的光子射束能量爲6MV，射束以前後(anterior-posterior，AP)方向垂直入射來量測。量測設備包括平行板游離腔、電量計、及固態水假體。將五種不同厚度但相同材質的熱塑性面模(1.6，2.0，2.2，2.4，3.2mm)擺放於假體表面固定位置，用三種不同照野大小(5×5，10×10，15×15平方公分)來量測假體表面所接受的照射劑量。另外也量測在假體固定深度的位置處，不同厚度的面膜所造成劑量衰減的影響。在相同條件下，1.6~2.4mm厚度的面膜所量測得到的表面劑量隨厚度增加呈現微幅上升且增加幅度不超過21%，但到量測厚度3.2mm面膜時表面劑量即明顯增加，且照野越小，增加幅度越明顯，在5×5平方公分照野下所量測到的表面劑量約爲厚度1.6mm面膜的125%，在15×15平方公分約爲120%。高能光子射束穿過不同厚度的熱塑性面模所產生的劑量衰減皆小於1%，當照野越小，面膜厚度的增加會造成表面劑量的增加幅度越大。

This study is to evaluate the dosimetric changes of skin dose when high-energy photon beam passes through the thermoplastic masks with different mask thickness and field sizes. We used 6MV photon beams emitted from an Elekta Synergy linear accelerator in anterior-posterior direction. The plane-parallel ionization chamber and Standards Grads Solid Water Phantom were used for dose measurement. Thermoplastic masks with different thicknesses of 1.6mm, 2.0mm, 2.2mm, 2.4mm, and 3.2mm were placed on the surface of phantom with a fixed source-surface distance. The surface dose was measured with 3 field sizes: 5×5, 10×10, and 15×15cm^2. We also evaluated dose decay from the thermoplastic masks. The surface dose increased with larger thickness of thermoplastic masks. The increase in surface dose was all less than 21% regardless of field sizes. The surface dose increased significant1y with the mask thickness ≧3.2mm, especially in smaller field sizes. In the 5×5cm^2 field size, the surface dose with a 3.2mm-thickness mask was 125% compared to that with a 1.6mm one. In the 15×15 cm^2 field size, the surface dose was 120%. The radiation decay after passing through thermoplastic masks of thickness from 1.6mm to 3.2mm is less than 1%. In conclusion, the increase of surface dose was noteworthy with a thicker thermoplastic masks and a smaller field size and the dose decay was less than 1% after high-energy photon beam passing through the thermoplastic masks in all different thickness.