肺癌患者進行立體定位放射治療時，因單次高劑量模式使患者需要良好的固定模具，然而治療床與固定模具都將造成患者表面劑量增加。因此，本研究分析治療床及固定模具對患者表面劑量之影響。研究使用平行板游離腔，固定射源到表面距離為100公分，改變照野大小：3×3 cm^2、5×5 cm^2、8×8 cm^2、10×10 cm^2及15×15 cm^2，分析不同能量：6 MV-FF、10 MV-FF、6 MV-FFF及10 MV-FFF，在有無治療床板與真空固定墊分別為5 mm、15 mm、30 mm、39 mm及55 mm厚度情況下，表面劑量的差異。研究結果顯示，6 MV-FF、10 MV-FF、6 MV-FFF及10 MV-FFF情況下，治療床平均增加表面劑量3.6、4.6、2.9、及3.7倍。5 mm、15 mm、30 mm、39 mm及55 mm厚度之固定模具則平均增加3.93、4.11、4.23、4.16及4.24倍之表面劑量。因此，肺癌患者執行立體定位放射治療時，在良好固定的前提下，應減少固定模具的厚度以降低表面劑量的增加。

As lung cancer patients treat by Stereotactic Body Radiation Therapy, they need better immobilization due to the high fraction size. However, the couch and vacuum bag will increase the surface dose in treatment. Therefore, this study investigates the influence of surface dose due to couches and vacuum bags. To investigate the difference of surface dose in 6 MV-FF, 10 MV-FF, 6 MV-FFF, and 10 MV-FFF with and without a couch and different thicknesses of vacuum bag: 5 mm, 15 mm, 30 mm, 39 mm, and 55 mm. This study take parallel ion-chamber into solid water phantom, sets SSD to 100 cm, changes field size to 3×3 cm^2, 5×5 cm^2, 8×8 cm^2, 10×10 cm^2 and 15×15 cm^2. The results show that the average increase for surface dose is 3.6, 4.6, 2.9, and 3.7-fold in 6 MV-FF, 10 MV-FF, 6 MV-FFF, and 10 MV-FFF, respectively. In addition, the average surface dose increased by 3.93, 4.11, 4.23, 4.16, and 4.24-fold respectively in 5 mm, 15 mm, 30 mm, 39 mm, and 55 mm thickness of vacuum bags. Therefore, when lung cancer patients are treated by Stereotactic Body Radiation Therapy, it’s better to reduce the thickness of the vacuum bag to decrease the surface dose under the premise of good immobilization. Using a couch and vacuum significantly increases the surface dose. For SBRT with a superficial target close to the couch and immobilization vacuum, careful attention to the impact of vacuum on skin dose in planning would be helpful in avoiding severe skin toxicity.