當病人做放射治療時，治療室內的相對濕度所代表的是空氣中”含水氣的量”和”飽和蒸氣壓”的比值，而相對濕度愈大，表示空氣中的含水量愈多；而高能光子射束在進入病患前會先經過空氣，愈多的含水量和高能光子射束碰撞會產生更多的散射電子，這些散射電子是導致病人表皮劑量增加的主因；所以本篇最主要的目的是探討治療室內相對濕度的大小對放射治療病人表皮劑量影響的研究。光子和空氣的碰撞數學模式主要是架構在Fermi-Eyges 的數學理論（Rossi and Greisen 1941, Eyges 1948）上；而相對濕度的改變如何影響光子和濕空氣的碰撞，所用到的數學模式是Goff-Gratch equation，由此式可以算出治療室當時溫度下的絕對飽和蒸氣壓，再把此飽和蒸氣壓乘上相對濕度就可算出在該溫度下的水蒸氣壓，有此水蒸氣壓就可算出當時光子射束所通過的空氣柱所碰撞出來的散射電子，進而估算當治療室內濕空氣的含量比乾空氣的含量的多寡，究竟會造成多少病患表皮劑量的變化量；此變化量再經由實驗量測驗證，以便探討數學模式推算結果的準確性，進而推展到臨床上的應用。

When a photon beam impinges onto air, Compton electron and electron-positron pairs will be expelled in the aircolumn between the source and the patient (phantom). The increasing surface dose emanating from relative high moisturehas been studied by numerical calculations and measurement in this report. The results were used to predict the causes ofserve skin reaction of relative high humidity in treatment room during treatment. By using Goff Gratch equation thesaturation water vapor pressure at a given treatment room temperature and pressure was calculated, then multiple by theroom relative moisture to derive the absolute moisture for calculating the linear scattering power in the air column. Thecross-beam dose profile of the build up region due to moisture induced scattering electron was calculated according to theFermi-Eyges theory. Measurements were performed with Elekta Precise medical linear accelerator at photon energy of 4,6 and 10 MV. Field sizes ranging from 10 cm x10cm to 40 cm x 40 cm, relative moisture from 40% to 70% wasinvestigated. EBT2 Gaf Chromic films were embedded in solid water phantom for the measurement of surface dose withand without moisture influence during megavoltage photon irradiation. The film calibration was carefully done in thebuild up region dose. The absorbed dose has been normalized to the absorbed dose from primary photons under conditionsof electronic equilibrium Dmax. Calculated and measurement absorbed dose profile of multiple scattering electronemanating from photon interaction with wet air for different relative moisture from 40% to 70% of the air column at a fixfield size are coincidence showing up to 10% difference in surface dose. The calculations indicated that the secondaryelectron from humidity air can explain a large part of the scattering electrons and may contribute significantly to thesurface to cause serve skin reaction.