直線加速器多葉式準直儀的位置點於放射治療計劃系統中有特定的定義點，此點跟實際照野位置點的相對關係是本篇探討和研究的主題。本篇探討的方法是讓多葉式準直儀開出某一照野之後，再按燈光照野和物理照野的定義找出劑量50%的位置，由此時的50%的位置點可藉由數學模型所提供的關係式反求出此時的照野精準數位位置，再由此照野精準數位位置算出治療計劃內所認可的多葉式準直儀的位置(Xmlc.p)，此相對關係可讓治療計劃系統掌握多葉式準直儀實際開出的照野位置，以便治療計劃系統做出正確的劑量計算。在光子能量6 MV及10 MV及MLC 在位置點於-8 cm 到+8 cm 的範圍內，其MLC 位置的修正量為正值；而在-12 cm 到-8 cm，及+8 cm 到+20cm 時MLC 位置的修正量為負值。如果根據廠商提供的修正量做為劑量計算的結果，則實測劑量和預期劑量會存在不可忽略的差異；如果有經過實驗量測後再做MLC 位置修正後可讓劑量計算差異小於0.5%

We present an analytical and experimental study for dose calculation correction by adjusting offset of round leaf end position in treatment planning system at different Source-Surface-Distance (SSD) in Intensity Modulated Radiation Therapy. Precise light field edge position (Xtang.p ) was derivated from 50% dose of central axis dose created by nominal light field with geometry and mathematic method. Leaf position (Xmlc.p) defined in treatment planning system for monitor unit calculation could be derivated from Xtang.p. Offset (correction) could be obtained by 50% dose of central axis dose position minus Xmlc.p position. For SSD from 90 cm to 120 cm of 6 MV and 10 MV, the dose 50% position is located at the outside of Xmlc,p in MLC leaf position range of +8 cm to - 8 cm, the offset correction is positively increased, whereas the offset correction is negatively increased as MLC leaf position of the ranges of -12 cm to - 8 cm and +20 cm to + 8 cm for 50 % position is located inside of Xmlc,p. The monitor unit calculation could come up under-dosage or over-dosage of 7.5 % per mm error offset correction. With careful measurement and an analytic offset correction, it is possible to archive dose calculation within 0.5%error for the adjusted MLC leaf edge location in treatment planning system.