電子射束有效SSD位置量測與劑量計算之研究

直線加速器所產生的射束，並無實際射源點，和遠隔治療機鈷－60相比，因鈷－60射源位置固定，射源至表面的位置(SSD)亦爲確定（一般爲SSD=65、85或100公分），其輻射劑量輸出可由標準議定書求得。依文獻[1]指出，在環境及輻射微劑量可忽略下，可依距離反平方定律求得其臨床上需求的延長SSD之劑量輸出；而直線加速器的電子射束，實際上並無射源點所在，其有效的SSD(effective source surface distance)也非定義上真正之射源點，若引伸爲射源的點，作爲臨床上延長SSD之反平方定律計算，其反平方定律的結果更關係臨床劑量計算應用需修正項。本研究計劃在量測探討埔里基督教醫院最新直線加速器(Siemens oncor)電子射束，將管套游離腔置於水假體中不同深度或不同的隙距(gap)3公分，並給予100單位輸出(MU)劑量，並量測其結果。以距離平方反比的方式回推，求得其VSP(Virtual Source Position)，也就是有效的SSD。電子射束不同能量之VSP(Virtual Source Position)，藉由各種不同方法，分析探討其差異性，藉由結果量測反平方定律之存在性並嘗試求得另一符合本治療機電子射束之數學修正式。

關鍵字

電子射束；有效表面射源距離；虛擬射源位置；反平方定律

並列摘要

The beams that are generated by linear accelerator have no actual radioactive source. Under the circumstances where environment and radiation dosage are negligible, clinically required extended SSD dosage output can be obtained by the Inverse Square Law. The linear accelerator beam has no actual radioactive source and the effective source surface distance is not the real source either. If we regard it as the point of source and calculate by the inverse square law of clinically extended SSD, the result of inverse square law will be more correlative with the clinical dosimetry application which needs correction. This study aims to measure and study the electron beam of a new linear accelerator (Siemens Impression oncor M5512) in Puli Christian hospital. Several ion chambers are placed in a water phantom in different depths or gaps (3 centimeters), and are supplied with 100 unit output (MU) dosage and the result is measured. Then, we adopt the inverse square law to deduce the Virtual Source Position (VSP), which is effective SSD. We analyze the difference by which we assess the existence of Inverse square law and try to seek another mathematically corrected formula.

並列關鍵字

Electron beam ； Effective SSD ； Inverse square law ； Virtual source position

國際替代計量

電子射束有效SSD位置量測與劑量計算之研究

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