本研究旨在探討NIPAM凝膠劑量計搭配臨床使用的Henschke裝療器，應用於近接治療劑量驗證之特性，以提高凝膠劑量計在臨床近接治療劑量驗證的實用性。
目前臨床執行的近接治療(Brachytherapy)計畫尚未將裝療器(Applicator)納入計算並考慮材質造成的劑量擾動，故此研究在填充有NIPAM (N-isopropylacrylamide)凝膠劑量計之假體(Phantom)中置入臨床用於治療子宮頸癌之Henschke裝療器，以放射核種銥(Ir-192)植入照射多射源位置之子宮頸癌治療計畫，透過自行研製之平行射束光學電腦斷層掃描儀(Parallel-beam optical-CT scanner )擷取凝膠劑量計所測得之三維劑量分布資料，並利用伽瑪評估法(Gamma evaluation)以3%劑量差異(Dose difference, DD)及3mm劑量吻合距離(Distance to agreement, DTA)之門檻，將此劑量計之量測值與治療計畫系統(Treatment planning system, TPS)模擬之估計值進行劑量驗證與比較。
研究結果顯示，假體中用以包覆裝療器的中心圓柱會導致光學電腦斷層掃描儀在收取凝膠劑量計影像時產生折射與散射，重組分析後的計算結果因此產生雜訊，進而導致劑量分布受到影響，且距離中心圓柱愈近之凝膠劑量計因介質不同使聚合作用受影響，故在伽瑪分析中不通過的比例較高。另外由等劑量曲線圖中可發現，相較於TPS的估計值，凝膠劑量計些微低估了劑量，可能原因是裝療器為不鏽鋼材質，其作為射源進入假體之管道，在進行輻射照射時衰減了輻射，使凝膠劑量計並未接受到原先所設定之劑量，故所測得的劑量與TPS不吻合。即便凝膠劑量計與治療計畫之間在特定區域有些許差異，整體而言伽瑪通過率大部分達95%以上。此研究亦針對NIPAM凝膠劑量計進行穩定性之探討，結果表明經由輻射照射後96小時內，此劑量計之穩定性及劑量再現性佳。
The aim of this study was to investigate the characteristics of the NIPAM gel dosimeter with a clinically used Henschke applicator for the application of brachytherapy dose verification so that the gel dosimeter could be employed in clinic in the future.
The treatment planning system (TPS) of brachytherapy currently calculate based on the TG-43 report which neither consider the dose disturbance caused by the material of applicator nor include a dose correction for it. Therefore, this research designed a phantom filled with NIPAM (N-isopropylacrylamide) gel and a Henschke applicator for clinical treatment of cervical cancer was implanted. A high dose rate (HDR) Ir-192 radioactive source was adopted to give this gel dosimeter phantom prescription dose for cervical cancer brachytherapy based on TPS by multiple-dwell position method, then three-dimension dose distribution data from gel dosimeter was measured by a self-created parallel-beam optical-CT scanner. The gamma evaluation technique was used to calculate the pass rate by comparing the measured data and TPS point by point, the criteria were 3% dose difference (DD) and 3 mm distance to agreement (DTA).
The results indicated that the central cylinder used to cover the applicator in the phantom would cause refraction and scatters when optical-CT was receiving the image of the gel dosimeter so that the reconstruction analysis data would have noise, which led to affect the dose distribution. Also, the polymerization of gel closed to the central cylinder was affected due to the deferent media between each other, so there was a low gamma pass rate surrounding to the central cylinder. On the other hand, the isodose curve revealed that the gel dosimeter slightly underestimated the dose compared to the calculated value of TPS. The possible reason was that the applicator was made of stainless steel, which played a role as a tract into the phantom to irradiate. Because the radiation was attenuated during irradiation, the gel dosimeter did not receive the originally set dose, so that the measured dose didn’t coincide with TPS. Even though there was some difference between the gel dosimeter and the treatment plan in particular area, the overall gamma pass rate is over 95%. This study also discussed the stability of the NIPAM gel dosimeter, the results show that the stability and dose reproducibility of the dosimeter were excellent within 96 hours after irradiation.
