全身皮膚電子束放射治療之劑量學

Dosimetry for Total Skin Electron Beam Radiation Therapy

謝忠志(Chung-Ching Hsieh) ； 戴宏旗(Hung-Chi Tai) ； 陳弘政(Hung-Cheng Chen) ； 奚正義(Cheng-I Hsi) ； 張國華(Kou-Haw Chang) ； 賴允亮(Yuen-Liang Lai) ； 吳孟浩(Meng-Hao Wu)

《放射治療與腫瘤學》 7卷1期 (2000/03) Pp. 35-44

https://doi.org/10.6316/TRO/200007(1)35

摘要

目的：描述本科考量客觀設備條件，參酌「六對照野法」進行全身皮膚電子束放射治療之劑量學實務經獻。
材料與方法：射源至表面距離(SSD)是323cm，並在治療平面前15cm處架設-0.8cm厚度之壓克力板以發散射束並衰減初能量為6MeV的電子能量。首先測定能使垂直方向達到最佳劑量均勻度的成對照野角(dual-field angle)，並量測水平和垂直15方向的照野平坦度。接著利用X光片劑量法求出電子在治療平面的劑量及相關數據，據以校正直線加速器的output。再利用熱發光劑量計(TLD)測量人形假體上以六個dual-fields和一個dual-field照射的吸收劑量之比值以決定倍數因子(multiplication factor，簡稱MF)，便可求得治療每單一照野所必須給予的劑量。再完全模擬實際治療條件，以TLD進行in vivo dosimetry，確認病人體表劑量並評估各點的劑量差翼(dose variation)。
結果：我們所實驗或測量得到的duel-field angle(20°)和multiplication factor(2.68)等參數，與他人經驗相符。電子的劑量子分佈及X-ray back ground(0.75克)亦令人滿意。直線加速器的output雖大幅減小(0.0684cGy/MU)，但藉著提高劑量率(600MU/min)使每次治療時間不致過長。病人體表的dose variation大致與文獻報告相符合。惟水平方向的照野平坦度略為不足(60cm的範圍內是±10%)。
結論：全身皮膚電子束放射治療的技術極繁復，對每位病人的劑量學考慮須兼顧個性、系統性與完整性。

關鍵字

全身皮膚電子束放射治療,六對照野法,劑量學

並列摘要

Purpose: Dependent on the equipment and facilities available, we adopted “six-dual-field technique” to administer total skin electron radiation therapy (TSET). Our experience with dosimetric measurements for this modality is reported.
Materials and Methods: The source-to-surface distance (SSD) was extended to be 323 cm, and an 0.8 cm-thick acrylic screen was installed at 15 cm in front of the treatment plane to scatter the beam and attenuate the electron energy. At first, the optimal dual-field angle was determined in order to achieve feasible dose uniformity in the vertical dimension, and then field flatness could be assured. Film dosimetry was applied to define depth dose distribution profile of electron beam at treatment plane. Accordingly, the output of the linear accelerator could be calibrated. Multiplication factor was deter-mined to be 2.68, represented as the ratio of the absorbed dose from all six dual-fields to that from one dual-field measured with thermoluminescent dosimeters (TLDs) taped at Rando phantom. Thus, the value of MU for each field could be calculated. TLDs were used for in vivo dosimetry to verify the skin dose and evaluate dose variation at different anatomic sites.
Results: The determined dual-field angle (20 ) and multiplication factor (2.68) were comparable to those described in the literature. Depth dose distribution and beam penetration were satisfying, and X-ray background level (0.75%) was also acceptable. Markedly decreased output of the Linac (0.0684 cGy/MU) could be partially compensated with the escalation of dose rate (600 MU/min). Thus, each treatment would not cost too much time. Does variation among different sites at skin is compatible with the experience of other institutions. Instead, field flatness in the horizontal dimension (±1O% within 60 cm) did not meet the requirement given by AAPM.
Conclusion: The technique of TSET is complex. Dosimetry for this modality should be studied case by case systematically and thoroughly.

並列關鍵字

Total skin electron radiation therapy (TSET),Six-dual-field technique,Dosimetry