比較在體積強度調控放射治療中有無使用準直儀追蹤技術對劑量分佈的影響

Dosimetric Comparison Between Jaw Tracking and Static Jaw Techniques in Volumetric-Modulated Arc Therapy

謝懿葶(Yi-Ting Hsieh) ； 黃炳勝(Bean-Son Huang) ； 蔡宗孝(Tsung-Shiau Tsai) ； 周振宇(Chen-Yu Chou) ； 吳宗原(Tsung-Yuan Wu)

《中華放射線技術學雜誌》 42卷3期 (2018/09) Pp. 141-144

摘要

在全身立體定位放射治療技術（Stereotactic Body Radiation Therapy, SBRT）中，因為給予的單次劑量很高，所以周圍正常組織接受到高劑量的風險也跟著提高。為了要保護正常組織，在治療計劃中必須盡可能地減少正常組織所接受到的劑量，因此其劑量限值必須比一般放射治療還要嚴謹。瓦里安公司的真光刀系統（TrueBeam^(TM)）提供一項準直儀追蹤技術（jaw tracking），使準直儀可以在執行體積強度調控放射治療（volumetric-modulated arc therapy, VMAT）時，隨著多葉式準直儀（MLC）開的孔徑大小而移動調整其位置，使其能儘量貼在多葉式準直儀開的孔徑外圍。如此將能大幅減少多葉式準直儀的滲漏劑量，就能降低周圍正常組織所接受到的劑量。為了評估在全身立體定位放射治療中使用此項技術對正常組織所帶來的優點，選定25個Hepatocellular Carcinoma （HCC）肝癌的病人，每位病患接給予50 Gy的總劑量並治療5次。利用真光刀系統的10 MV FFF射束搭配體積強度調控放射治療，比較病患有使用準直儀追蹤技術與沒有使用此項技術在劑量分佈上的差異。本研究選用真光刀系統10 MV FFF射束的最高劑量率2400 MU/min。所使用的治療計劃系統為瓦里安公司的Eclipse planning system，而劑量計算演算法為Analytical Anisotropic Algorithm （AAA, version 13.0.26），選用的解析度（grid size）為2.5 mm。每個治療計劃的設計都是完整三圈弧形治療，以達到相同的腫瘤包覆率（CTV 100 %及PTV至少95 %）的前提下，比較有無使用準直儀追蹤技術，其正常組織接受到劑量的差異。所評估的正常組織包括：脊髓、腎臟、胃、十二指腸、小腸以及肝臟。所有有使用及沒有使用準直儀追蹤技術的治療計劃都是使用相同的治療計劃限值條件，單純比較有無使用此項技術對正常組織的影響。在相同的腫瘤包覆條件下，看到有使用準直儀追蹤技術治療計劃的計量分佈，其正常組織接受的劑量明顯較低，且腫瘤外圍中低劑量的分佈明顯下降的比較快。較明顯的正常組織接受到劑量的差異包括：肝臟的平均計量低至少130 cGy；V_(15Gy)也至少低100 c.c.。脊髓所接受的最大劑量至少低300 cGy，腎臟的平均劑量也至少低200 cGy。在肝癌病人高單次劑量的全身立體定位放射治療手術中，使用真光刀的準直儀追蹤技術搭配體積強度調控放射治療，的確大幅降低了肝臟腫瘤周圍正常組織所接受到的劑量。

關鍵字

準直儀追蹤技術 ； 真光刀 ； 體積調控放射治療 ； 全身立體定位放射治療

並列摘要

To compare the dosimetric differences between jaw tracking (JT) and non-jaw-tracking (nJT) techniques in volumetric-modulated arc therapy (VMAT) for Hepatocellular Carcinoma (HCC) treatment. The dose distribution around critical organs close to the tumor in HCC is critical owing to its high fractionation dose in Stereotactic Body Radiation Therapy (SBRT). Varian TrueBeam^(TM) (Varian Medical Systems) system provides the JT technique, which keeps the main jaws of the LINAC dynamically as close as possible to the MLC aperture during dose delivery, hence minimize the leakage and transmission of the MLC and further reduces the dose to the organ at risk (OAR). In order to validate the advantage of OAR dose sparing by using JT technique in SBRT for HCC, treatment plans using JT and nJT techniques were designed for dosimetric comparison. Fifteen HCC patients were selected, all treated with doses of 50 Gy in 5 daily fractions with VMAT using 10 MV FFF beams. The maximum dose rate enabled for FFF beams is 2400 MU/min for 10 MV. Each VMAT plan was individually designed using three full arcs with collimator no rotation to obtain the best adherence to planning objectives for each patient. All dose distributions were computed with the Analytical Anisotropic Algorithm (AAA, version 13.0.26) implemented in the Eclipse planning system with a calculation grid resolution of 2.5 mm. Planning objectives to target coverage aimed to cover CTV with 100 % and PTV with at least 95 % of the prescribed dose. The main OARs considered were: spinal cord, kidneys, stomach, duodenum, small bowel, and liver. JT and nJT Planning were performed with the same objectives constraint while plan optimization and the mean doses of OARs were obtained for plan comparison. At the same CTV and PTV dose coverage, every comparative result shows that JT plan did lower the dose received by OAR and improve the dose fall-off around the low dose region. The significant OAR dose reduction for the JT plan in this study are as follows: the mean doses to the liver was 130 cGy less, the V_(15Gy) of liver was 100 c.c. smaller, the maximum spinal cord dose was 300 cGy lower, and the mean kidney dose was 200 cGy lower than that in the nJT plan. JT technique did show superior OARs sparing than nJT plans in HCC. The dose reduction is of clinical importance, especially for high fractionation dose SBRT treatment.

並列關鍵字

Jaw tracking ； TrueBeam ； VMAT ； SBRT