如何以旋轉準直儀的方式來改善頭頸部強度調控放射治療

To ReDuCe DoSe InHoMogeneITy aT neCk JunCTIon By uSIng

吳東和(Tung-Ho Wu) ； 邱仲峰(Jeng-Fong Chiou) ； 朱曉榮(Ron Zhu) ； 黃光偉(Kuang-Wei Huang) ； 翁培峰(Pei-Fong Wong) ； 董世民(Sam Tung) ； 趙坤山(K.S. Clifford Chao) ； 蔡若婷(Jo-Ting Tsai)

《放射治療與腫瘤學》 13卷3期 (2006/09) Pp. 205-215

https://doi.org/10.6316/TRO/200613(3)205

摘要

目的：針對頭頸部癌症的放射治療，嘗試採用準直儀旋轉式的強度調控放射線治療（Collimator Rotated IMRT, CR IMRT ）來改善頸部照野接合處的劑量均勻度， 並比較與傳統IMRT 方式的差異性。
材料與方法：本研究選擇10 位頭頸部癌症患者的CT 影像，分別以傳統的IMRT 及新設計的CR IMRT電腦治療計劃進行分析，並比較其順形參數與劑量均勻度的差異。CR IMRT 治療方式為旋轉準直儀九十度，主靶區及上頸部以九個照野涵蓋；鎖骨及下頸部的淋巴結則以三個照野，照野上下緣相接處分別呈階梯狀互相重疊，用以減少相接處局部劑量不均勻的情形。依據美國醫學物理師學會TG-40 報告的要求， 準直儀的誤差應在2公釐以內，所以本研究分析照野接鄰處偏差的間隔（gap）±1公釐、±2公釐， 重疊（overlap）±1公釐、±2公釐時對劑量的影響。X光片驗證也於本研究進行，用以分析這兩種強度調控治療方式在照野相接處的劑量不均勻度。
結果：傳統IMRT與CR IMRT在計劃品質分析上，幾乎達到相同的順形參數（conformity index ）和劑量均勻程度（homogeneity index ），重要正常器官的劑量評估上， 包括腦幹、脊髓、兩側唾液腺的最大劑量或平均劑量均在器官容許劑量範圍內。以X光片實際照射頸部相接處發現傳統IMRT治療方式在上下緣1公釐、2公釐、4公釐誤差時相接處劑量為±12%、±22% 、±41%，以CR IMRT 的治療方式則發現上下緣1公釐、2公釐、4公釐誤差時相接處劑量大幅改善為±6%、±8%、±12% 。
結論：CR IMRT 在治療計畫品質幾乎與傳統IMRT 相同， 且上下頸接鄰區的劑量不均勻性可以大幅改善， 為一值得用以治療頭頸部癌症之IMRT 治療方式。

並列摘要

Purpose : To improve dose homogeneity at the neck junction in head-and-neck cancer radiotherapy, we employ a rotating collimator and compare the differences between it and conventional intensity-modulated radiation therapy (IMRT).
Materials and Methods : CT images for 10 head-and-neck cancer patients previously treated with SIHB technique in our clinic were used for this planning study. We generated a new CR IMRT in each case and compared the differences between the conformity index and homogeneity index. The new planning technique also used beams with equally-spaced gantry angles but only for the neck above the shoulder. We made beams above the shoulder with a slightly lower field border (normally 3 to 5 mm), to have a built-in feathering. Three of the nine beams treated the entire neck, including the lower neck and supraclavicular lymph nodes. Only three beams are required to split and therefore the total number of fields is 12. The AAPM TG-40 recommended an accuracy of field size and jaw symmetry calibration to be within 2 mm. Hence, for an accelerator operating within these guidelines,
two abutting fields can overlap or gap up to 2 mm. Field mismatches of ±1 mm and ±2 mm because of imperfect jaw/MLC calibration were simulated. We used film to measure junction inhomogenity.
Results : The new technique could be used to successfully generate IMRT plans for head and neck cancers. Both conventional IMRT and CR IMRT had almost the same conformity index and
homogeneity index. Upon evaluating the dose distribution of critical organs, we found all of them were within the tolerance ranges. Film measurements showed that dose inhomogeneities that resulted from 1 mm, 2 mm, 4 mm junction area as jaw/MLC calibration errors were reduced from as large as ±12%,
±22%, ±41% with the single-isocenter and half-beam (SIHB) technique to less than ±6% 、±8%, ±12% with this newly developed technique.
Conclusion : Compared with the conventional SIHB technique, the new technique provides superior dose homogeneity in the abutment region between the supraclavicular and head-and-neck IMRT
fields. With a modulating lower border of fields, the feathering mechanism substantially reduces dose inhomogeneities that result from imperfect jaw/MLC calibration.