目的：此篇回溯性研究的目的是為了比較左冠狀動脈前降支（LAD）以及重要器官在左側乳癌 患者接受術後全乳房強度調控式放射治療（IMRT）以及傳統順形放射治療（3DCRT）時，劑量 分布上的差異性。 材料與方法：本研究篩選 14 名接受術後放射治療之左側乳癌患者，其中 9 名接受全乳房相切 照野放射治療；5 名接受全乳房相切照野放射治療及鎖骨上窩強度調控放射治療，此 5 名病患 我們只比較全乳房相切照野放射治療的部分。因此我們比較傳統順形治療和兩種強度調控式放 射治療劑量分布上的差異性（其中一種沒有使用左冠狀動脈前降支劑量限制，另一種則有）， 並分析 3 個組別間的劑量分布， 分別命名為：相切照野組（tangential field, TF），IMRT 無左冠 狀動脈前降支劑量限制組（non-LAD constraint，NLC） 和 IMRT 使用左冠狀動脈前降支劑量限 制組（use LAD constraint，LC）。劑量分布的分析項目包括：劑量順形率（conformity index）， 劑量同質率（homogeneity index），以及左冠狀動脈前降支，心臟和左側肺臟的劑量- 體積圖 （dose-volume histogram，DVH）。使用重複性測量 ANOVA 以及 Bonferroni post hoc test 評估全乳 房強度調控式放射治療以及傳統順形放射治療對於左冠狀動脈前降支，心臟和左側肺臟的劑量 分布是否有顯著的差異。 結果：IMRT 對於劑量同質率和劑量順形率的改善有統計上顯著的差異（p < 0.0001）。我們發 現使用 IMRT 時，若有使用左冠狀動脈前降支劑量限制條件時，可以有效降低 LAD 平均接受劑 量，V20 Gy 體積百分比，V30 Gy 體積百分比和 LAD area（由 LAD 在各方向加 1 公分形成）平 均接受劑量（p 值分別為 <0.0001， <0.0001，0.004，<0.0001），但強度調控式放射治療相較於傳 統順形放射治療會明顯增加 LAD V5 Gy 體積百分比（p 值 < 0.05）。但 LAD Dmax 在三組間則 無明顯差異。使用強度調控式放射治療會使心臟平均接受劑量， 左側肺臟平均接受劑量和 V5 Gy 體積百分比明顯高於傳統順形放射治療（3DCRT）（p 值皆 < 0.0001），且於兩個 IMRT 組別間無 顯著差異。 結論：強度調控式放射治療可以提供較佳的劑量同質率以及劑量順形率。 若需使用 IMRT 治療 患者，則建議使用適當的左冠狀動脈前降支劑量限制條件（在此篇研究中，我們使用的左冠狀 動脈前降支劑量限制條件為 V20 Gy 體積百分比小於50%），以期減少左冠狀動脈前降支高劑量 分布的區域。但 IMRT 會明顯增加低劑量分佈的區域，因此使用 IMRT 須謹慎評估此風險。未 來需要更深入的研究以期能有更趨理想的左冠狀動脈前降支劑量限制條件。

Purpose : The intent of this study was to compare the dosimetry of the left anterior descending coronary artery (LAD) and organs at risk (OARs) in left breast cancer patients in two treatment techniques, intensity-modulated radiation therapy (IMRT) and 3-dimentional conformal radiotherapy (3DCRT). Materials and Methods : Fourteen patients with left breast cancer who had received post-operative radiotherapy (RT) to the breast between October 2012 and November 2013 were enrolled in the study. All patients were irradiated with 6- or 10-MV photon beams using 3-dimentional conformal tangential technique to the left whole breast. For dosimetric comparison, two sets of six-field IMRT plans, with or without LAD constraint of V20 Gy < 50%, on each patient were performed. These three RT plans were named tangential field (TF), non-LAD constraint (NLC) and LAD constraint (LC), respectively. Five patients also received supraclavicular fossa (SCF) irradiation, but only whole breast dosimetry parameters were analyzed. The conformity index (CI), homogeneity index (HI) and dose-volume histogram (DVH) for the LAD, heart and ipsilateral lung were calculated for analysis. Repeated measures of one-way ANOVA with Bonferroni post hoc test (software SPSS 21.0) was used for statistics. Results : For the 14 patients given left breast irradiation, there was an obvious statistical benefit for HI (average 12.67%) and CI (average 92.95%) by IMRT (both p < 0.0001). According to the data, it was found that an LAD mean dose (Dmean), V20 Gy, V30 Gy and LAD area mean dose (LAD of 10 mm in all directions) were reduced by using the LC arm (average 20.28 Gy, 41.19%, 29.60% and 21.46 Gy, respectively) if using IMRT technique (all p < 0.05). However, there was no statistical difference between TF and LC arms (p > 0.05). Additionally, although the low dose bath (LAD V5 Gy average 82.81%) was a drawback in IMRT (p < 0.05), the LC arm was still better than the NLC arm if the LAD constraint was used (p = 0.013). There was no statistical significance between the 3 arms in regard to LAD maximum dose (Dmax) (3-arm average 47.21 Gy). 3DCRT had lower heart Dmean (average 3.77 Gy), ipsilateral lung Dmean (average 7.14 Gy) and V5 Gy (average 19.21%) than with IMRT (average 6.10 Gy, 9.31 Gy and 38.57%, respectively) (all p <0.0001), and there was no statistical difference between IMRT arms (p >0.05).Conclusion : In the light of our dosimetric data, IMRT may offer much better HI and CI than conventional tangential-field RT. In order to decrease the high-dose area of LAD, we had to select an appropriate LAD constraint (in this study, LAD V20 Gy <50% was used), if IMRT planning was done. IMRT technique results in a low dose bath (V5 Gy) and we should use this treatment option carefully. Further investigation for LAD constraint is needed.