摘要目的:比较乳腺癌保乳术后仰卧位CT与MRI定位图像的靶区和剂量的差异，探讨MRI定位在保乳术后部分乳腺照射中的应用价值。方法:29例早期乳腺癌患者保乳术后放疗前在仰卧位下行CT及MRI定位扫描，分别对CT与MRI图像上的瘤床进行术腔可视化评分(CVS)，勾画瘤床(TB)、临床靶区(CTV)、计划靶区(PTV)，并基于CT图像制定部分乳腺照射计划。比较CT与MRI图像上TB的CVS、靶区差异与一致性，探讨40　Gy处方剂量等剂量线覆盖PTV比例差异。结果:CT、MRI图像上TB的CVS分别为2.97±1.40、3.10±1.40( P＝0.408)。MRI图像上勾画的TB、CTV、PTV较CT均明显增加，分别为(24.48±16.60) cm 3∶(38.00±19.77) cm 3、(126.76±56.81) cm 3∶(168.42±70.54) cm 3、(216.63±81.99) cm 3∶(279.24±101.55) cm 3(均 P<0.001)，但CTV、PTV增加的比例较TB明显减小(均 P<0.001)。CT与MRI图像勾画的TB、CTV、PTV一致性指数分别为0.43±0.13、0.66±0.11、0.70±0.09( P<0.001)。放疗计划40　Gy等剂量线覆盖PTV-CT与PTV-MRI比例中位数分别为95.6%(95.0%～97.5%)与81.9%(62.3%～92.4%)( P<0.001)。 结论:CT与MRI对于TB术腔的可视化相近，但MRI图像勾画的TB、CTV、PTV范围明显增加，基于CT图像制定的放疗计划对于PTV-MRI剂量覆盖不充分。作为CT定位的补充，保乳术后MRI定位的应用可能可以提高TB勾画的准确性。

abstractsObjective:To compare magnetic resonance imaging (MRI)-based and computed tomography (CT)-based target volume delineation and dose coverage in partial breast irradiation (PBI) for patients with breast cancer, aiming to explore the application value of MRI localization in PBI after breast-conserving surgery.Methods:Twenty-nine patients with early breast cancer underwent simulating CT and MRI scans in a supine position. The cavity visualization score (CVS) of tumor bed (TB) was evaluated. The TB, clinical target volume (CTV), planning target volume (PTV) were delineated on CT and MRI images, and then statistically compared. Conformity indices (CI) between CT- and MRI-defined target volumes were calculated. PBI treatment plan of 40 Gy in 10 fractions was designed based on PTV-CT, and the dose coverage for PTV-MRI was evaluated.Results:The CVS on CT and MRI images was 2.97±1.40 vs. 3.10±1.40( P=0.408). The volumes of TB, CTV, PTV on MRI were significantly larger than those on CT, (24.48±16.60) cm 3vs. (38.00±19.77) cm 3, (126.76±56.81) cm 3vs. (168.42±70.54) cm 3, (216.63±81.99) cm 3vs. (279.24±101.55) cm 3, respectively, whereas the increasing percentage of CTV and PTV were significantly smaller than those of TB. The CI between CT-based and MRI-based TB, CTV, PTV were 0.43±0.13, 0.66±0.11, 0.70±0.09( P<0.001), respectively. The median percentage of PTV-MRI receiving 40 Gy dose was 81.9%(62.3% to 92.4%), significantly lower than 95.6%(95.0%~97.5%) of PTV-CT. Conclusions:The CVS between CT and MRI is not significantly different, but the MRI-based TB, CTV, PTV are significantly larger than CT-based values. The PTV-MRI is of underdose if PBI treatment plan is designed for PTV-CT. As a supplement of CT scan, MRI can enhance the accuracy of TB delineation after breast-onserving surgery.