Chen Katherine S, Glaser Scott M, Kim Hayeon, Beriwal Sushil
University of Pittsburgh School of Medicine, Pittsburgh, PA.
Department of Radiation Oncology, University of Pittsburgh Cancer Institute, Pittsburgh, PA.
Brachytherapy. 2017 Sep-Oct;16(5):964-967. doi: 10.1016/j.brachy.2017.06.002. Epub 2017 Jul 8.
In image-based brachytherapy (IBBT), the dosimetry of small structures may be particularly sensitive to changes in contouring between imaging modalities. We therefore sought to characterize differences in urethral dosimetry in vaginal brachytherapy based on contouring on MRI vs. CT.
We retrospectively identified our most recent 15 patients treated with intracavitary brachytherapy for distal vaginal malignancies. On T2-weighted MRI, both the lumen and urethral wall were contoured. On CT, the urethral lumen alone was contoured, as the wall is indistinguishable from surrounding tissue. High-dose-rate (HDR) IBBT plans were generated for all patients.
Mean urethral volume was higher on MRI than CT at 3.7 cc vs. 1.1 cc (p < 0.0005). As a result, there were statistically significant increases on MRI in D and D, as well as EQD2 D and EQD2 D when applied to a full course of treatment (45 Gy EBRT + 25 Gy IBBT).
We have quantified the expected differences in urethral volume and dosimetry when contoured on MRI vs. CT. Inclusion of the urethral wall on MRI, with its average thickness of 2.2 mm, likely more accurately reflects the true organ at risk and results in an increase in reported dose compared to CT.
在基于图像的近距离放射治疗（IBBT）中，小结构的剂量测定可能对成像模态之间的轮廓勾画变化特别敏感。因此，我们试图基于MRI与CT上的轮廓勾画来描述阴道近距离放射治疗中尿道剂量测定的差异。
我们回顾性地确定了最近15例接受腔内近距离放射治疗的阴道远端恶性肿瘤患者。在T2加权MRI上，勾画了尿道管腔和尿道壁。在CT上，仅勾画了尿道管腔，因为尿道壁与周围组织无法区分。为所有患者生成了高剂量率（HDR）IBBT计划。
MRI上的平均尿道体积高于CT，分别为3.7立方厘米和1.1立方厘米（p < 0.0005）。因此，当应用于整个疗程（45 Gy外照射放疗 + 25 Gy IBBT）时，MRI上的D和D以及等效剂量2（EQD2）D和EQD2 D有统计学显著增加。
我们已经量化了在MRI与CT上进行轮廓勾画时尿道体积和剂量测定的预期差异。MRI上包含平均厚度为2.2毫米的尿道壁，可能更准确地反映真正的危险器官，并且与CT相比导致报告剂量增加。
