Chapman Christina Hunter, Prisciandaro Joann I, Maturen Katherine E, Cao Yue, Balter James M, McLean Karen, Jolly Shruti
Department of Radiation Oncology, University of Michigan, Ann Arbor, Michigan.
Department of Radiology, University of Michigan, Ann Arbor, Michigan.
Int J Radiat Oncol Biol Phys. 2016 Jun 1;95(2):743-50. doi: 10.1016/j.ijrobp.2016.01.042. Epub 2016 Jan 28.
Although recurrences and toxicity occur after vaginal cuff (VC) brachytherapy, little is known about dosimetry due to the inability to clearly visualize the VC on computed tomography (CT). T2-weighted (T2W) magnetic resonance imaging (MRI) is superior to CT in this setting, and we hypothesized that it could provide previously unascertainable dosimetric information.
In a cohort of 32 patients who underwent cylinder-based brachytherapy for endometrial cancer with available MR simulation images, the VC was retrospectively contoured on T2W images, and cases were replanned to treat the upper VC to a dose of 7 Gy/fraction prescribed to 5 mm. Relevant dose-volume parameters for the VC were calculated.
T2W MRI identified significant underdosing not observed on CT or T1-weighted imaging. Over two-thirds (69%) of patients had at least 1 cm(3) of VC that received less than 75% of the prescription dose and half (50%) of patients had a least 1 cm(3) of VC that received less than 50% of the prescription dose. The mean minimum point dose to the VC was 2.4 Gy, or 34% of the intended prescription dose (range: 0.53-6.4 Gy).
We identified previously unreported VC underdosing in over two-thirds of our patients, with most of these patients having volumes of undistended VC that received less than half of the prescription dose. The maximum dimension was along the craniocaudal axis in some patients or left-right/anterior-posterior axis in others, suggesting that suture material may be restricting access to the vaginal apex and that alternative applicators may be needed when the diameter of the apex is larger than the introitus. Additional follow-up will be needed to determine whether underdosing is associated with isolated VC failure or whether low failure rates across the cohort suggest that some patients are being exposed to excessive dose and unnecessary risk of toxicity.
尽管阴道断端近距离放射治疗后会出现复发和毒性反应，但由于在计算机断层扫描（CT）上无法清晰显示阴道断端，因此对其剂量测定了解甚少。在这种情况下，T2加权（T2W）磁共振成像（MRI）优于CT，我们推测它可以提供以前无法确定的剂量信息。
在一组32例接受基于施源器的子宫内膜癌近距离放射治疗且有可用MR模拟图像的患者中，在T2W图像上对阴道断端进行回顾性轮廓勾画，并对病例重新规划，将阴道断端上部治疗至规定剂量为7Gy/分次，处方点为5mm。计算阴道断端的相关剂量体积参数。
T2W MRI显示出CT或T1加权成像未观察到的明显剂量不足。超过三分之二（69%）的患者至少有1cm³的阴道断端接受的剂量低于处方剂量的75%，一半（50%）的患者至少有1cm³的阴道断端接受的剂量低于处方剂量的50%。阴道断端的平均最小点剂量为2.4Gy，即预期处方剂量的34%（范围：0.53 - 6.4Gy）。
我们在超过三分之二的患者中发现了以前未报告的阴道断端剂量不足，这些患者中的大多数未扩张阴道断端体积接受的剂量低于处方剂量的一半。在一些患者中，最大尺寸沿头尾轴，而在其他患者中沿左右/前后轴，这表明缝合材料可能限制了对阴道顶端的照射，并且当顶端直径大于阴道口时可能需要替代施源器。需要进一步随访以确定剂量不足是否与孤立的阴道断端失败相关，或者整个队列中的低失败率是否表明一些患者正接受过量剂量和不必要的毒性风险。
