Department of Radiation Oncology, Virginia Commonwealth University, Richmond, Virginia, USA.
Med Phys. 2022 Jun;49(6):3926-3935. doi: 10.1002/mp.15666. Epub 2022 Apr 22.
High-dose-rate (HDR) vaginal cuff brachytherapy is an effective adjuvant therapy for women with stage I endometrial cancer. Although infrequent, failures do occur, most frequently at the vaginal vault. A potential cause of failure is insufficient dosimetric coverage at the vaginal apex due to cold spots from the anisotropic dose distribution of the source. Here, we propose a novel direction modulated brachytherapy (DMBT)-concept vaginal cylinder (VC) applicator that resolves this dosimetric issue.
The novel DMBT-VC applicator was designed and simulated with the GEANT4 Monte Carlo code. The outer cylinder material chosen was polyphenylsulfone (PPSU) plastic, and the central part was a detachable rod, housing a single lumen made of either polyether ether ketone (PEEK) plastic or an MR-compatible tungsten alloy. The PPSU-based outer cylinder, together with the inner PEEK rod provides the dose distribution of a conventional VC applicator. The PEEK rod is then replaced with an MR-compatible tungsten alloy rod of the same dimensions to generate directional "pencil-like" beams to compensate for the anisotropic cold spots. Two widely used Ir HDR sources, VS2000 and GammaMedPlus, were also simulated.
The novel DMBT-VC applicator was able to remove the underdosage at the apex due to the anisotropy effect regardless of the HDR sources without unnecessarily increasing the dose to the periphery of the applicator. Also, further directional modulation to reach deeper in the apex by up to 14 mm beyond the VC surface was achievable, again without increasing the peripheral doses. Total treatment dwell times increased only by 7-13%.
The novel DMBT-VC applicator provides improved dose coverage at the vaginal apex by overcoming the classical anisotropy issue ubiquitous to all HDR brachytherapy sources. The next step in development of the device is manufacturing a prototype for clinical testing.
高剂量率（HDR）阴道袖口近距离放疗是治疗 I 期子宫内膜癌患者的有效辅助治疗方法。尽管这种情况很少见，但确实会出现治疗失败，最常见的是在阴道穹窿部位。失败的一个潜在原因是由于源的各向异性剂量分布导致冷点，阴道顶点处的剂量覆盖不足。在这里，我们提出了一种新的方向调制近距离放疗（DMBT）-概念阴道圆柱（VC）施源器，以解决这个剂量问题。
新的 DMBT-VC 施源器是使用 GEANT4 蒙特卡罗代码设计和模拟的。选择的外筒材料是聚苯硫醚（PPSU）塑料，中央部分是一个可拆卸的杆，内部有一个单腔，由聚醚醚酮（PEEK）塑料或一种磁共振兼容的钨合金制成。基于 PPSU 的外筒与内部的 PEEK 棒一起提供了传统 VC 施源器的剂量分布。然后，用相同尺寸的磁共振兼容的钨合金棒替换 PEEK 棒，以产生定向“铅笔状”光束，补偿各向异性冷点。还模拟了两种广泛使用的 Ir HDR 源，VS2000 和 GammaMedPlus。
新的 DMBT-VC 施源器能够消除由于各向异性效应导致的在顶点处的剂量不足，而与 HDR 源无关，同时不会不必要地增加施源器外围的剂量。此外，通过在 VC 表面之外再增加 14mm 深度，可以实现进一步的定向调制，而不会增加外周剂量。总的治疗驻留时间仅增加了 7-13%。
新的 DMBT-VC 施源器通过克服所有 HDR 近距离放疗源普遍存在的经典各向异性问题，提供了在阴道顶点处更好的剂量覆盖。该设备的下一步开发是制造一个原型进行临床测试。
