Department of Radiation Oncology, Asan Medical Center, Seoul, Korea.
Proton Therapy Center, National Cancer Center, Goyang, Korea.
PLoS One. 2020 Jul 28;15(7):e0236585. doi: 10.1371/journal.pone.0236585. eCollection 2020.
The aim of this work is to build a framework that comprehends inverse planning procedure and plan optimization algorithm tailored to a novel directional beam intensity-modulated brachytherapy (IMBT) of cervical cancer using a rotatable, single-channel radiation shield. Inverse planning is required for finding optimal beam emitting direction, source dwell position and dwell time, which begin with creating a kernel matrix for each structure based on Monte-Carlo simulated dose distribution in the rotatable shield. For efficient beam delivery and less transit dose, the number of source dwell positions and angles needs to be minimized. It can be solved by L0-norm regularization for fewest possible dwell points, and by group sparsity constraint in L2,p-norm (0≤p<1) besides L0-norm for fewest active applicator rotating angles. The dose distributions from our proposed algorithms were compared to those of conventional tandem-based intracavitary brachytherapy (ICR) plans for six cervical cancer patients. The algorithmic performance was evaluated in delivery efficiency and plan quality relative to the unconstrained algorithm. The proposed framework yielded substantially enhanced plan quality over the conventional ICR plans. The L0-norm and (group sparsity+L0-norm) constrained algorithms reduced the number of source dwell points by 60 and 70% and saved 5 and 8 rotational angles on average (7 and 11 angles for highly modulated cases), relative to the unconstrained algorithm, respectively. Though both algorithms reduced the optimal source dwell positions and angles, the group sparsity constrained optimization with L0-norm was more effective than the L0-norm constraint only, mainly because of considering physical constraints of the new IMBT applicator. With much fewer dwell points compared to the unconstrained, the proposed algorithms led to statistically similar plan quality in dose volume histograms and iso-dose lines. It also demonstrated that the plan optimized by rotating the applicator resulted in much better plan quality than that of conventional applicator-based plans.
这项工作的目的是建立一个框架，该框架理解针对使用可旋转单通道辐射屏蔽的新型宫颈癌定向射束强度调制近距离放射治疗（IMBT）的逆规划过程和计划优化算法。逆规划需要找到最佳的射束发射方向、源驻留位置和驻留时间，这是从基于旋转屏蔽中蒙特卡罗模拟剂量分布为每个结构创建内核矩阵开始的。为了实现高效的射束传递和减少转运剂量，需要最小化源驻留位置和角度的数量。可以通过 L0 范数正则化实现尽可能少的驻留点，通过 L2,p 范数（0≤p<1）中的组稀疏约束以及 L0 范数实现尽可能少的有源施源器旋转角度。通过与六位宫颈癌患者的传统的基于 tandem 的腔内近距离放射治疗（ICR）计划相比，比较了我们提出的算法的剂量分布。通过与无约束算法相比，评估了算法在输送效率和计划质量方面的性能。与传统的 ICR 计划相比，所提出的框架大大提高了计划质量。L0 范数和（组稀疏+L0 范数）约束算法将源驻留点数分别减少了 60%和 70%，平均节省了 5 个和 8 个旋转角度（对于高调制情况为 7 个和 11 个角度），相对于无约束算法。尽管两种算法都减少了最佳源驻留位置和角度，但具有 L0 范数的组稀疏约束优化比仅 L0 范数约束更有效，这主要是因为考虑了新型 IMBT 施源器的物理约束。与无约束相比，驻留点少得多，所提出的算法在剂量体积直方图和等剂量线中产生了统计学上相似的计划质量。它还表明，通过旋转施源器优化的计划比传统的基于施源器的计划具有更好的计划质量。
