Author: Zachary Diamond, Pretesh Patel, Sibo Tian, Yinan Wang, Xiaofeng Yang, David Yu, Ahmal Jawad Zafar, Jun Zhou 👨🔬
Affiliation: Emory University, Department of Radiation Oncology and Winship Cancer Institute, Emory University 🌍
Purpose: Active breath-hold techniques effectively mitigate respiratory motion but poses challenges for patients who are unable to tolerate the procedure. Conventional planning relies on multiple energy layers, extending delivery time due to slow layer switching. We propose to use pin ridge filters (pRFs), initially developed for FLASH radiotherapy, to construct a monoenergetic beam plan, minimizing dose delivery time.
Methods: The conventional ITV-based IMPT plan (free-breathing treatment) served as the reference. A new beam model was commissioned using the maximum energy of the IMPT plan, enabling the development of a GTV-based IMPT-DS plan (breath-hold treatment) with a downstream energy modulation strategy. To streamline the IMPT-DS plan, a nested pencil-beam-direction (PBD) spot reduction process iteratively removed low-weighted spots along each PBD, generating pRFs with coarser resolution. The IMPT-DS plan was then converted into an IMPT-pRF plan, using a single-energy beam with optimized spot positions and weights. 5mm setup and 3.5% range uncertainties were used for all plans. This approach was validated on lung and liver SBRT cases (10 Gy RBE ×5), comparing the relative total volume receiving the prescription dose (R100=VRx/VGTV+5mm) and mean lung-GTV and liver-GTV doses between the conventional IMPT and IMPT-pRF plans.
Results:For the lung case, the mean lung-GTV dose decreased from 10.3 Gy to 6.9 Gy, with delivery time reduced from 188.79 to 36.16 seconds and R100 from 1.39 to 0.68. The largest time reduction was at 150°, from 47.4 to 3.99 seconds. For the liver case, the mean liver-GTV dose decreased from 5.7 Gy to 3.8 Gy, with delivery time reduced from 111.13 to 30.54 seconds and R100 from 2.1 to 0.95. The largest time reduction was at 180°, from 38.57 to 3.94 seconds.
Conclusion: This method significantly reduces dose delivery time and organ-at-risk dose. Further analysis is needed to validate its clinical feasibility.