Author: David Byun, Ting Chen, Paulina E. Galavis, Allison McCarthy, Hesheng Wang, Michael J Zelefsky 👨🔬
Affiliation: NYU Langone Health 🌍
Purpose: To summarize a MR guided adaptive workflow developed for pelvic lymph nodes (PLN) stereotactic radiotherapy using large field size on Elekta Unity© MR Linac system, and to quantitatively analyze the dosimetry benefits obtained with the proposed adaptive workflow.
Methods: 30 patients who were treated at our institute for prostate+PLN SBRT on Unity underwent simulation on Philips Ingenia 1.5T MR simulator with MR-compatible radio opaque surface markers to establish a triangulation reference for patient setup. Average superior/inferior target volume length was 17.0 cm (range 13.0cm to 18.4cm). We used SNC MICRO+ fixed laser system to facilitate patient setup in the Unity treatment room. For all patients, T2 3D MR scans (TR 1300ms, TE 100ms) were acquired. MR images were first sent to MIM for semi-automatic target volumes and OARs contouring, then exported to offline Monaco with contours for preplanning operations. Finally, adaptive to shape optimization was performed in online Monaco using T2 MR with forced electron density (ED). At the end of the treatment, another T2 3D scan was acquired, in which physicians and physicists re-contoured targets and OARs, respectively. Dose distribution was reconstructed in selected post-treatment scans using adaptive plan and initial reference plan parameters with the updated contours for dosimetry comparison.
Results: The use of laser system helped to reduce the setup uncertainty from 6±7.5mm (range 7mm sup to 30mm inf from reference) to 0±3.1mm (range 7mm sup to 5mm inf), which was essential for large field size treatment on Unity. Dosimetry in post-treatment scans showed significant improvement (paired t-test < 0.05 criteria comparison) of target coverage and reduced OAR radiotoxicity in online adaptive plans comparing to offline reference plans.
Conclusion: We have successfully established an adaptive workflow for large field prostate+PLN SBRT on Unity. There is significant dosimetry improvement over non-adaptive treatment to the same site.