Individual lymph nodes: “See it and Zap it”

Abstract

Background and purpose: With magnetic resonance imaging (MRI)-guided radiotherapy systems such as the 1.5T MR-linac the daily anatomy can be visualized before, during and after radiation delivery. With these treatment systems, seeing metastatic nodes with MRI and zapping them with stereotactic body radiotherapy (SBRT) comes into reach. The purpose of this study is to investigate different online treatment planning strategies and to determine the planning target volume (PTV) margin needed for adequate target coverage when treating lymph node oligometastases with SBRT on the 1.5T MR-linac. Materials and methods: Ten patients were treated for single pelvic or para-aortic lymph node metastases on the 1.5T MR-linac with a prescribed dose of 5x7Gy with a 3 mm isotropic GTV-PTV margin. Based on the daily MRI and actual contours, a completely new treatment plan was generated for each session (adapt to shape, ATS). These were compared with plans optimized on pre-treatment CT contours after correcting for the online target position (adapt to position, ATP). At the end of each treatment session, a post-radiation delivery MRI was acquired on which the GTV was delineated to evaluate the GTV coverage and PTV margins. Results: The median PTV V 35Gy was 99.9% [90.7-100%] for the clinically delivered ATS plans compared to 93.6% [76.3-99.7%] when using ATP. The median GTV V 35Gy during radiotherapy delivery was 100% [98-100%] on the online planning and post-delivery MRIs for ATS and 100% [93.9-100%] for ATP, respectively. The applied 3 mm isotropic PTV margin is considered adequate. Conclusion: For pelvic and para-aortic metastatic lymph nodes, online MRI-guided adaptive treatment planning results in adequate PTV and GTV coverage when taking the actual patient anatomy into account (ATS). Generally, GTV coverage remained adequate throughout the treatment session for both adaptive planning strategies. ''Seeing and zapping" metastatic lymph nodes comes within reach for MRI-guided SBRT.

Adaptive radiotherapy: The Elekta Unity MR-linac concept

“…Before introducing a new patient category with a specific fractionation scheme on the Unity, a careful evaluation of the various options has to be made to choose the appropriate trade-off. For oligometastatic lymph node treatments with 5 × 7 Gy, this is done in Winkel et al (2019) [15], [16].…”

Impact of a vacuum cushion on intrafraction motion during online adaptive MR-guided SBRT for pelvic and para-aortic lymph node oligometastases

“…Multiple target lymph nodes can move independently [39], which calls for more elaborate plan adaptation strategies (re-start adapt to shape workflow: again contour GTVs and perform online planning) to correct for motion observed on the PV scan [26,38]. Thus, SBRT on an MR-linac without vacuum cushion immobilization seems feasible for patients with single targets; for patients with multiple targets larger, anisotropic PTV margins could be considered for SBRT without vacuum cushion immobilization [40].…”

Target coverage and dose criteria based evaluation of the first clinical 1.5T MR-linac SBRT treatments of lymph node oligometastases compared with conventional CBCT-linac treatment

“…If necessary, the contours of the target lymph node(s) and OARs within 2 cm of the PTV(s) were manually adapted by a radiation oncologist [22]. Based on the daily MRI and adapted contours, a new plan was created [25]. Radiation delivery according to the new plan was performed after MRI based position verification.…”