Session Item

Gastric lymphoma is a tumor site that is very difficult to visualize on CBCT. This results in difficulty in using CBCT for both image matching for setup and the evaluation of daily anatomical variations of the stomach, which can be significant. Therefore, adaptive radiotherapy on the MR-linac could benefit patients with gastric lymphoma by providing high soft tissue contrast for setup, gating on internal anatomy, and the possibility to adapt to the daily anatomy. However, appropriate PTV margins to account for intra-fraction variations for this tumor site are unknown. Therefore, the purpose of this work was to evaluate the intra-fraction variation in the position of the stomach in a few pilot cases and to establish a workflow for determining patient-specific PTV margins for adaptive radiotherapy of the stomach on the MR-linac.

Two gastric lymphoma patients and one healthy volunteer underwent simulation on the MR-linac (MRIdian, ViewRay). Simulation MRIs were acquired in inspiration breath hold at two separated timepoints within the same session (MRI1 and MRI2) to represent the changes that could happen intra-fractionally, and sagittal cine images and tracking were performed after each MRI to test the feasibility of tracking. The CTV was contoured offline on both scans (CTV1 and CTV2) using a commercial TPS (Eclipse, Varian Medical Systems). Registration of the MRIs was performed with focus on the region of the target used for tracking on the sagittal cine images. PTVs were created using expansions ranging from 5-10 mm from CTV1. CTV2 was copied to the MRI1 structure set, and it was assessed if the PTVs created from CTV1 were sufficient to also cover CTV2.

Gating on the cine images was successful with tracking on the superior part of the stomach (Figure 1) to simulate gating during treatment. The average time between scans was 21 minutes (range 15-33). CTV volumes ranged from 397 to 592 cc. All three cases had complete coverage of CTV2 by the a PTV expansion from CTV1 of 8 mm (Table 1). Furthermore, the superior region of CTV2 near the diaphragm was always covered by a 5 mm expansion (Figure 2), which was expected due to focus on that region during registration.

Figure 1

Figure 2

Table 1

We found that, as a starting point for patients with gastric lymphoma treated adaptively on the MR-linac, PTV margins should be 5 mm superiorly (in the region of the target near the diaphragm) and 8 mm in other directions. We also developed a methodology for patient-specific evaluation of margins; the registration process described in this study will be performed for each patient to verify the sufficiency of the margins. Furthermore, images from treatment sessions will be retrospectively analyzed to assess if individual margins should be adjusted.