2122 - Impact of Abdominal Compression in MR-Guided Online Adaptive Planning for Abdominal Cancers Treated with a 1.5 Tesla MR-Linac

Purpose/Objective(s): Abdominal compression is an effective approach in managing respiratory motion in treating abdominal cancers and has been adopted by many centers to treat patients with online adaptive planning on a 1.5 Tesla MR-Linac. However, pneumatic compression belt (CB) is often in the radiation beam path causing online planning uncertainties. This study will investigate the impact of CB usage on planning accuracy in treatment planning system (TPS) for 1.5 Tesla MR-Linac treatment of abdominal cancers.

Materials/Methods: Twelve patients (9 kidney, 2 liver, and 1 pancreas cancers) with abdominal compression were analyzed. Clinically approved CT-based plans including the CB served as ground truth. To simulate MR-based online adaptive plans in treatment planning system, which uses bulk density override to create synthetic CT for dose calculation, body and bony structures were overrode with mean electron density (ED) calculated by the TPS. The ED of CB was tested with different values from 0 (no CB) to 0.30 to determine an optimal value. The clinical plans were then recalculated for simulated MR-based plans with different CB EDs. Plan similarity was assessed using gamma analysis (1%/1mm) with further dosimetric comparisons for planning target volume (PTV) metrics (minimum, maximum, mean, D95%) and maximum doses to the small bowel, large bowel, and stomach using paired student t-tests.

Results: Of the 12 cases, 9 cases achieved the best gamma results with an ED of 0.30 for CB in MR-based plans. The PTV dosimetric parameters did not show any significant difference (p>0.05) for CB ED of 0.30, while having significant difference (p=0.05) for at least one PTV dosimetric metric for CB ED values in [0, 0.25]. There is no significant difference (p > 0.05) for maximum dose to the stomach, small bowel, or large bowel with any CB ED value (except for large bowel with CB ED of 0.30).

Conclusion: We recommend including CB in online adaptive planning if a CB is used and the electron density override for CB should use a value of 0.3 to reduce planning uncertainty, if the CB is in the beam path. The mean ED calculated by the system does not provide a reliable approximate ED value for planning.