Room: AAPM ePoster Library
Purpose: Determine the dosimetric impact of using post-operative MRI imaging, acquired weeks before the start of radiotherapy (RT), for RT treatment planning.
Methods: 13 patients enrolled in a prospective clinical trial to investigate MR imaging for guidance of adaptive RT for high grade gliomas were retrospectively evaluated. T1 post-contrast and FLAIR images were acquired within 1-2 days post-surgery (MR-PO) and again within 1 day of the treatment planning CT (MR-TP) with 1mm isotropic resolution. The gross tumor volume (GTV) was contoured by an expert radiation oncologist and expanded by 1.5cm to create a clinical tumor volume (CTV) on both the MR-PO (MR-PO-GTV and MR-PO-CTV) and MR-TP (MR-TP-GTV, MR-TP-CTV). The MR-PO and the MR-TP were rigidly registered to the planning CT. The Dice Similarity Coefficient (DSC) and volume change was calculated between the MR images. Both the GTV and CTV were expanded by 0.5cm to form the PTV60 (MR-PO-PTV60, MR-TP-PTV60) and PTV50 (MR-PO-PTV50, MR-TP-PTV50), respectively. A 6MV VMAT plan was developed with 2-3 arcs to achieve 60Gy min dose to the MR-PO-PTV60 and 50Gy to the MR-PO-PTV50 and acceptable dose to normal tissues.
Results: The median time between scans was 22 days (range: 12-34). 5 patients showed progression between the MR-PO and MR-TP. The mean DSC between the MR-PO and MR-TP was 0.78 (0.64-0.95) for the GTV and 0.79 (0.46-0.97) for the CTV. The average volume undertreated (overtreated) if the MR-PO was used was 8.5cc (7.7cc) and 58.2cc (26.1cc) for the GTV and CTV, respectively. When planned based on the MR-PO targets, the average D95 was 58.3Gy and 42.3Gy for the MR-TP-PTV60 and MR-TP-PTV50, respectively. The average D98 was 59.3Gy and 41.2Gy for the MR-TP-GTV and MR-TP-CTV, respectively.
Conclusion: These results reveal substantial tissue changes between surgery and RT-planning that could adversely affect delivered dose, highlighting the importance of a TP-MRI.
Funding Support, Disclosures, and Conflict of Interest: Research reported was supported in part by the Helen Black Image-Guided Fund, the Carter Numismatics Fund, RaySearch Alliance, Dr. Marnie Rose Foundation and an MD Anderson Institutional Research Grant.