Dosimetric and Motion Analysis of Margin-Intensive Radiation Delivery by Stereotactic Body Radiation Therapy for Operable Pancreatic Cancer

Abstract

Preoperative margin-intensive radiation delivery (MIRD) delivers a single high dose of ablative radiotherapy (30 Gy) focused on the surgically inaccessible retroperitoneal margin (RM) in operable pancreatic cancer, utilizing stereotactic techniques, in an effort to reduce local failure following surgery. We investigated the motion of regional organs at risk (OAR), evaluated the dosimetric effects of abdominal compression (AC) to reduce motion, and compared various planning techniques for MIRD. Ten patients were evaluated with 4DCT acquired both with and without AC. The peak respiratory regional organ and vessel centroid excursion was measured. A “sub-GTV” was defined by a radiation oncologist and surgical oncologist encompassing the RM typically lateral and posterior to the superior mesenteric artery (SMA), and expanded by 5 mm for the PTV. Identical 3D non-coplanar SBRT (3DSBRT) plans were designed for the average compression and non-compression scans. AC scans were planned with 3DSBRT, coplanar IMRT (IMRT), and CyberKnife (CK) planning techniques. Dose volume analysis compared OAR doses with and without AC and for different planning methods. Mean PTV size was 20.2 cc. Motion of the SMA, celiac trunk, and renal vessels were <5mm and not significantly impacted by AC. AC did not significantly increase OAR dose including the stomach and transverse colon (TC). Several differences were observed in the doses to OARs as a function of planning method. 3DSBRT was superior to IMRT for the mean dose to the left renal vessels (6 vs.10 Gy) (p = <0.001), and mean (5 vs. 9 Gy) (p = 0.004) and maximum (15 vs. 21 Gy) (p = 0.003) doses to the right renal vessels. 3DSBRT was superior to CK for the mean dose to the left renal vessels (6 vs 8 Gy) (p = 0.002). CK was superior to IMRT for the mean dose to the TC (1.8 vs. 2.6 Gy) (p = 0.009), and the dose to 20cc of the TC (4 vs. 8 Gy) (p = 0.003). CK was superior to 3DSBRT with respect to PTV conformality index (CI) (1.19 vs. 1.29) (p = 0.008) and the dose to 20cc of the TC (4 vs. 6 Gy) (p = 0.003). IMRT was superior to 3DSBRT for the mean dose to the liver (82 vs. 109 cGy) (p = <0.001), max dose to the SMA (30.7 vs. 36.0 Gy) (p = <0.001), and CI (1.15 vs. 1.29) (p = 0.001). IMRT was superior to CK for the maximum dose to the celiac trunk (30 vs. 34 Gy) (p = 0.007) and the maximum dose to the SMA (31 Gy vs. 36 Gy) (p = 0.001). AC does not significantly reduce the limited motion of structures in close proximity to the MIRD target and does not increase the dose to OARs that can be displaced by AC. The treatment planning techniques evaluated have different advantages with no clear superior method in our analysis. Dose to adjacent vessels may be reduced with 3DSBRT or IMRT techniques, while CI is increased with IMRT or CK.