Room: AAPM ePoster Library
Purpose: To quantify intrafractional gastrointestinal (GI) peristalsis for determination of a planning-at-risk margin (PRVintra) to account for GI-intrafraction peristalsis during online adaptive radiotherapy (ART) re-planning.
Methods: Twenty patients (10 abdominal and 10 pelvic) underwent MR-guided online adaptive radiotherapy (MRgoART) to 5 fraction SBRT. Patients received a 3D MR for on-table adaptive planning (pre-ART MR). Post-delivery, patients received another 3D MR (post-ART MR). Deformations within 3 cm of PTV were evaluated. Specifically, peristalsis of the GI organ-at-risk (OAR) was quantified with respect to the PTV proximal border through Hausdorff distance on pre-ART MR (HDpre-ART_ OAR-PTV), post-ART MR (HDpost-ART_OAR-PTV), and intrafractional change between pre/post-ART MR (?HDintra_OAR-PTV). Volume of OAR within 3 cm of PTV was quantified on pre-ART MR (Vpre-ART_OAR-PTV), post-ART MR (Vpost-ART_OAR-PTV) and intrafractional difference (?Vintra_OAR-PTV). OAR peristalsis with respect to itself was quantified through Dice similarity coefficient (?DSCintra_OAR-OAR), Hausdorff distance (?HDintra_OAR-OAR), and mean distance to agreement (?MDAintra_OAR-OAR).
Results: Forty MRgoART fractions (80 MR scans) across 20 patients were evaluated. Median time between pre/post-ART MRs were 64.5 min (abdominal) and 50 min (pelvic). HDpre-ART_OAR-PTV was 68.6 mm and 46.5 mm for abdominal and pelvic cases, respectively. HDpost-ART_OAR-PTV was 65.3 mm and 64.06 mm for abdominal and pelvic cases, respectively. ?HDintra_OAR-PTV was 3.30 mm and 17.5 mm for abdominal and pelvic cases, respectively. GI OAR volume within 3 cm of PTV was 44.8/29.1 cm3 (Vpre-ART, OAR-PTV), 68.8/49.2 cm3 (Vpost-ART_OAR-PTV), 24.0/20.2 cm3 (?Vintra_OAR-PTV) for respective abdominal/pelvic patients. GI peristalsis of OAR to itself was 0.5/0.43 ?DSCintra_OAR-OAR, 32.8/40.61 mm ?HDintra_OAR-OAR, and 7.9/15.98 mm, ?MDAintra_OAR-OAR for respective abdominal/pelvis.
Conclusion: Pelvic patients had greater intrafractional GI OAR motion likely due to proximal bladder-filling (?HDintra_OAR-PTV) compared to abdominal patients. For abdominal MRgoART, a 3 mm PRVintra to account for intrafractional peristalsis may be considered. Further investigation of pelvic PRVintra is needed to understand the bladder-to-GI OAR deformations.
Funding Support, Disclosures, and Conflict of Interest: Dr Mittauer reports honorarium and travel expenses by ViewRay Inc., and co-founder of MR Guidance, LLC.