Purpose: To quantify the impact of organ deformation on the ablative treatment for pancreatic cancer.
Methods: We retrospectively analyzed fourteen patients treated with volumetrically-modulated arc therapy in 2017—2019 for pancreatic cancer. Thirteen patients each had one or more fiducial markers attached to the target and matched by image guidance for treatments. The other one patient was treated with bone match. We generated 72 virtual computed-tomography scans (vCT) by deforming planning CTs (pCT) to cone beam CTs (CBCT). Areas that were matched for treatment were used to guide the deformable registration. Daily dose distributions on the vCT scans were calculated and mapped back to the pCT, where we characterized the daily target coverage by the percentage of gross tumor volume receiving the full dose. The daily dose distributions of each patient were further summed up allowing evaluation of cumulative coverage and other relevant dosimetric quantities, which were compared with the treatment plans.
Results: The median daily coverage was 65% (range 12—74%). The daily coverage varied by <10% (standard deviation, SD) for all patients except the bone-matching patient, for whom the SD was 29%. The median cumulative coverage was 58% (range 6—74%), compared to median 79% (range 10—96%) from the original plans. The cumulative target coverage of the bone-matching patient degraded by the largest amount (88% drop). For that patient, the cumulative stomach dose was 62 Gy, which was 5 Gy higher than the planned value and exceeded our limit of 60 Gy. For another patient, the image registration was significantly affected by the CBCT artifacts.
Conclusion: Organ deformation significantly affected target coverage for ablative treatment of pancreatic cancer. Daily image guidance with internal marker localization provided some mitigation but is limited by the drawbacks of CBCT imaging.
Funding Support, Disclosures, and Conflict of Interest: This research was funded in part through the NIH/NCI Cancer Center Support Grant P30 CA008748.