2360 - Intra- and Inter-Fraction Errors and Deformation during Pancreas SBRT: Analysis and Predictability of Dose Variations for Upper Gastrointestinal Organs

L. Masi¹, M. Loi², R. Doro¹, M. Zani³, G. Simontacchi², V. Salvestrini¹, G. Francolini², D. Greto², and L. Livi⁴; ¹Istituto Fiorentino di Cura e Assistenza (IFCA), CyberKnife Center, Florence, Italy, ²Radiation Oncology, Careggi University Hospital, University of Florence, Florence, Italy, ³Medical Physics, Careggi University Hospital, University of Florence, Florence, Italy, ⁴Department of Experimental Clinical and Biomedical Sciences “Mario Serio”, University of Florence, Florence, Italy

Purpose/Objective(s): Prescription of ablative doses in pancreas SBRT requires steep dose fall-off to fulfill OARs dose limits. Both geometric errors and organ deformation may lead to OARs doses violation. We aimed at evaluating effects of intra-fraction errors and inter-fraction deformation on OARs doses, investigating their predictability in terms of target-to -OAR proximity Materials/

Methods: 20 patients, treated for pancreatic cancer in 5 fraction prescribing 40Gy/50Gy to PTV/GTV, were considered. Plan optimization required for duodenum, stomach and bowel V35Gy< 0.5 cc. Target-to-OAR proximity was quantified for each patient computing the expansion-intersection volume (EIV) on the planning CT (pCT), as the intersection between the PTV expanded by 5 mm and duodenum, stomach and bowel. All patients were treated at a frameless robotic radiosurgery system using fiducial real time respiratory tracking modality. Residual tracking errors were extracted for 100 fractions using the system log-files. Standard uncertainties (mean values + 2 standard deviations) along the three directions (SI, LR; AP) were used as descriptive statistics. To simulate the effects of tracking errors on OARs doses, a perturbed plan was created modifying each CK beam coordinates by the corresponding errors and a perturbed dose distribution was calculated. Inter-fraction OARs deformation was evaluated acquiring pre-fraction CT scans with a maximum of 2 repeated scans per patient. Deformed OARs were contoured by the same physician who contoured the pCT. Planned doses were re-calculated on the deformed anatomy using the fiducials alignment center, to rigidly register the pCT to each pre-fraction scan. OARs constraints of perturbed plan and deformed anatomy were considered met at V35Gy<1 cc.
Results: EIV values ranged from 0.5 to 28 cc (median 9.6 cc). Tracking errors standard uncertainties ranged from 0.6 mm to 7.2 mm, with respective median values 3.5 mm (SI), 2.4 mm (LR), 2.0 mm (AP). For plans perturbed by tracking errors, OARs constraints were violated only in 2 / 20 patients (V35Gy respectively 1.4 cc and 1.2 cc). For these two patients, EIV values were above 25 cc, and errors uncertainties were above 5 mm along at least one direction. The recalculated doses on deformed OARs, contoured on repeated CT scans, showed constraints violation in 13/20 cases with V35Gy values up to 5 cc (median 2.1 cc). The 7 patients for whom constraints were still met on the deformed anatomy (V35Gy<1cc) showed EIV values below 1.5 cc.
Conclusion: CK tracking errors have small effects on OAR doses, but attention must be paid to minimize these errors for patients with large EIV values. Although OARs deformation has larger effects, constraints were still met in 33% of patients. EIV is a promising parameter to predict who is at risk of exceeding OARs constraints during pancreas SBRT, needing plan adaptation.