Intra-fraction errors and inter-fraction deformation during pancreas SBRT: Analysis and predictability of dose variations for upper gastrointestinal organs.

[PURPOSES] /Objectives. Effects on OAR doses of intra-fraction respiratory tracking errors and inter-fraction deformation were evaluated for pancreas SBRT and predictability in terms of target-to -OAR proximity investigated.

[MATERIALS] /Methods. 31 pancreatic cancer patients, treated at CyberKnife (CK) in 5 fractions prescribing 40 Gy/50 Gy to PTV/GTV, were considered. Plan optimization required for duodenum, stomach and bowel V35Gy < 0.5 cm. Target-to-OAR proximity was quantified by the expansion-intersection volume (EIV), computed on each planning CT as the intersection between the PTV expanded by 5 mm and duodenum, stomach and bowel. Treatments were delivered using CK real-time respiratory tracking. Tracking errors occurring during each fraction were computed using the system log-files. To simulate their effects on OAR doses, a perturbed plan was created modifying each beam coordinates by the corresponding errors. Inter-fraction OARs deformation was retrospectively evaluated on pre-fraction CT scans. Deformed OARs were contoured and planned doses re-calculated on the deformed anatomy. OAR constraints of perturbed plan and deformed anatomy were considered met at V35Gy < 1 cm. EIV thresholds were defined and their predictive accuracy evaluated.

[RESULTS] EIV values ranged from 0.54 to 28.11 cm. Tracking errors standard uncertainties ranged from 0.6 to 6.4 mm. For plans perturbed by tracking errors, OAR constraints were violated (V35Gy = 1.2 cm) in a single patient showing the largest EIV value (28.11 cm) and large tracking uncertainties (>5 mm). Doses recalculated on deformed OARs still fulfilled constraints for 14/31 patients. For the remaining 16 cases, V35Gy values up to 8.0 cm (median 2.0 cm) were observed. An EIV threshold of 3.5 cm showed 77% prediction accuracy.

[CONCLUSIONS] CK tracking errors have small effects on OAR doses, but they must be minimized for those patients with large EIV values. OAR deformation has larger effects, but constraints were still met in 45% of patients. EIV is a promising predictive parameter.