Practical radiation oncology | 2019
Setup management for stereotactic body radiation therapy of patients with pancreatic cancer treated via the breath-hold technique.
Abstract
PURPOSE/OBJECTIVES\nActive Breathing Coordinator (ABC) (Elekta AB, Crawley, UK) is a motion management strategy for radiation treatment. During setup, aligning the patient to the bony spine alone does not necessarily lead to an accurate alignment to soft tissue targets and further adjustment is necessary. Determining a safe range of values for such adjustments is an important quality assurance measure and was the purpose of this study, with focus on stereotactic body radiation therapy (SBRT) in patients with pancreatic cancer.\n\n\nMATERIALS/METHODS\nThe retrospective study included 19 previously treated patients. For each fraction, a free-breathing cone beam computed tomography (CBCT) scan was registered to a reference breath-hold CT for alignment to the spine. Two perpendicular breath-hold kV projection images were then acquired and compared with corresponding reference digitally reconstructed radiographs (DRR) for additional alignment with a surrogate fiducial marker. By comparing the breath-hold kV projection images from subsequent treatment fractions with those from the first fraction, we derived the three-dimensional variability of the fiducial position with respect to the reference image.\n\n\nRESULTS\nWe observed intra-fraction setup error to be within 2.0 mm. For inter-fraction, we observed average reproducibility of 1.7 ± 0.8 mm, 2.0 ± 1.4 mm, and 3.2 ± 2.5 mm in the left-right (LR), anterior-posterior (AP), and superior-inferior (SI) directions, respectively. The average excursion values from free breathing spine to breath-hold fiducial alignment were 1.5 ± 1.4 mm, 2.0 ± 1.9 mm, and 3.0 ± 2.0 mm in the LR, AP and SI directions, respectively. The observed ranges of average excursions among all patients were 0.2 to 5.1 mm, 0.1 to 5. 9 mm, and 0.6 to 7.8 mm in the LR, AP and SI directions, respectively.\n\n\nCONCLUSIONS\nThis study demonstrates that intra-fraction targeting errors can be within 2mm while inter-fraction shifts from free-breathing spine to ABC breath-hold target can be as high as 8mm. Values that deviate significantly would need further investigation to rule out factors such as local progression, bowel gas or fiducial shift prior to treatment.