Retrospective Analysis of Treatment Setup Errors in Abdominal Stereotactic Body Radiotherapy (SBRT)
C Kumaran Nair*, G Khelashvili, I Das, P Yadav, Northwestern University Feinberg School of Medicine, Chicago, IL
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PO-GePV-M-147 (Sunday, 7/10/2022) [Eastern Time (GMT-4)]
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Purpose: Dose delivery in stereotactic body radiation therapy (SBRT) depends to a great extent on daily setup reproducibility. This study aims at characterizing the daily shifts and impact on resulting dose coverage to the planning target volume (PTV) for SBRT patients.
Methods: We retrospectively reviewed the daily Cone Beam CTs (CBCT) of ten abdominal patients treated with 33 Gy to 50 Gy in 3-5 fractions. Radio-opaque fiducial markers were implanted for seven patients prior to the planning CT scan acquisition. During treatment planning, the fiducial markers were contoured. All patients were treated on Elekta Axesse/Versa HD machines. Prior to treatment, CBCTs were acquired and registered to the planning CT scans. Necessary couch shifts (translational and rotational) were applied before treatment delivery. A second CBCT was acquired as a verification to ensure precise 6D couch movements. In order to assess the dosimetric impact, the daily CBCTs were registered to planning CT in MIM by adopting the shifts used for the treatment. Dose volume histogram (DVH) of PTV was evaluated for coverage differences between the original and CBCT accumulated plans.
Results: The average inter-fraction shift in the left/right, anterior/posterior and superior/inferior axes were (0.06±0.21, 0.12±0.19 and 0.18±0.22 cm) respectively. The larger degree of movements were observed in the rotational directions with pitch 0.47±0.63, roll 0.08±0.56 and yaw 0.05±0.92 degrees, respectively. Dosimetric parameters V100 (cc), V95(cc) and V90(cc) were evaluated for all the selected patients. For nearly 80% of the cases, these parameters were comparable between the original and accumulated plans. Only 20% of the patient cohort had deviations >3% in the dose coverage (V100:3.6-5% for 3 cases, V95: 2.8-3.7% for 2 cases).
Conclusion: Even with a comprehensive image guidance and fiducial tracking strategy, the inter-fractional setup variations remain significant for patients undergoing SBRT for abdominal tumors.
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