SU-E-T-518: Dosiemtric Impact of Patient Positioning Uncertainty in SBRT Treatments




To investigate the dosimetric impact of patient positioning uncertainties on conventionally fractionated IMRT, hypofractionated and single-fraction SBRT in light of the recently published AAPM practice guideline on IGRT quality assurance (MPPG2a) .


Treatment plans of three prostate IMRT, three lungs SBRT and three spine SBRT patients were selected for this study. For each patient, uncertainties in isocenter positioning were simulated by displacing the planned isocenter with variable amount in three cardinal directions. The resulting dose distribution was obtained by recalculating the original treatment plan for each displaced isocenter position with the same planned monitor units. Dosimetric impact was assessed by comparing the isocenter-shifted plan with the original plan with respect to the image-treatment isocenter coincidence tolerance recommended in MPPG2a (1 mm for SRS and 2 mm for SBRT) .


For prostate IMRT, a 2-mm uncertainty in lateral and superior-inferior direction had minimal dosimetric impact on either PTV or rectum. However, a 2-mm deviation in the anterior-posterior position could Result in up to 8% change in PTV minimum dose (PTVDmin) and up to 7% increase in rectum D25. For lung SBRT, uncertainties of ≤ 2mm had minimal impact on PTV mean dose and mean lung dose. However, up to 21% reduction in PTVDmin was observed with a 2-mm uncertainty in superior-inferior direction. For single-fraction spine SBRT, a 1-mm uncertainty could cause up to 10% change in PTVDmin and cord maximum dose. Formal statistical analysis for patient-specific impact over a given course of treatment is being performed .


In addition to SRS, single-fraction SBRT should require isocenter alignment tolerance to be ≤ 1 mm. The 2-mm positioning tolerance for SBRT may not be sufficient for certain patients. Our analysis can be integrated into the treatment planning system for prospective prediction of the susceptibility of individual plans to setup uncertainties.