SU-E-J-173: Dosimetric Effect of Patient Positioning Uncertainties for IMRT and SBRT




To quantify the dosimetric effects of patient positioning uncertainties on PTV coverage and normal tissue sparing and find the tolerance for IMRT and SBRT positioning errors.Method and Materials: Patient positioning uncertainties were represented by isocenter shifts. Three head and neck IMRT, three prostate IMRT, and two lung SBRT plans were included in this study. The potential positioning uncertainties were introduced by shifting the isocenters by 0.1 cm, 0.3 cm, 0.5 cm, .0 cm, and 1.5 cm in Eclipse. Actual isocenter shifts (clinical shift) based on OBI/CBCT were also applied to the plans. The plans with isocenter shifts were recalculated in Eclipse and the results were compared with the original plans.


The dose errors increased with isocenter shifts. For head and neck patients, the PTV D95 dropped by 3.3% and the cord dose increased by 3.0% for 3-mm shift, but D95 dropped 6.8 % with 5 mm shift, and spinal cord dose increased by 6.9%. For prostate, 3-mm shift reduced 3.4% in D95 and increased 1.2% rectum dose and 2.7% bladder dose, while 5 mm shift caused 9.8% of reduction in D95 and 14.2% increase in bladder dose, but 7.1% decrease in rectum mean dose. For lung SBRT, 1-mm shift caused 2.2% decrease in D95 and 0.7% decrease in spinal cord, but 3 mm shifts led to 14.9% of decrease in D95, while spinal cord dose also decreased by 1.5%.ConclusionThe patient positioning uncertainties reduced the PTV coverage and changed the dose to the normal tissues. However, the significance of dosimetric effect of patient positioning uncertainties varied with different tumor sites. This study suggested that the positioning uncertainty should be within 3 mm for regular IMRT treatment, and 1 mm for SBRT to avoid significant dose errors.