Abstract
Purpose: The purpose of this study was to characterize the uncertainty in dose measurements by optically‐stimulated luminescent dosimeters (OSLD) as a Results of image‐guided isocenter shifts in volumetric modulated arc therapy (VMAT) of prostate cancer patients. Methods: Daily image‐guided couch shifts were extracted from the record and verify system for 25 pre‐op prostate cancer patients (1130 fractions) with implanted fiducials and 7 post‐op prostate cancer patients (255 fractions) registered using bony anatomy. Standard deviations were calculated to quantify typical isocenter shifts in the superior‐inferior, anterior‐posterior and lateral directions. Typical shifts were applied to treatment plans for 13 pre‐op and 9 post‐op prostate cancer patients treated with VMAT and tracked with weekly OSLDs. Keeping the number of monitor units constant, the dose to the OSLD was recalculated for all 27 combinations of isocenter shifts. All shifted measurements were compared to the original location. Standard deviations of these percent difference distributions quantified the dosimeter uncertainty due to isocenter shifts for fiducial‐matched and bone‐matched image guidance. Clinical OSLD readings were compared with calculated uncertainties and correlated with actual image‐guided shifts. Results: Shifts for fiducial‐matched and bony‐landmark matched image guidance were 4 mm/5 mm/5 mm, and 3 mm/2 mm/3 mm in the superior‐inferior, anterior‐posterior, and lateral directions respectively. Standard deviations of 7.4% for prostate patients matched to the fiducial marker and 5.6% for the patients matched to bony anatomy were calculated. Clinical OSLD readings often exceeded these values and showed little correlation with actual image‐guided shifts. Conclusion: Image‐guided isocenter shifts contribute substantial uncertainty in expected dose to OSLDs that must be taken into account if the dosimeter is placed before image guidance. Additional clinical uncertainties must be investigated before appropriate tolerances can be applied to OSL dosimetry.
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