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Dosimetric Impact of Placement Errors in Optically Stimulated Luminescent in Vivo Dosimetry

M Tariq

M Tariq1,2*, C Gomez2 , A Riegel1,2 , (1) Northwell Health, Lake Success, NY, (2) Hofstra University, Hempstead, NY


SU-I-GPD-T-307 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Optically stimulated luminescent dosimeters (OSLDs) are increasingly used for in vivo dosimetry of intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT). Given the high-dose gradients in IMRT and VMAT, placement uncertainty may have an outsize role in variability of measured-to-planned dose agreement. The purpose of this work was to characterize placement errors using cone-beam computed tomography (CBCT) and to measure the potential dosimetric impact of misplaced OSLDs.

Methods: We examined 293 CBCTs where OSLDs were visible of 128 patients of varying anatomical sites. The CBCTs were registered with the treatment plan in which the planned location and actual placement of the OSLD were analyzed. A reference beam was created en face to the skin at the planned OSLD location. Using the beam’s eye view, placement error was measured as the two-dimensional distance between the planned and actual OSLD location. Dosimetric impact was assessed by projecting the actual placement point to the skin on the treatment plan, comparing this point dose to the planned dose and expressing the difference as percent error. Dosimetric errors were correlated with placement errors.

Results: OSLDs were grossly misplaced for 19 CBCTs and were excluded. For the remaining 274 CBCTs, average placement error was 9.7±9.5mm with average dosimetric error of -2.37±19.29%. Dosimetric change was weakly correlated with placement error (R²=0.3933). There were several outliers for which small placement errors yielded large dosimetric differences and vice versa. The former scenario was caused by placement near field edges or high intensity-modulated dose gradients.

Conclusion: We observed an average placement error of approximately 1 cm and a large variability of dosimetric impact with small placement errors potentially producing large measured-to-planned percent dose errors. Accurate OSLD placement for IMRT and VMAT treatment is critical for minimizing measured-to-planned dose variance.

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