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The Effect of Different Beam Filters On the KV CBCT Image Dose Response Curves of NanoDot OSL Dosimeters

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T Giaddui

T Giaddui*1,Y Cui2, J Galvin1, Y Yu1, Y Xiao1, (1) Thomas Jefferson University, Philadelphia, PA,(2) Duke University Medical Center, Durham, NC.

SU-E-I-7 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To evaluate the effect of different beam filters on the dose response curves of nanoDot OSL dosimeters at kV CBCT energy.

Methods:The InLight™ OSL dosimetry system (Landauer,Inc., Glenwood, IL, USA) consisting of nanoDot dosimeters and MicroStar reader was calibrated using the X-ray Volume Imager (XVI) mounted on the Elekta Synergy (Elekta, Crawley, UK) linac and the On Board Imager(OBI) mounted on the Varian True Beam (Varian Medical systems, Palo Alto, CA) linac. Doses ranged between 0 and 12 cGy were delivered to nanoDot OSLDs on 5 cm thick PMMA slab using 120kVp F0(no filter) and F1(Bowtie filter)XVI CBCT beams, 125 kVp half fan (HF) and full fan (FF) OBI beams. Dose points were correlated to absolute air kerma levels measured in air using the 0.6 cc Farmer ionization chamber and converted to dose in water at the phantom surface according to AAPM TG61.

Results:Dose response curves for all imaging settings were linear. The dose response curves for the 120 kVp F0 and F1 VXI CBCT beams differed by 8 to 9 %. The dose response curves for the 125 kVp FF and HF OBI CBCT beams also differed by 7 to 8 %. These results suggest that nanoDot dosimeters are sensitive not only to differences in kVp of the beam but also to differences in spectra resulted from the use of different filters.

Conclusion:Dose response curves of nanoDot OSLDs for photon beams of same kVp but filtered with different filters are different. Hence,the use of correction factors (based solely on differences in the response of the OSLDs at different kVps) to allow the reading of the MicroStar reader to be converted from one set of reference condition to another is not sufficient to ensure accurate estimation of measured doses.It is important to create dose response curve at every irradiation setting.

Funding Support, Disclosures, and Conflict of Interest: This project is funded, in part, under a grant with the Pennsylvania Department of Health. The Department specifically declaims responsibility for any analyses, interpretations or conclusions

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