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KV OBI Spectroscopy: Measurement Vs Monte Carlo Simulation

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Z Grelewicz

Z Grelewicz*, R Wiersma, The University of Chicago, Chicago, IL

SU-D-141-2 Sunday 2:05PM - 3:00PM Room: 141

Purpose: With IGRT there has been a dramatic increase in the use of kV based on-board imaging (OBI). Although, numerous Monte Carlo (MC) simulation studies on the OBI kV source exist, the only validation of the actual OBI photon energy spectrum has been in the form of half-value-layers. Realistic MC OBI simulation requires correct modeling of the kV beam spectrum. Here, we present for the first time, actual experimentally measured OBI spectra together with their MC simulated counterparts.

Methods: The spectra produced by a commercial LINAC OBI were measured using a thermoelectric cooled CdTe detector and a multichannel analyzer. To reduce detector over saturation, the kV radiation source photon flux was reduced by the use of collimating disks placed before the CdTe surface and by setting the kV source to its minimum mAs. Preliminary measured spectra for 60 kVp, 80 kVp, 100 kVp, and 125 kVp beams for the OBI were compared to MC results based on a model of the OBI. For the 125 kV beam, a percent depth dose (PDD) curve was measured with a diamond detector, and compared to a PDD generated by MC based on the simulated spectrum.

Results: The corrected measured spectra correspond roughly to the MC generated spectra, with the MC spectra generally showing more high energy photons and fewer low energy photons. The MC PDD and measured PDD agree to within 2% of absolute dose.

Conclusion: We present new measurements of kV OBI spectrometry to advance kV dosimetry and more sophisticated modeling of kV imaging systems. The discrepancy between measured and simulated spectra is small, and the dosimetric effect of the discrepancy is also small.

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