(Sunday, 7/14/2019)
Room: ePoster Forums
Purpose: To determine the mean out-of-field photon energy during radiotherapy treatments in a linac, using the properties of the TLD-300 thermoluminescent (TL) dosimeter. This material has the property of a glow curve whose shape is dependent on the radiation field quality. Previous investigations by our group have studied this feature in fields of X-rays used in diagnostic radiology. This work reports results for secondary radiation fields created by a 6 MV linac.
Methods: The glow curve of TLD-300 dosimeters was calibrated at different photon qualities: a mammography beam, Tc-99m source, F-18 source annihilation photons, Cs-137 and Co-60 sources. The thermal protocol for the TL chips has been optimized to increase signal stability. The glow curve was analyzed in terms of its component individual peaks, obtained by deconvolution. The ratio between high- and low- temperature peaks, called HLTR, was related to the incoming energies by an analytical expression. Irradiations took place under CPE at the linac head, and inside a PMMA phantom at lateral distances of 1-15 cm from the field edge.
Results: The HLTR parameter follows the energy dependence previously observed at diagnostic radiology qualities. On the linac head shielding, a mean energy of (396 ±44) keV was measured. Out-of-field mean energies were (362 ±30) keV at 1 cm from the edge, and (300 ±22) keV at 5-15 cm away. The main limitation of the method is the decreased sensitivity of the glow curve to changes in energy above 700 keV.
Conclusion: We have extended the characterization of the TLD-300 glow curve to photon megavolt therapy energies. In spite of reduced sensitivity, we’ve been able to quantify the secondary field mean energy at the linac head and out-of-field. We continue working to reduce uncertainties and searching for a TL protocol-associated feature that would increase the sensitivity of the method.
Funding Support, Disclosures, and Conflict of Interest: PAPIIT-UNAM, Grant 103219
Keywords
Radiation Therapy, TLDs, Energy Spectrum
Taxonomy
TH- External beam- photons: Contaminant neutron/out of field dosimetry
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