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Mean Energy, Dose, and Sources of Out-Of-Field Radiation in Flattening-Filter and Flattening-Filter-Free 6 MV Beams, Measured with TL Dosimeters

VM Lopez-Guadalupe1, A Rodriguez-Laguna2, MA Poitevin-Chacon2, E Lopez-Pineda1, ME Brandan1*, (1) Instituto De Fisica, UNAM, Mexico City, MX (2) Unidad de Radioterapia, Hospital Medica Sur, Mexico City, MX

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: To measure the mean photon energy and dose imparted by secondary radiation generated by flattening-filter (FF) and flattening-filter-free (FFF) 6 MV beams. We´ve used the properties of TLD-300 and TLD-100 dosimeters and have identified the main sources of out-of-field radiation: head leakage, secondary collimator, and phantom scattering.

Methods: TLD-300 dosimeters’ glow-curve shape is an indicator of photon energy and chips were calibrated exposing to mammography X-rays, Tc-99m, F-18(annihilation), Cs-137, and Co-60; the deconvoluted glow-curve shape was quantified through high-to-low-temperature-ratio (HLTR). TLD-100 measure dose and chips were calibrated in the FF beam for absorbed doses in water. Simultaneous TLD-100/TLD-300 irradiations took place on the linac head and in a PMMA phantom, at the isocenter plane, 1-15 cm away from the field edge (FE). As previously reported three experimental configurations were used to identify the three main sources of out-of-field radiation.

Results: Energies below 700 keV were determined with relative uncertainties below 15% and above 1250 keV could not be quantified due to decreased sensitivity of the method. Typical energies of head leakage were 500 keV for FF and FFF beams. For collimator scattered radiation, mean energies were equal to or larger than 1250 keV. For phantom scattered radiation, mean photon energies were 400 keV (FF) and 300 keV (FFF). On the phantom surface, at 1-15 cm from FE, 78%-89% of out-of-field dose came from the secondary collimator. At 4.5 cm in phantom, at 1-5 cm FE, 48%-61% of out-of-field dose originated in the same phantom. Head leakage imparted less than 0.1% of dose at isocenter. The FFF beam always delivered less out-of-field doses than the FF beam.

Conclusion: This TLD300/TLD100 dosimetric method enabled to quantify energy and dose imparted by out-of-field radiation generated in 6 MV linac FF and FFF beams, and identified/quantified the main sources of the secondary radiation.

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Funding Support, Disclosures, and Conflict of Interest: CONACYT Mexico for VMLG Master degree scholarship; UNAM PAPIIT-IN103219, for partial funding; no conflicts of interest.

Keywords

Energy Spectrum, Scatter, TLDs

Taxonomy

TH- Radiation Dose Measurement Devices: TLD

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