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Correction Method to Determine Surface Dose for OSL Detectors

T Reynolds

T Reynolds*, P Higgins , Univ Minnesota, Minneapolis, MN

Presentations

SU-E-T-91 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: OSL detectors are commonly used in clinic due to their numerous advantages, such as linear response, negligible energy, angle and temperature dependence in clinical range, for verification of the doses beyond the dmax. Although, due to the bulky shielding envelope, this type of detectors fails to measure skin dose, which is an important assessment of patient ability to finish the treatment on time and possibility of acute side effects. This study aims to optimize the methodology of determination of skin dose for conventional accelerators and a flattening filter free Tomotherapy.

Methods: Measurements were done for x-ray beams: 6 MV (Varian Clinac 2300, 10x10 cm² open field, SSD = 100 cm) and for 5.5 MV (Tomotherapy, 15x40 cm² field, SAD = 85 cm). The detectors were placed at the surface of the solid water phantom and at the reference depth (dref=1.7cm (Varian 2300), dref =1.0 cm (Tomotherapy)). The measurements for OSLs were related to the externally exposed OSLs measurements, and further were corrected to surface dose using an extrapolation method indexed to the baseline Attix ion chamber measurements. A consistent use of the extrapolation method involved: 1) irradiation of three OSLs stacked on top of each other on the surface of the phantom; 2) measurement of the relative dose value for each layer; and, 3) extrapolation of these values to zero thickness.

Results: OSL measurements showed an overestimation of surface doses by the factor 2.31 for Varian 2300 and 2.65 for Tomotherapy. The relationships: SD₂₃₀₀ = 0.68 x M₂₃₀₀-12.7 and SDτομο = 0.73 x Mτομο-13.1 were found to correct the single OSL measurements to surface doses in agreement with Attix measurements to within 0.1% for both machines.

Conclusion: This work provides simple empirical relationships for surface dose measurements using single OSL detectors.


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