Program Information
An Experimental 2D Computed Radiography (CR) Dosimeter for IMRT. Are In-Field Measurements Affected by the Low Energy Photon Overresponse?
W Crijns1*, D Vandenbroucke2 , P Leblans2 , T Depuydt4 , (1) University Hospital Leuven, Leuven, Belgium,(2) Agfa Healthcare N.V., Mortsel, Belgium, (3) KU Leuven, Leuven, Belgium
Presentations
SU-F-T-299 (Sunday, July 31, 2016) 3:00 PM - 6:00 PM Room: Exhibit Hall
Purpose:
Computed Radiography (CR) dosimetry could offer film dosimetry resolution and flexibility but with reusability and instantaneous processing. For an experimental CR-plate, designed for radiotherapy (Zeff=18), CR's typical out-of-field over-response to low energy photons was previously reduced to 8%. The present work assesses the impact of the residual over-response when open-fields are combined or when intensity modulated fields are used.
Methods:
Agfa Healthcare’s experimental CR-plate was scanned and erased 4min after each irradiation using a flying-spot CR-15-X-engine based reader, which was adapted for radiotherapy dosimetry. A CR-plate specific calibration and uniformity correction was used.
For open-fields two abutting half beams (5x10cm²) captured out-of-field and in-field doses in a single image. Additionally, both half beams were measured individually as well as a 3x18Gy open-field SBRT-lung treatment. For intensity modulated fields standard test patterns (Chair and Pyramid) and a clinical 5x5Gy rectal VMAT plan were captured. All measurements were compared to the corresponding dose calculations.
Results:
For open-fields the out-of-field overdose was clearly larger than the in-field overdose (10% vs. 4%). The sum of the individual measurements corresponded well with the combined measurement (dose difference, ΔD<-2.2%). The SBRT case had no overdose in the high dose region; ΔD=-5.6%±3.3%, the deviation was attributed to CR-fading effects (-0.3%/min) which were not corrected for.
Compared to open-fields, intensity modulated deliveries had a further increased over-response out-of-field (ΔD=+58% to +125% [Chair] +43% [Pyramid]), due to the increased amount of low energy photons for IMRT. However, this effect was not measured in-field where even decreased dose signals were observed (ΔD=-0.3% to +2.25% [Chair], -4.5% to -0.1% [Pyramid]). The rectal VMAT treatment had a dose difference +2.4%±6.0%. The in-field deviations were attributed to a residual non-uniformity.
Conclusion:
The experimental CR-plate’s out-of-field over-response does not propagate in in-field over-response errors when static or dynamic (IMRT/VMAT) abutting fields are used.
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