SU‐F‐T‐299: An Experimental 2D Computed Radiography (CR) Dosimeter for IMRT. Are In‐Field Measurements Affected by the Low Energy Photon Overresponse?

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 CRplate 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 (5×10cm 2 ) captured out‐offield and in‐field doses in a single image. Additionally, both half beams were measured individually as well as a 3×18Gy open‐field SBRT‐lung treatment. For intensity modulated fields standard test patterns (Chair and Pyramid) and a clinical 5×5Gy 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‐offield (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 CRplate's out‐of‐field over‐response does not propagate in in‐field overresponse errors when static or dynamic (IMRT/VMAT) abutting fields are used.