ESTRO 2022 - Abstract Book

S1312

Abstract book

ESTRO 2022

Materials and Methods Six high-resolution radiation detectors (EPID, Pinpoint 3D ionization chamber, W2 plastic scintillators, microDiamond, microSilicone and Edge detector) were used for the small field measurements. Output factors, profiles, depth doses were measured for various beam energies (6MV,6MV FFF, 10MV and 10MVFFF) and field sizes (10x10cm2, 5x5cm2, 4x4cm2, 3x3cm2, 2x2cm2, 1x1cm2, 0.5x0.5cm2) in a Varian True Beam Linear accelerator. Acrylic plates of appropriate depth were placed on the EPID during measurements, whereas Sun nuclear 3D water tank was used with the five point detectors. EPID measured data were compared with the reference golden beam data provided by Varian for field sizes up to 3x3cm2 and remaining small field data were compared with published data, as well as with other five high-resolution detector measurements. Results Results showed that EPID output factors, profiles and depth dose deviations for fields >3x3cm2 were up to ±3%, compared to golden beam data and other high resolution detector data. For the remaining small fields (0.5x0.5cm2 - 3x3cm2), the difference was observed to be 4%, compared to published data and other small field detectors. Conclusion The EPID can be used a small field dosimetry tool for the commissioning and verification measurements with the effective correction factors. 1 Portsmouth Hospitals University NHS Trust, Medical Physics Dept., Portsmouth, United Kingdom; 2 Portsmouth Hospitals University NHS Trust, Medical Physics Dept, Portsmouth, United Kingdom Purpose or Objective Film dosimetry is an important technique for the verification of complex radiotherapy delivery for audit or individual patient plan quality assessment. However, uncertainties including lateral scan artefact inherently limit achievable accuracy with conventional transparent radiochromic films. This study is the first to consider a completely new film and associated analysis software: Orthochromic (OrthoChrome Inc., USA), which mitigates many of these limitations. We characterise the film response to radiotherapy irradiation, evaluate improvements in the scanning process, and investigate results with a test treatment plan verification. Materials and Methods Orthochromic film OC-1 and Orthochrome Pro software (OrthoChrome Inc.) were evaluated in this study. The opaque film is scanned in reflective mode on a conventional flatbed scanner (Epson 12000XL). We characterised the film for use in routine radiotherapy clinic dosimetry applications and plan verifications: Including calibration accuracy, response as a function of dose level (0 to 200 Gy) and dose rate (0.4 to 2.4 Gy/min), effects of film orientation on scanner, lateral position of film on scanner, with repeated scanning, and post-exposure changes with time. The film was used in an IMRT Thorax Phantom (CIRS, USA) to verify the treatment delivery of a 10 MV FFF (flattening filter free) Pinnacle (Philips, NL) lung SABR plan (54Gy/3 fractions), with the film positioned in a transverse plane through the target. Profile comparison and gamma analysis within specified isodose (50% and 5%) were used. Results A reliable calibration of the OC-1 film was obtained using the Orthochrome Pro software: individual calibrations were obtained for multiple lateral scan positions on the scanner. The film exhibited a logarithmic post exposure darkening, and non-linear response as a function of dose level, consistent with radiochromic film. There were no dose rate effects observed (<0.1% change). There were no film orientation effects at scanning on pixel value (<0.1% change), and much reduced lateral scan effects across the scanner compared to radiochromic film (<0.7% change across scanner at up to 20 Gy). Repeated scanning of the film up to the tested 20 times did not change the scanned pixel value. Comparison of the measured and planned dose distributions for the SABR plan showed good agreement. Comparison profiles through the high dose region are shown in Figure 1. The gamma comparison between measured and planned is shown in figure 2. PO-1532 Evaluation of a new film dosimetry system for radiotherapy; orthochromic A. Palmer 1 , D. Nash 2