Abstract Book

S1241

ESTRO 37

EP-2371 Equipment and phantom variability in cone beam CT quality control A. Antón-Jiménez 1 , C. Delgado-Soler 1 , D. Sánchez- Artuñedo 1 1 Hospital Universitari Vall d'Hebron, Servei de Física i Protecció Radiològica, Barcelona, Spain Purpose or Objective Commercial CT phantom, Catphan®, provides several modules to assess image quality for cone beam CT (CBCT). CBCT images in radiotherapy are registered to the planning CT data set to evaluate setup or anatomical changes. The Varian Medical Systems On-Board Imager (OBI) includes default imaging protocols for head, thorax and pelvis. The aim of this study is to evaluate differences in image quality parameters comparing two different OBI equipments and two Catphan® phantoms. Material and Methods For five OBI CBCT default protocols (standard-dose head, high-quality head, low-dose head, pelvis and low-dose thorax) five CBCT of each Catphan®504 were acquired. This procedure was repeated in two clinacs with the same OBI model. A total set of 100 CBCT were analyzed and sensitometry, contrast-to-noise ratio (CNR), uniformity, geometry and spatial resolution were studied in all of them. To calculate sensitometry HU values of the different materials of interest were measured in the central slice of module CTP404 and two more slices separated 0.5 cm from the central one. CNR for each material was calculated using these values. Uniformity was measured in module CTP486-2 in three different slices to study variability too along the LONG axis. To assess geometry in CTP404, distance between the four holes, slice width and distance between radio-opaque markers in LONG direction were measured. Finally, spatial resolution was measured in module CTP528. Differences between the two phantoms and between the two OBIs were analyzed. Results • Sensitometry: The average absolute phantom variation of all protocols and structures is 4 HU. Figure 1 shows that OBIs variations are maximum for high-quality head and low-dose thorax. Teflon, delrin and acrylic are the materials that present the higher HU variations.

Conclusion Variations between OBIs in sensitometry module can exceed 50 HU. Consequently, a specific HU versus electronic density table for every OBI is recommended as a reference instead of a generic one. Except for geometry, differences are observed for all the parameters tested. We recommend maintaining the phantom used during commissioning during all the quality control. Ep-2372 High-Precision Salvage Re-Irradiation For Local Recurrence Of Prostate Cancer: Series Of 64 Patients D.P. Rojas 1 , B.A. Jereczek-Fossa 1 , D. Zerini 2 , C. Fodor 2 , A. Viola 1 , G. Fanetti 1 , S. Volpe 1 , G. Marvaso 2 , R. Luraschi 3 , A. Bazani 3 , E. Rondi 3 , F. Cattani 3 , A. Vavassori 2 , A. Bazzani 1 , S. Ronchi 1 , A. Maucieri 2 , S. Alessi 4 , G. Petralia 4 , O. De Cobelli 5 , G. Musi 5 , O. Orecchia 6 1 European Institute of Oncology - University of Milan, Department of Radiation Oncology - Department of Oncology and Hemato-oncology, MIlan, Italy 2 European Institute of Oncology, Division of Radiation Oncology, MIlan, Italy 3 European Institute of Oncology, Division of Medical Physics, MIlan, Italy 4 European Institute of Oncology, Division of Radiology, MIlan, Italy 5 European Institute of Oncology, Division of Urology, MIlan, Italy 6 European Institute of Oncology, Department of Medical Imaging and Radiation Sciences, MIlan, Italy Purpose or Objective To evaluate high-precision external beam re-irradiation (re-EBRT) for local relapse of prostate cancer (PCa) after radiotherapy.

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CNR: although values vary significantly between protocols, variations between phantoms or OBIs are mostly below 5%, except for Acrylic where variations between machines are over 10%. Uniformity: Although HU varies substantially between protocols, similar behaviors were observed for all the combinations of phantom, OBI and protocol. Low-dose thorax protocol presents the most uniform image. Table 1 summarizes the maximum absolute differences towards central slice, being all these differences below 20 HU. Geometry: all differences between measured values and nominal are below 1 mm. No differences between phantoms or machines are observed. A difference of one pair of lines is found in Low-dose thorax and Pelvis protocols when changing the phantom for both OBIs.

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