ESTRO 2021 Abstract Book

S623

ESTRO 2021

In general, the scanners at the participating centers showed maximum distortions below 0.5 mm within 200 mm from the iso-center of the scanner. The reason for the high level of distortion of the MRI scanner at one center has not yet been found, but the detection of an outlier proves the value of the national collaborative MRI quality assurance network. The next steps include analysis of clinical MRI sequences, other image quality metrics, independent distortion calculation, and a web-based quality assurance system for monitoring. PD-0789 A global quality index for prostate SBRT with and without SIB: a multiplanning study E. Villaggi 1 , S. Russo 2 , V. Hernandez 3 , C. Moustakis 4 , O. Blanck 5 , M. Esposito 2 , N. Hardcastle 6 , R. de Blas Piñol 7 , J. Saez 8 , R. Doro 9 , L. Masi 10 , L. Strigari 11 , S. Strolin 11 , M.D. Falco 12 , V. Silvestri 12 , B. Nardiello 13 , S. Broggi 14 , A. Savini 15 , M. Stasi 16 , P. Mancosu 17 1 AUSL Piacenza, Medical Physics, Piacenza, Italy; 2 Azienda USL Centro Toscana, Medical Physics, Florence, Italy; 3 Hospital Universitari Sant Joan de Reus, Medical Physics, Tarragona, Spain; 4 University Hospital Münster, Medical Physics, Münster, Germany; 5 University Center Schleswig-Holstein, Department of Radiation Oncology, Kiel, Germany; 6 Peter MacCallum Cancer Centre, Medical Physics, Melbourne, Australia; 7 Institut Catala d'Oncologia-Hospital Duran i Reynals, Medical Physics, Barcelona, Spain; 8 Hospital Clinic de Barcelona, Medical Physics, Barcelona, Spain; 9 University of Florence Istituto Fiorentino di Cura ed Assistenza IFCA, Medical Physics, Florence, Italy; 10 University of Florence Istituto Fiorentino di Cura ed Assistenza IFCA, , Medical Physics, Florence, Italy; 11 Policlinico S. Orsola-Malpighi, Medical Physics, Bologna, Italy; 12 SS. Annunziata Hospital, Medical Physics, Chieti, Italy; 13 Ospedale San Pietro FBF, Medical Physics, Rome, Italy; 14 HSR San Raffaele, Medical Physics, Milan, Italy; 15 Istituto Scientifico Romagnolo per lo Studio e la Cura dei Tumori IRST IRCCS, Medical Physics, Meldola, Italy; 16 Azienda Ospedaliera Ordine Mauriziano, Medical Physics, Turin, Italy; 17 Humanitas Clinical and Research Hospital, Medical Physics Unit of Radiation Oncology, Milan, Italy Purpose or Objective Sterereotactic Body Radion Therapy (SBRT) is an emerging treatment modality in clinically localized prostate cancer (PCa), but limited data is available on what plan quality is achievable. The interest to implement simultaneous integrated boost (SIB) to the dominant intraprostatic lesion (DIL) highlights the challenges of achieving optimal plans and increasing the dose conformity. The Italian Association of Medical Physics (AIFM) and the German Society for Medical Physics (DGMP) working groups for SBRT joined conducting an international study with the aim to quantify the goals of providing adequate target coverage whilst minimizing the normal tissue dose and exploring overall quality indices. Materials and Methods Two anonymized and pre-contoured PCa cases with DIL boost volumes were shared and 2 plans with different prescriptions were optimized: (I) 35 Gy (7Gyx5fr) homogeneously to the planning target volume (PTV) and (II) 50 Gy (10Gyx5fr) as SIB to the DIL and 35 Gy to the remaining PTV with pre-defined plan objectives. Statistical methods were developed using R language from the original RTplan-dose-files. Plans were compared using a range of conformity metrics which were both dependent of the targets coverage (R50, Paddick Conformity Index PCI and Conformity Index CI) and independent of target coverage (Prescription Dose Spillage PDS, Modified Gradient Index MGI). Furthermore, plans were ranked according to a customized quality index QI defined as the ratio of constraints to be raised and constraints to be lowered: D98(PTV) * D98PTV(PTV-DIL) / D2(PTV) * D2(PTV-DIL) * D05(Rectum) * D05(Bladder) * V18 (Rectum) * V18(Bladder). Results Forty-four treatment plans from 11 participating centers with various platforms technologies (2 robotic-based and 9 c-arm-based Linacs) from Italy, Germany, Spain and Australia were collected and reviewed. Despite the significant increase of dose between the classical and the DIL prescriptions, almost all planners achieved the protocol constraints to the PTVs and OARs with minor deviations. Nevertheless, conformity metrics varied over a wide range (Table 1). Figure 1 shows the ranking of plans according to the various quality indices. The customized QI was not related to any other conventional quality metric according to Pearson correlation analysis. Moreover, QI presented a wide range that could usefully rank plans. Table 1.