Abstract Book

S1141

ESTRO 37

1 UMC Utrecht, Department of Radiation Oncology, Utrecht, The Netherlands 2 UMC Utrecht, Center for Image Sciences, Utrecht, The Netherlands 3 Leeds Teaching Hospital, Department of Medical Physics and Engineering, Leeds, United Kingdom Purpose or Objective Magnetic Resonance only (MR-only) radiotherapy workflows can reduce: cost, radiation exposure, and possible registration errors compared to a CT-MRI workflow. Currently, synthetic CT (sCT) datasets must be generated to enable dose calculation and treatment planning within an MR-only workflow. Commercial solutions are available for sCT generation, but only within the scope of prostate radiotherapy. This work investigates if a commercial automatic solution, “MR for Correction Attenuation” (MRCAT), designed for MR-only dose calculation on patients with prostate cancer, is also suitable for accurate dose calculations on patients with rectal cancer. Material and Methods For twenty patients with rectal cancer, conventional CTs (Philips Brilliance Big Bore, Philips Medical Systems, Ohio, USA) were rigidly registered and resampled to bulk- assigned sCTs generated with MRCAT (Ingenia 3T MR-RT 5.1.7.3, Philips Healthcare, The Netherlands). Following the current clinical pathway at UMC Utrecht, gaseous regions intersecting the irradiation beam were electron density filled with water both on CT and sCT images. Dose calculations were performed through Monaco (v 5.11.02, Elekta AB, Sweden) on registered CT (Figure 1a) and sCT (Figure 1b) images utilising VMAT plans composed by two coplanar arcs of 10 MV irradiating between 50º and 310º. Plans were recalculated on CT and sCT without further optimisation (Monaco QA modality). The patient cohort included three clinical dose schemes: 25x2.0Gy (16 patients), 25x2.4Gy (2 patients), and 5x5Gy (2 patients). Image similarity was evaluated in terms of Mean Absolute Error (MAE) in the CT and sCT body contour intersections. Dose distributions were subsequently analysed through voxel-based dose differences (CT minus MRCAT relative to prescribed dose, Figure 1d) and gamma analysis. Set-up errors and geometrical accuracy of sCT against CT was evaluated identifying deviations in body contours in terms of beam depth from skin to the isocentre.

Conclusion Results suggest that despite body contour deviations due to set-up and anatomical differences, dose distributions calculated were accurate. Therefore, MRCAT appears feasible for use in a clinical radiotherapy workflow for patients with rectal cancer. Future investigations will evaluate dose to clinically relevant volumes as well as evaluate the reliability of MR images for position verification purposes. EP-2080 Dual-energy computed tomography and prediction of response to radiotherapy treatment in lung cancer V. Gonzalez-Perez 1 , E. Arana 2 , J. Cruz 3 , M. Barrios 2 , F. Blázquez 1 , L. Oliver 1 , C. Bosó 1 , D. Moratal 4 , J. Sánchez 1 , M. Chust 5 , L. Arribas 5 , V. Crispín 1 1 Fudación Instituto Valenciano de Oncología, Medical Physics Department, Valencia, Spain 2 Fudación Instituto Valenciano de Oncología, Radiology Department, Valencia, Spain 3 Fudación Instituto Valenciano de Oncología, Anatomic Pathology Department, Valencia, Spain 4 Universitat Politècnica de València, Center for Biomaterials and Tissue Engineering, Valencia, Spain 5 Fudación Instituto Valenciano de Oncología, Radiation Therapy Department, Valencia, Spain Purpose or Objective The iodine content quantification parameter can be generated by Dual Energy Computed Tomography (DECT). In lung cancer, the iodine contrast absorbed by a tumour (I abs ) is related to the angiogenesis process and, therefore, to the tumour aggressiveness and overall survival. In this study, the relationship of I abs to the survival of lung cancer patients treated with radiation therapy is assessed. Material and Methods A total of 38 lung cancer and metastatic patients treated with radiation therapy who underwent initial diagnostic DECT following the suspicion of lung cancer were prospectively studied. 9 patients were treated with hypofractionated schemes (40-70 Gy in 5-10 fractions) and 29 with conventional fractionation (60-70 Gy in 30-35 fractions). The lesions were adenocarcinoma (55.3%), metastasis (13.2%), squamous cell carcinoma (21.1%), and small cell lung cancer (10.5%). The CT examinations were performed on a Discovery CT750 HD scanner (GE Healthcare, USA). The patients were injected with 1.35 ml/kg of body weight of an iodinated contrast material (Iopamidol, 300 mg/ml; Bracco, Italy). The iodine measurement study (quantified in mg/cm 3 ) was reconstructed with workstation ADW4.6 (GE Healthcare, USA), and the lesion volumes were semi- automatically segmented using the Dexus® lung nodule function. Minimum, mean, maximum, and standard deviation I abs values were registered as (I abs ) minimum , (I abs ) mean , (I abs ) maximum , and (I abs )σ respectively. The cohort was stratified into two groups according to threshold values above or below the percentile 25%, 50%, and 75% for each parameter.

Results Across twenty patients (Table 1), average MAE was 52.5±3.4 HU (±1σ). An average dose deviation of - 0.1±0.1% of the prescribed dose was obtained using a threshold of 10% of the prescription dose (D10). Within this volume, a gamma analysis (2%, 2mm) pass rate of 95.9±2.7% was obtained. The average difference (CT minus MRCAT) in beam depth over the irradiation angles was 1.0±0.8 mm.