ESTRO 36 Abstract Book

S494 ESTRO 36 _______________________________________________________________________________________________

Material and Methods Structural MRI-scans (T1-weighted) from 18 glioma patients (grade II and III), who underwent high dose radio(chemo)therapy (54-60 Gy) with curative intent have been analyzed. MRIs were acquired before treatment and at several time intervals thereafter. Because of the individual characteristics of these data e.g., voxel size (0.5…6 mm³) and the field strength (1…3 T) a standardized image processing approach was developed. For bias field correction, registration with atlas data, resampling, and segmentation of different tissue types, image processing methods from the ANTs-, FSL- and SPM-toolbox were used, respectively. Based on these images the volumes of white matter and gray matter have been longitudinally analyzed. Results

quantitative

image-based

structural

tissue

characterization was performed. Material and Methods

T2-weighted and T1-weighted MRI after contrast agent (CA) injection at 1.5T were acquired in thirteen patients before RT (MRI1) and at about 12 months of follow-up (MRI2). In order to reduce possible errors due to non- quantitative values of signal intensity, a normalization step was performed between MRI1 and MRI2 of each patient, using a histogram matching method. Right and left internal obturator muscle contours were manually delineated upon T2w MRI1 by an expert and then automatically propagated on MRI2 by an elastic registration method. The following textural features were extracted in each volume: histogram-based indices (mean intensity, variance, 95 th percentile, entropy, skewness, kurtosis), GLCM (Grey-Level Co-occurrence Matrix)-based indices (energy, correlation, homogeneity, entropy, contrast, dissimilarity), NGTDM (Neighborhood Grey-Tone Different Matrix)-based indices (coarseness, contrast, busyness, complexity, strength) and fractal dimension. To assess changes in internal obturator muscles, a comparison of the parameters extract on MRI1 and MRI2 was carried out by Wilcoxon test, with significant p-value Exemplificative T1w MRI1 and MRI2 with relative muscles histograms were shown in Figure 1. From a qualitative assessment, a homogenous higher enhanced area (red circle in Figure 1) was localized in MRI2 in a region near the prostate. Quantitatively, significant increase in mean, variance and 95 th percentile values on both T1w MRI and T2w MRI2 was also found, as well as variation of indices describing histogram shape as visible by the histograms reported in Figure 1. Moreover, changes of GLCM and NGTDM-based indices confirmed that the spatial distribution of this intensity enhancement was concentrated in a homogeneous local area, as suggested by increased homogeneity and correlation indices and decreased complexity and fractal dimension (Table 1). < 0.05. Results

Figure 1 shows the changes of brain tissue volume depicted as box plots with the median values highlighted in red. While the entire brain volume on average remains constant over two years after therapy, in the same time period the volume of gray matter and white matter varies conversely in a wide range. Noteworthy, this work points out the difficulties of retrospectively analyzing clinically acquired data due to differences in acquisition parameters and in investigation intervals. Conclusion The observed changes over time underpin the importance of exact follow-up protocols in quantitative e valuation of structural brain changes after radiotherapy. Together with the data on interpatient heterogeneity, our findings allowed to design a prospective study in a larger cohort of patients treated by photons vs. protons for assessing the dependence of MRI-detected volumetric changes with delivered dose. PO-0896 Quantitative MRI-based characterization of obturator muscles after prostate cancer radiotherapy E. Scalco 1 , I. Pirovano 1 , T. Rancati 2 , A. Cicchetti 2 , F. Palorini 2 , B. Avuzzi 3 , A. Messina 4 , R. Valdagni 2 , (3), (5), G. Rizzo 1 1 Istituto di Bioimmagini e Fisiologia M olecolare, CNR, Segrate Milano, Italy 2 Fondazione IRCCS Istituto Nazionale dei Tumori, Prostate Cancer Program, Milano, Italy 3 Fondazione IRCCS Istituto Nazionale dei Tumori, Radiation Oncology 1, Milano, Italy 4 Fondazione IRCCS Istituto Nazionale dei Tumori, Radiology, Milano, Italy 5 Università di Milano, Department of Oncology and Hemato-oncology, Milano, Italy Purpose or Objective To investigate radiation-induced alterations in periprostatic muscles, such as internal obturators, in prostate cancer patients treated with external-beam radiotherapy (RT). These tissues are usually included in the high dose radiation field and can be involved in genito- urinary toxicity. In this work, a texture analysis for