ESTRO 37 Abstract book

S1162

ESTRO 37

Aim: To follow treatment response on DWI of head and neck tumors using functional diffusion maps. Material and Methods Twelve patients treated with radiotherapy, with or without concurrent chemotherapy, underwent MRI prior to and during week 2, 3, 4 and 5 of the radiotherapy treatment. The MRI protocol contained SPLICE DW-MRI sequences with b-values of 0, 200 and 800 s/mm 2 . All imaging was obtained with the patient positioned in the radiotherapy mask. The tumor was solely de lineated on the pretreatment diffusion weighted MR images and ADC map resulting in a tumor mask. ADC maps were all registered rigidly to the MRI pretreatment . Functional diffusion maps were created for all patients using a threshold of 0.3 10 -3 mm 2 /s for ADC changes (figure 1). Results During (chemo)radiotherapy more tumor voxels showed increase (mean 33%) in ADC than a decrease (9%) (figure2). ADC increase was already apparent in the second week of treatment except in one patient. ADC decrease mostly points to volume decrease, which was most apparent from week 4 during treatment on. Conclusion During (chemo)radiotherapy, tumor response using DW- MRI mostly showed increase of apparent diffusion coefficient in around one third of voxels as soon as the second week into treatment. This can be interpreted as reduction of cell density by necrosis, replacement of tumor by normal tissue and edema in and around remaining tumor tissue. Decrease in ADC is most evident from week 4 into treatment on. This suggests a reduction of tumor voxels replaced by air containing voxels. Functional diffusion maps provide an objective measure to follow response on DW-MRI during treatment provided a good geometric accuracy as is offered by the SPLICE technique. Figure 1 Functional diffusion map of a patient. Showing the ADC maps acquired prior to and during treatment (upper row). The tumor mask is shown in the lower left corner. Further the increase (red) and decrease (blue) on a per-voxel basis is shown in the lower row. EP-2110 Target volume definition with MRI and 68Ga- DOTATOC-PET/CT for patients with meningiomas C. Garibaldi 1 , M. Ferrari 2 , M. Colandrea 3 , A. Cascio 4 , M. Ciocca 5 , A. Iannalfi 6 , E. D'Ippolito 6 , S. Pesente 7 , C. Grana 3 , M. Cremonesi 1 1 European Institute of Oncology, Unit of Radiation Research, Milano, Italy 2 European Institute of Oncology, Unit of Medical Physics, Milano, Italy 3 European Institute of Oncology, Nuclear Medicine, Milano, Italy 4 University of Milano, Radiology and Radiotherapy Techniques, Milano, Italy 5 Centro Nazionale di Adroterapia Oncologica CNAO, Medical Physics Unit, Milano, Italy 6 Centro Nazionale di Adroterapia Oncologica CNAO, Radiation Oncology, Milano, Italy 7 Tecnologie Avanzate T.A. s.r.l., Research & Development, Udine, Italy

The median of the relative volume difference of the primary tumor segmented on PET/CT and PET/MR was 4.8% (range 0.4% to 39.9%) for the gradient-based method and 18.0% (range 0.7% to 71.2%) for the threshold-based method. The fraction of features stable between the scans is shown in Table 1. A larger number of radiomic features was stable when segmentation was performed using the gradient-based compared to the threshold- based method, which is in agreement with the improved reproducibility of tumor volume using gradient-based method. More than 75% of shape and intensity features yielded an ICC>0.9 between the scans for both segmentation methods. However, only 51.5% of the texture and 34.2% of the wavelet features reached this criterion (for gradient-based method and even less in threshold-based method). In the wavelet features analysis, more features were robust in the smoothed images (low-pass filtering) in comparison to images with emphasized heterogeneity (high-pass filtering) (Fig.1). Hierarchical clustering revealed 11 uncorrelated groups of stable features. Conclusion Shape and intensity radiomic features were robust when comparing the two types of [18F]-FDG PET scans (PET/CT and PET/MR). In contrast, substantially worse stability was observed for texture and wavelet features, which needs to be considered for their use in prognostic modelling. This instability can be caused by different factors, such as the different attenuation correction methods or test-retest variability. EP-2109 Functional diffusion maps to assess treatment response in head and neck tumors B. Peltenburg 1 , T. Schakel 1 , C.H.J. Terhaard 1 , R. Bree 2 , M. Philippens 1 1 UMC Utrecht, Radiation Oncology, Utrecht, The Netherlands 2 UMC Utrecht, Head and Neck Surgical Oncology, Utrecht, The Netherlands Purpose or Objective The apparent diffusion coefficient (ADC) determined by diffusion weighted magnetic resonance imaging (DW-MRI) is a surrogate measurement of cellularity and stromal component of a tumor. ADC changes during treatment might be of prognostic value in patients with head and neck squamous cell carcinoma (HNSCC) and allows for treatment modification. However, tumor visibility on MRI decreases during treatment hampering tumor delineation and increasing observer variation. To avoid this, functional diffusion maps can be used to objectively analyze ADC changes in time on a per-voxel basis. This requires images with good geometric accuracy to enable image registration of the time series. Split-echo acquisition of FSE signals (SPLICE) sequences have superior geometric performance to echo planar imaging (EPI) sequences and can therefore provide these distortion free DW-MRI images.