ESTRO 36 Abstract Book

S898 ESTRO 36 2017 _______________________________________________________________________________________________

Healthcare, Forchheim, Germany). DECT images were acquired at 100/Sn140 kVp, and SECT images were obtained as a weighted summation of the low and high DECT images. The DECT scans were acquired at the same day as the control CT scan midway through the treatment course and using the same dose settings as used for the control scan. The CT scans covered the whole anatomical region of the head down to the top of lungs – the SPR comparison was thereby performed over very heterogeneous tissue regions. SPR images were calculated from both the DECT and SECT scans for the four first patients included in the study. For DECT, SPR images were calculated using a noise-robust method previously developed in our group. For SECT, the stoichiometric method was used. Based on SPR images, difference maps were calculated. Seven regions of interest (ROIs) were placed, each covering a single tissue type. Relative SPR differences between the DECT and SECT calculations were extracted from the ROIs. Results For bone, SECT systematically underestimated the SPR compared with DECT, while the reverse was the case for the soft tissues (Fig. 1). The relative SPR differences ranged from -2.2% to 0.9%, with a mean difference of - 0.6% (Fig. 2). Large variations of up to 1.5 percentage points were seen for the SPR difference across the patients. However, the differences for the individual patients were systematically either positive or negative for each ROI (Fig. 2).

Conclusion The calculation of the skin dose-of-the-day using planning CT-to-MVCT DIR is sufficiently reliable. The method was proven to be able of pointing out early superficial overdosing, to inform adaptive strategies. Preliminary results suggest that clinically relevant changes at half treatment should occur in a minority of patients, reinforcing the utility of our approach to select patients who may really benefit from adaptive replanning.

Electronic Poster: Physics track: CT Imaging for treatment preparation

EP-1672 Dual energy CT for improved proton stopping power estimation in head and neck cancer patients V. Taasti 1 , L. Muren 1 , K. Jensen 2 , J. Petersen 1 , J. Thygesen 3 , A. Tietze 4 , C. Grau 2 , D. Hansen 1 1 Aarhus University Hospital, Dept. of Medical Physics, Aarhus, Denmark 2 Aarhus University Hospital, Dept. of Oncology, Aarhus, Denmark 3 Aarhus University Hospital, Dept. of Clinical Engineering, Aarhus, Denmark 4 Aarhus University Hospital, Dept. of Neuroradiology, Aarhus, Denmark Purpose or Objective Pre-clinical and phantom studies have established that dual energy CT (DECT) improves estimation of the proton stopping power ratio (SPR) compared to single energy CT (SECT), leading to increased accuracy in treatment planning dose calculations. However, proton SPR estimation using DECT vs. SECT has only been compared in a single study of tumours in the cranial region with limited anatomical variations, with inconclusive results. We have therefore initiated a clinical imaging study of proton SPR estimation in the head and neck region comparing DECT and SECT. The aim of this study was to investigate if SPR differences between the two CT modalities were found when evaluating heterogeneous tissues of the head and neck region. Material and Methods The patients were CT scanned with a 2 nd generation dual source CT scanner, SOMATOM Definition Flash (Siemens