ESTRO 36 Abstract Book

S911 ESTRO 36 _______________________________________________________________________________________________

The patients were CT scanned with a 2 nd generation dual source CT scanner, SOMATOM Definition Flash (Siemens 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 Large differences in proton SPR estimation were found between DECT and SECT, although these were within the uncertainties which are currently used for dose calculation in particle therapy. These differences indicate that DECT will allow for reduction of treatment margins, resulting in better dose conformity. We are currently performing proton treatment planning for the patients comparing DECT- and SECT-based proton SPRs to investigate the dose difference in the tumour and in the surrounding healthy tissues, as well as potential impact on the range uncertainty margins used in proton treatment planning.