Application of single- and dual-energy CT brain tissue segmentation to PET monitoring of proton therapy

The purpose of this work was to evaluate the ability of single and dual energy computed tomography (SECT, DECT) to estimate tissue composition and density for usage in Monte Carlo (MC) simulations of irradiation induced β ⁺ activity distributions. This was done to assess the impact on positron emission tomography (PET) range verification in proton therapy. A DECT-based brain tissue segmentation method was developed for white matter (WM), grey matter (GM) and cerebrospinal fluid (CSF). The elemental composition of reference tissues was assigned to closest CT numbers in DECT space (DECT_dist). The method was also applied to SECT data (SECT_dist). In a validation experiment, the proton irradiation induced PET activity of three brain equivalent solutions (BES) was compared to simulations based on different tissue segmentations. Five patients scanned with a dual source DECT scanner were analyzed to compare the different segmentation methods. A single magnetic resonance (MR) scan was used for comparison with an established segmentation toolkit. Additionally, one patient with SECT and post-treatment PET scans was investigated. For BES, DECT_dist and SECT_dist reduced differences to the reference simulation by up to 62% when compared to the conventional stoichiometric segmentation (SECT_Schneider). In comparison to MR brain segmentation, Dice similarity coefficients for WM, GM and CSF were 0.61, 0.67 and 0.66 for DECT_dist and 0.54, 0.41 and 0.66 for SECT_dist. MC simulations of PET treatment verification in patients showed important differences between DECT_dist/SECT_dist and SECT_Schneider for patients with large CSF areas within the treatment field but not in WM and GM. Differences could be misinterpreted as PET derived range shifts of up to 4 mm. DECT_dist and SECT_dist yielded comparable activity distributions, and comparison of SECT_dist to a measured patient PET scan showed improved agreement when compared to SECT_Schneider. The agreement between predicted and measured PET activity distributions was improved by employing a brain specific segmentation applicable to both DECT and SECT data.