Validation of proton stopping power ratio estimation based on dual energy CT using fresh tissue samples

Abstract

Dual energy CT (DECT) has been shown, in theoretical and phantom studies, to improve the stopping power ratio (SPR) determination used for proton treatment planning compared to the use of single energy CT (SECT). However, it has not been shown that this also extends to organic tissues. The purpose of this study was therefore to investigate the accuracy of SPR estimation for fresh pork and beef tissue samples used as surrogates of human tissues. The reference SPRs for fourteen tissue samples, which included fat, muscle and femur bone, were measured using proton pencil beams. The tissue samples were subsequently CT scanned using four different scanners with different dual energy acquisition modes, giving in total six DECT-based SPR estimations for each sample. The SPR was estimated using a proprietary algorithm (syngo.via DE Rho/Z Maps, Siemens Healthcare, Forchheim, Germany) for extracting the electron density and the effective atomic number. SECT images were also acquired and SECT-based SPR estimations were performed using a clinical Hounsfield look-up table. The mean and standard deviation of the SPR over large volume-of-interests were calculated. For the six different DECT acquisition methods, the root-mean-square errors (RMSEs) for the SPR estimates over all tissue samples were between 0.9% and 1.5%. For the SECT-based SPR estimation the RMSE was 2.8%. For one DECT acquisition method, a positive bias was seen in the SPR estimates, having a mean error of 1.3%. The largest errors were found in the very dense cortical bone from a beef femur. This study confirms the advantages of DECT-based SPR estimation although good results were also obtained using SECT for most tissues.

Theoretical and experimental analysis of photon counting detector CT for proton stopping power prediction

“…To evaluate the new PCD method experimentally, we used the proton pencil beam measurement data for organic tissues described in Taasti et al …”

“…VOI sizes are stated in Table S1.III in Data S1. Most tissues were found to be very homogeneous . The SPR estimates were compared to measured SPR values, which were taken as the ground truth in this evaluation.…”

“…The SPR estimates were compared to measured SPR values, which were taken as the ground truth in this evaluation. The SPR values were measured in proton pencil beams with an energy of 185.2 MeV using a multilayer ionization chamber, further details on the experimental setup and uncertainty calculations for the SPR measurements were described by Taasti et al …”

“…These studies include investigations on how to perform a robust calibration of the conversion from CT numbers in conventional single‐energy CT (SECT) to SPR, the influence of beam hardening (BH), the use of dual‐energy CT (DECT), as well as the use of proton CT . It has been shown in several studies that DECT gives more accurate SPR estimates than SECT using the stoichiometric method proposed by Schneider et al …”

Helium CT: Monte Carlo simulation results for an ideal source and detector with comparison to proton CT

“…However, these studies were performed using computer simulations or phantom data, never on real patients. Recent studies on fresh animal meat showed for six different DECT acquisition methods that the root‐mean‐square errors for RSP evaluation over all tissue samples were between 0.9% and 1.5%, versus 2.8% for conventional x‐ray CT . On the other hand, computed tomography with charged particles (protons and ions) can provide a 3D map of RSP of the object under study, directly reconstructed from energy loss data without any conversion needed .…”