Initial Validation of Proton Dose Calculations on SPR Images from DECT in Treatment Planning System

Abstract

Purpose To investigate and quantify the potential benefits associated with the use of stopping-power-ratio (SPR) images created from dual-energy computed tomography (DECT) images for proton dose calculation in a clinical proton treatment planning system (TPS). Materials and Methods The DECT and single-energy computed tomography (SECT) scans obtained for 26 plastic tissue surrogate plugs were placed individually in a tissue-equivalent plastic phantom. Relative-electron density (ρe) and effective atomic number (Zeff) images were reconstructed from the DECT scans and used to create an SPR image set for each plug. Next, the SPR for each plug was measured in a clinical proton beam for comparison of the calculated values in the SPR images. The SPR images and SECTs were then imported into a clinical TPS, and treatment plans were developed consisting of a single field delivering a 10 × 10 × 10-cm3 spread-out Bragg peak to a clinical target volume that contained the plugs. To verify the accuracy of the TPS dose calculated from the SPR images and SECTs, treatment plans were delivered to the phantom containing each plug, and comparisons of point-dose measurements and 2-dimensional γ-analysis were performed. Results For all 26 plugs considered in this study, SPR values for each plug from the SPR images were within 2% agreement with measurements. Additionally, treatment plans developed with the SPR images agreed with the measured point dose to within 2%, whereas a 3% agreement was observed for SECT-based plans. γ-Index pass rates were > 90% for all SECT plans and > 97% for all SPR image–based plans. Conclusion Treatment plans created in a TPS with SPR images obtained from DECT scans are accurate to within guidelines set for validation of clinical treatment plans at our center. The calculated doses from the SPR image–based treatment plans showed better agreement to measured doses than identical plans created with standard SECT scans.

An evaluation of the use of DirectSPR images for proton planning in the RayStation treatment planning software

“…For all dose distributions validated here, the measurement depth was specifically chosen to be near the distal edge of the dose distributions, where the largest differences have been reported between SECT-based and DECT-based predictions.For all of our measurements, the pass rates were comparable even for the very tight (1%, 1 mm) passing criterion. These results differ from those from Mossahebi et al 6 where they observed a large change in gamma pass rates between SECT and SPR-based predictions, even for a very simple plan. We believe that this difference may be due to the fact that their predictions were done in a different treatment planning system and their SPR datasets were from a different source.…”

“…For all of our measurements, the pass rates were comparable even for the very tight (1%, 1 mm) passing criterion. These results differ from those from Mossahebi et al 6 . where they observed a large change in gamma pass rates between SECT and SPR‐based predictions, even for a very simple plan.…”

“…Given that one of the major benefits of proton therapy over photon therapy is the lack of exit dose on the distal edge of the target,such methods of range uncertainty mitigation can represent a major challenge in cases where an organ at risk resides near the distal target edge. 6 With the advent of dual-energy CT (DECT) scanners, it has now become possible to use the differences in images acquired using two different spectral distributions of x-rays to obtain more information on the composition of materials within each voxel and, in turn, calculate the stopping-power ratio (SPR) for that voxel more accurately. Recently a commercial solution (Direct-SPR™, Siemens Healthineers, Erlangen, Germany) has become available where the SPR data is calculated from DECT into a purportedly more accurate image set that can then be used for dose planning.…”

“…While multiple previous studies 6,[8][9][10][11] have investigated the feasibility of using SPR data generated from DECT for treatment planning,the purpose of this study is to specifically investigate the use of the newly available commercial SPR data within RayStation, and compare the results to the traditional approach of using a single-energy CT (SECT) for proton planning.…”

“…The traditional approach to dealing with such uncertainties has been to add distal and proximal margins to the target, leading to increased amounts of normal tissue being treated, and an effective undermining of much of the potential advantages of proton beam therapy. Given that one of the major benefits of proton therapy over photon therapy is the lack of exit dose on the distal edge of the target, such methods of range uncertainty mitigation can represent a major challenge in cases where an organ at risk resides near the distal target edge 6 …”