Abstract
PurposeTo 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 MethodsThe 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.ResultsFor 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.ConclusionTreatment 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.
Highlights
To address range uncertainties associated with proton therapy, additional margins are added to clinical target volumes, which are treated with the full prescription dose [1,2,3]
The SPR images of 10 tissuesurrogate plugs individually inserted into a high-density polyethylene (HDPE) phantom were imported into a commercial treatment planning system (TPS), and clinical proton treatment plans were developed
On average, the smallest difference between measured and dual energy computed tomography (CT) (DECT)-generated SPR values was obtained with the Maryland Proton Treatment Center (MPTC) model, the SPR images generated with that model were used for subsequent treatment planning and dose-delivery studies presented in this article
Summary
Current standard clinical practice involves the use of a 3.5% þ 1–2 mm [1, 2] margin with respect to the distal end of all treatment beams, which is added to the clinical target volume. As a result, these extra margins increase the volume of healthy tissues surrounding the tumor that receives a therapeutic dose. Many researchers are actively pursuing methods of reducing the intrinsic uncertainties in proton and heavy-ion beam radiation therapy. These include the use of in vivo imaging and range-monitoring techniques to verify daily beam delivery [4,5,6,7,8], as well as methods to improve the precision of dose calculation in the patient, such as improved Monte Carlo models [2, 9], proton radiography and proton computed tomography (CT) [10,11,12], and the use of dual energy CT (DECT) for patient CT simulations [13,14,15,16,17]
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