MRI-Based Prostate SBRT Treatment Planning Using Synthetic CT
J Scholey1*, T Nano2, X Miao3, P Larson4, M Descovich5, (1) University of California San Francisco, San Francisco, CA, (2) University of California San Francisco, San Francisco, CA, (3) Siemens Medical Solutions USA Inc., Boston, MA (4) University of California San Francisco, San Francisco, CA,(5) University of California San Francisco, San Francisco, CA
Presentations
PO-GePV-M-187 (Sunday, 7/25/2021) [Eastern Time (GMT-4)]
Purpose: To evaluate the feasibility of using a commercial MRI-derived synthetic CT (sCT) protocol for CyberKnife- and VMAT-based treatment planning for prostate SBRT.
Methods: Same-day CT and MRI simulations in the treatment position were performed on ten patients who were prospectively enrolled into this IRB-approved study and treated with SBRT to the prostate (40Gy in 5 fractions). Dixon in-phase and out-of-phase MRIs were acquired on a 3T scanner (MAGNETOM Vida, Siemens Healthcare) using a 3D T1-weighted gradient echo sequence (TE1/TE2/TR=1.23/2.46/4ms, readout bandwidth=1090 Hz/pixel, 1x1x1.5mm³, scan time=5mins) to generate sCT datasets with a bulk-assignment algorithm based on in-phase, out-of-phase, fat-only, and water-only MRIs. CT and sCT datasets were co-registered and HU values compared using mean absolute error (MAE). CyberKnife and VMAT treatment plans were generated on each patient CT and recomputed onto corresponding sCTs using the same plan parameters and CT HU calibration curve. Dose distributions were compared using 3%/3mm local gamma criteria and dose-volume-histograms.
Results: Relative electron density values computed from CT datasets and assigned to sCT datasets were 1.04, 0.95, 1.1, and 1.7g/cc for soft tissue, adipose, inner bone, and cortical bone, respectively. The proposed sCT protocol produced total MAE of 94±20HU. As expected, disagreement could be seen in instances of variable bowel/rectal gas filling, fiducials or calcifications appearing in the CT but not the sCT, and/or bone distribution differences, a common challenge in mapping sCT from MRI. Gamma criteria values were 98.9±0.9% (97.1-100%) and 97.7±1.3% (95.3-99.3%) for VMAT and CyberKnife plans, respectively. Dose delivered to 95% PTV volume in sCT plans was 100.5±0.8% (99.8-102.5%) and 97.0±6.0% (81.9-103.0%) of the prescription dose in the CT plans for VMAT and CyberKnife plans, respectively.
Conclusion: MRI-derived sCT using the proposed approach shows excellent dosimetry agreement with conventional CT simulation, demonstrating the feasibility of using MRI-derived sCT for prostate SBRT treatment planning.
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