SU-E-T-557: Dose Calculation Accuracy Using Cone-Beam CT (CBCT) for Lung Stereotactic Body Radiotherapy (SBRT)

Abstract

Purpose: To evaluate the dose calculation accuracy using Varian's cone‐beam CT(CBCT) for lung stereotactic body radiotherapy(SBRT). Methods: Treatment plans of 2 lungSBRT patients (1 and 2) were retrospectively studied. Both plans used 3 or 4 6MV coplanar IMRT fields, generated on an in‐house Treatment Planning System (TPS) using Pencil‐Beam (PB) dose calculation with radiological path‐length correction. CBCTimages were taken prior to each treatment and a total 7 sets of CBCTimages were exported into TPS. Each CBCTimages set was registered with the planning CT based on matching of PTV PTV. Patient contours from planning CT were also imported into CBCT and only external contours were adjusted accordingly. CT numbers (HU+1000) were compared at chosen spots between CBCT and planning CT. Treatment fields were copied to CBCT and CBCT plans were generated. Results: CBCT CT‐numbers agree with planning CT‐number at low CT‐number (< 1100) region. For CT‐number 99, 887 and 1049, CBCT shows 83±24, 836±48 and 1054±26. For high CT‐ number 1571 and 1981, the CBCT CT‐numbers are 1730±55 and 2248±174. Mean dose for CBCT plans are slightly higher than those in treatment plans, which are 101.8% and 100.8% vs 102.0±1.1% and 102.1±0.1% of prescribed dose. Average D95 in CBCT plans are closed to those in treatment plan: 100.2±0.2%, 100.2±0.8% in CBCT 99.3% and 100.8% in treatment plan. However target inhomogeneity is higher in the CBCT plans, with average PTV D05 of 104.1±0.5% and 103.9±1.4% compared to 102.4% and 103.0% respectively. Critical normal tissuedoses in CBCT plans were approximately the same as the acceptable values predicted by treatment plan. Conclusions: The dosimetric differences between CBCT and planing‐CT were within 2% of each other. CBCT may be used to patient radiation dose verification besides patient setup and target verification for SBRT patients.