SU-E-T-550: Significant Improvement in Agreement Between Calculated and Measured Dose Distributions: Proper Modeling of Elekta LINAC Couchtops in Pinnacle Treatment Planning System

Abstract

Purpose: To demonstrate the importance of properly modeling the treatment couch to dose calculation in patient treatment using arc therapy. Methods: Two treatment couch tops (Aktina AK550 and Elekta iBeam evo) of Elekta LINACs were scanned using a CT simulator. Various parts of the couch tops were contoured and their densities were measured and recorded by the Pinnacle treatment planning system (TPS) using the established CT density table. These contours were saved as organ models to be loaded into a treatment plan, and placed beneath the patient/phantom. 10 different patient plans (5 VMAT, 5 SBRT) using arc therapy were evaluated using the Delta4 dosimetry system. The Delta4 phantom measurements were compared to calculated plans both with and without modeling the couch, and the differences were evaluated on an individual beam basis for a total of 24 arcs. Student's t test was utilized to determine statistical significance. Results: The improvement in average pass rate within a dose difference of 3% was 3.1% (76.9% to 80%) for VMAT beams with p = 0.046. The improvement in pass rate with λ-index < 1 (3%, 3mm) was 1.4% (96.8% to 98.1%) with p=0.018. For SBRT, only the dose difference improvement was significant at an average improvement of 3.3% (73.4% to 76.7%) and p = 0.0027. The maximum improvement in dose and λ-index pass rate for a single VMAT beam was 16.5% and 5.0%, respectively. The same for a single SBRT arc was 10.5% and 15.8%, respectively. Conclusion: Ignoring the treatment couch, a standard of practice in 3D and static IMRT planning in Pinnacle TPS, can overestimate the dose delivered especially using arc therapy. This work shows that modeling the couch during treatment planning can improve the agreement between calculated and measured dose distributions. Consequently, we have implemented these couch models clinically.