ESTRO 2021 Abstract Book

S1301

ESTRO 2021

Sweden), were included for analysis. IMRT plans of 3x20Gy were created on the mid-position planning CT. A patient-specific PTV margin was applied, accounting for target delineation (TD) uncertainty, residual target misalignment and intra-fraction motion. A daily 4D-MRI was used to correct for target misalignment with adapt-to-position. Intra-fraction motion monitoring was performed with repetitive 4D-MRIs. Retrospectively, the dose distribution of each delivered beam was shifted according to the tumor displacement in each phase of the corresponding intra- fraction 4D-MRI and finally accumulated over 3 fractions. To assess adequacy of D DA to the target, we performed two evaluations: 1) Creating a dose evaluation volume (DEV) by expanding the GTV(=CTV) with margin M : M = α (Σ Tot – Σ DA ) + β (σ Tot – σ DA ) , where the subscript Tot refers to the root-sum-squares of all components of the systematic ( S ) and random ( s ) geometric errors taken into account in the total PTV margin and DA to those accounted for in the dose accumulation. In this work, the TD error ( S TD =3mm) was the only component not taken into account in the DA and a =2.5, resulting in M »5mm. DEV_D 98% was evaluated. 2) Simulating residual uncertainties for 10.000 virtual treatments per patient with Monte Carlo (MC) error simulations. The delineation error was approximated by 10.000 random 3D shifts (Gaussian distribution; SD=3mm) of the GTV within D DA . The probability (P) that 100% of the prescribed dose (D) was delivered to 98% of the target volume (V) was calculated (P D=100%,V=98% ). Additionally, we evaluated D P=90%,V=98% and V P=90%,D=100% . Results Results are displayed in Table 1. DEV_D 98% and D P=90%,V=98% were corresponding within 2.4%. Pearson’s correlations between DEV_D 98% and P D=100%,V=98% , D P=90%,V=98 and V P=90%,D=100% were 0.97, 1.00 and 0.96. Mean P D=100%,V=98% over all patients was 92.5%.

Conclusion Residual uncertainties in DA were evaluated with a DEV and MC error simulations. Both methods provide similar information for individual patients. The MC probabilities can be meaningfully averaged over patients to evaluate target coverage on a population level. For this cohort, the mean P D=100%,V=98% was 92.5%, which is in good agreement with the 90% population for which the margins were designed.

Digital Poster: Basic dosimetry and phantom and detector development

PO-1576 A novel anthropomorphic phantom for the commissioning of MLC-based stereotactic radiosurgery. T. Brown 1 , C. Beck 1 , C. Holloway 1 , J. Kerns 2 , J. Fagerstrom 1 , D. Kaurin 1 , K. Kielar 2 1 Northwest Medical Physics Center, Department of Education & Research, Lynnwood, WA, USA; 2 Varian Medical Systems, Global Portfolio Solutions, Palo Alto, CA, USA Purpose or Objective A new anthropomorphic head phantom has been designed by Varian Medical Systems for the purposes of commissioning MLC-based SRS treatments on TrueBeam and Edge linear accelerators. The validation of the phantom prototype and development of an end-to-end testing procedure was performed by Northwest Medical Physics Center, a non-profit, clinical physics consulting group, at two independent community cancer centers with active SRS programs. Materials and Methods The initial phantom prototype was designed to accommodate four interchangeable target cassettes for CT-MRI fusion verification, Winston-Lutz and hidden target tests, an ion chamber, and two perpendicular EBT3 film segments. A 2-cm, contrast-enhanced target located at the center of the ion chamber and film cassettes allowed for single-target SRS verification. Phantom testing and end-to-end procedure development were performed using a Varian TrueBeam and Edge linear accelerators equipped with Millennium and high-definition MLC, respectively. Treatment plans at 6FFF and 10FFF were designed and tested using a nominal prescription dose of 18 Gy. End-to-end testing comprised of phantom simulation, isocenter size determination, and treatment delivery using a pinpoint ion chamber and EBT3 film. Results Initial prototype testing resulted in design changes for a revised, final design, including a change in the Winston-Lutz central marker material, addition of a second (1 cm) target to the film cassette, inclusion of fiducial points to the EBT3 film for registration with DICOM dose planes, and an adjustment to the external dimensions of the film cassette. Pinpoint ion chamber measurements showed agreement with the planned dose to within 3% for all plans tested. DoseLab software was used to perform a relative gamma analysis of the film dose planes compared to extracted data from the treatment planning system. Film irradiations showed gamma-passing results >90% for 3% and 1 mm, using auto-registration shifts ≤1 mm in any direction. Conclusion Initial testing of a new anthropomorphic SRS head phantom indicates that the system is robust for verifying end-to-end MLC-based SRS treatments. Ongoing work is now concerned with collecting end-to-end data, using the revised phantom design, from additional clinical sites for the purposes of establishing acceptance criteria for end-to-end measurements.