ESTRO 2023 - Abstract Book

S1443

Digital Posters

ESTRO 2023

Conclusion Instead of using single energy, energy-specific isocentricty must be measured and maintained within AAPM MPPG-9a guidelines of 1.0 mm to ensure the safety of treatment delivery. As shown here, if not measured individually, the isocentricity can drift to values above acceptable levels. Further studies will be performed to quantify the clinical impact.

PO-1726 Portal dosimetry pitfall for verification of 6 MV unflattened modulated plans

J. Calvo-Ortega 1,2 , P. Greer 3,4 , S. Moragues-Femenía 1 , M. Pozo-Massó 1

1 Hospital Quirónsalud Barcelona, Radiation Oncology, Barcelona, Spain; 2 Hospital Quirónsalud Málaga, Radiation Oncology, Málaga, Spain; 3 Calvary Mater Hospital Newcastle, Radiation Oncology, Newcastle, Australia; 4 University of Newcastle, School of Mathematical and Physical Sciences, Newcastle, Australia Purpose or Objective Breast cancer radiotherapy is planned in our department by using the single-isocenter multiple-field IMRT technique. In order to speed up the treatment delivery, an important issue when deep inspiration breath hold technique is used, 6X unflattened beams can be delivered using a repetition rate (RR) higher than in the case of flattened 6 MV beams. The aim of this study is to describe the sub-optimal performance of a portal dosimetry (PD) model used for patient specific quality assurance of 6X unflattened modulated plans. Materials and Methods Seven IMRT plans (66 fields) of breast cancer were selected. They were designed using the Acuros algorithm of the Eclipse v 16.1 treatment planning system, using 6X FFF photons beams (RR= 1400 MU/min) from a TrueBeam linac equipped with a Millennium 120 MLC and an aSi 1200 EPID. A PD verification was created for each clinical plan using the portal dose image prediction algorithm (PDIP v. 16.1), that was configured using the Van Esch package (J Appl Clin Med Phys. 2013 Nov 4;14(6):4286). For each individual beam, the PDIP algorithm calculates the expected EPID dose image to be compared with the measured one. 2D gamma index analysis was performed by the Varian Portal Dosimetry application. In addition, each clinical plan was checked by mapping every individual beam onto a polystyrene phantom (PTW Octavius top LINAC QA Top) with a PTW 1500 array at 5 cm depth. Planar dose calculated by Eclipse was compared with the measured one, by using the 2D gamma analysis of the PTW Verisoft v 4.1 software. Modulation metric (MUs per Gy prescribed) was computed for each field. Gamma passing rates (GPRs) resulting from the PD and Octavius-based verifications were compared for the 66 fields included in this study. The criteria recommended by the AAPM Task Group No. 218 were applied (3% global/2 mm and a 10% dose threshold), as well as the universal action limit of GPR ≥ 90%. Results 1) Octavius-based verification (Fig 1): GPR ≥ 90% was obtained for all fields. Excellent average GPR of 99.2% (SD: 1.5%) was obtained over all fields. 2) PD-based verification (Fig 1): 26 fields resulted in GPRs less than 90%. Over all fields, the average GPR was 82.7% (SD: 22.8%).

3) Modulation was not identified as a reason to obtain PD-based GPR < 90%, Fig 2.