ESTRO 2022 - Abstract Book

S650

Abstract book

ESTRO 2022

The effect of the ASC settings on the complexity of the leaf modulation is examined by calculating the modulation complexity score (MCS). In a next stage, all 30 treatment plans were delivered by a TrueBeam STx with high-definition MLC (HD120) to an OCTAVIUS 1000SRS (PTW, Freiburg) to measure the delivered dose. The measurements were then compared to the planned dose using gamma volume analysis with criteria of 2%/2mm in the Verisoft software (PTW). Results It was shown that a linear correlation exists between gamma pass rate and modulation complexity score (fig.1). The MCS increased (less complexity) when a higher ASC setting was used. The mean gamma pass rates for the different ASC settings are: 97.4%, 97.6%, 97.6% 97.8%, 98.1%, 98.5% for respectively, the original plan, very low, low, moderate, high and very high ASC setting (fig.2). The dose constraints of the original plan were still clinically accepted when using the ASC settings.

Fig.1.

Fig.2.

Conclusion The mean gamma pass rates as a function of different ASC settings show a slight increase when going from the very low to very high ASC setting, suggesting that higher ASC settings might provide a higher passing rate. Therefore ASC can be an interesting tool to be used in treatment planning of head and neck patients. [1] Nguyen M., Chan G.H. Quantified VMAT plan complexity in relation to measurement-based quality assurance results. J.Appl.Clin.Med.Phys. 2020;21:11:132-140.

PD-0738 Assessing the extent of treatment delivery errors among IROC H&N and lung phantoms

S. Edward 1 , R. Howell 1 , P. Balter 1 , C. Peterson 2 , J. Pollard-Larkin 1 , S. Kry 1

1 MD Anderson Cancer Center, Radiation Physics, Houston, TX, USA; 2 MD Anderson Cancer Center, Biostatistics, Houston, TX, USA Purpose or Objective The Imaging and Radiation Oncology Core (IROC) head and neck (H&N) and lung phantoms, used as an end-to-end test of an institution's radiotherapy treatment process, has an average failure rate of 14%. This work investigates whether delivery log files predict the observed dose deviations in these phantom irradiations. Materials and Methods Delivery log files were collected for IROC lung and H&N phantoms irradiated by various institutions from January 2019 to present. We evaluated these files in Mobius FX to determine the difference between the TPS-calculated and machine- delivered doses (as a perturbation of the TPS). We evaluated whether this perturbed dose more accurately predicted the actual delivered dose, measured by the thermoluminescent dosimeter (TLD). A total of 41 lung phantoms (82 TLDs) and 36