ESTRO 2023 - Abstract Book

S1715

Digital Posters

ESTRO 2023

Conclusion We successfully quantified plan quality and evaluated the robustness of aperture-based CPBs, LPBs, and LPMBs for brain metastases. The LPB and LPMB stand as excellent alternatives to CPB or photon therapy and can significantly increase the preservation of normal tissue. Aperture-based IMPT plans can also achieve better plan quality than Gamma Knife plans.

PO-1954 Estimating the RBE of carbon ion therapy by common models using measurable microdosimetric inputs

S. Hartzell 1 , F. Guan 2 , P. Taylor 1 , C. Peterson 3 , S. Kry 1

1 MD Anderson Cancer Center, Radiation Physics, Houston, USA; 2 Yale University School of Medicine, Department of Therapeutic Radiology, New Haven, USA; 3 MD Anderson Cancer Center, Department of Biostatistics, Houston, USA Purpose or Objective Inconsistencies in dose delivered during carbon radiotherapy is due largely to uncertainties in RBE, which is calculated using one of several models that typically require Monte Carlo computed input parameters. While RBE by Microdosimetric Kinetic Model (MKM) can be calculated using a measured microdosimetric values, there exist no direct means of measuring RBE by other common models, including Repair Misrepair Fixation (RMF) and Local Effect Model I (LEM). This study investigates estimating RBE using a uniform, microdosimetric measurement for each model, allowing measurement-based validation of model implementation and RBE comparison across models and institutions. Materials and Methods Monte Carlo simulations were used to calculate the biophysical properties of carbon ions that serve as RBE input parameters: microdosimetric quantities, double-strand break yields, and kinetic energy distributions for 10 clinically realistic carbon beams (7 monoenergetic/3 SOBP). For each MKM, RMF, and LEM, we calculated particle α and β , and subsequent RBE according to model definitions using proper input parameters. Next, model-specific particle α and β values from two beams were plotted as a function of the corresponding positional microdosimetric value, y*, as scored within a TEPC to represent a case for which the input had just been measured, and fit with a polynomial (example in Figure 1). This fit was used to estimate particle α and β for remaining beams to validate the robustness of the fit. The percent difference between true RBE and RBE calculated with TEPC-simulated y* was calculated to quantify the uncertainty in this measurement-based approach to estimating RBE. A ±5% threshold was selected to determine acceptability.

Results The estimation method had a ±5% accuracy in 91% of data points across all beams and all three models. A histogram of estimation accuracy is shown in Figure 2 for RMF and LEM. For all models, accuracy in the entrance region was typically <1%, while greatest error was seen in areas of rapidly changing physical dose and LET (just proximal to Bragg peak/SOBP). This was tolerable, particularly in a clinical audit framework, as these regions are already areas of high dosimetric uncertainty. The effect of physical dose on the estimation method was also evaluated, and increases in dose were found to notably improve accuracy.