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Introducing Uncertainty to Measurement Based RBE Assessment in Carbon Ion Radiotherapy

S Hartzell1*, F Guan2 , O Vassiliev3 , C Peterson4 , P Taylor5 , S Kry6 , The University of Texas MD Anderson Cancer Center, Houston, TX


(Wednesday, 7/17/2019) 10:30 AM - 11:00 AM

Room: Exhibit Hall | Forum 7

Purpose: One of the largest inconsistencies in dose delivered during carbon ion therapy is due to uncertainties in relative biological effectiveness (RBE), a value that is calculated via one of several clinically implemented algorithms. This study investigates the uncertainty in the measured input parameters for RBE calculation by the Microdosimetric Kinetic Model (MKM) and Repair Misrepair Fixation model (RMF).

Methods: Microdosimetric spectra were calculated using Monte Carlo (GEANT IV) for monoenergetic carbon beams of clinical energy. From these spectra, both dose and frequency mean lineal energy values were calculated as functions of initial beam energy and depth for clinically relevant beam energies. These values were used to calculate RBE based on two clinically implemented algorithms, MKM and RMF. The impact on the RBE from six unique sources of uncertainty associated with Tissue Equivalent Proportional Counter (TEPC) measurements were simulated: electronic uncertainty, gas pressure, W-value, energy calibration, low energy cut-off, and counting statistics. These sources were quantified by statistically introducing uncertainty into the simulated measurements 100 times and sampling the resultant RBE (associated with each of the 100 perturbations).

Results: The uncertainty introduced by the sources of physical noise varied depending on the model used (MKM vs. RMF), the depth of the measurement, and the beam energy. The largest source of uncertainty was associated with the W-value (i.e., calibration of the detector), which had an uncertainty of typically 2% (1-sigma). Overall, the total 1-sigma uncertainty in the RBE based on uncertainty in TEPC measurements were 2-4%.

Conclusion: The overall uncertainty in RBE was typically less than 3% and was, at most, 4.0% (1-sigma). While the true RBE has extensive uncertainty associated with it, the modeled RBE can be measured with good accuracy, within a 5% deviation, which is the tolerance reasonable goal for assessing delivered dose.


Not Applicable / None Entered.


TH- External Beam- Particle therapy: Carbon ion therapy - quality assurance

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