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Assessment of Microdosimetric Measurement Uncertainty in Carbon Ion Radiotherapy

S Hartzell*, F Guan, O Vassiliev, P Taylor, C Peterson, S Kry, The University of Texas MD Anderson Cancer Center, Houston, TX


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: is an important tool to assess mixed radiation fields produced in carbon radiotherapy, and serves as a direct input parameter to multiple common models of biological effectiveness. The most common detector used to make microdosimetric measurements is the Tissue-Equivalent Proportional Counter (TEPC), which is subject to various inherent uncertainty resulting from detector design and beam configuration. This work quantifies the magnitude of micrododsimetric measurement uncertainty in TEPC measurements in carbon beams.

Methods: spectra and subsequent lineal energy values, frequency-, dose-, and saturation corrected dose-mean lineal energy (y(F),y(D),y*), were calculated using Geant4 Monte Carlo for five monoenergetic and three SOBP carbon beams incident on a water phantom. Data was collected at one millimeter intervals, yielding 9,600 unique microdosimetric values. The impact on microdosimetric spectra from eight random and systematic sources of uncertainty associated with TEPC measurements were simulated: wall effects, pulse pile-up, electronics, gas pressure, W-value, gain instability, low energy cut-off, and counting statistics. Uncertainty was quantified by statistically introducing each source into simulated spectra 200 times and sampling resultant data. Variance was quantified as the 1s standard deviation about the 200 perturbed values for each source. Bias was quantified as the percent difference between default lineal energy values and the mean of perturbed values for each systematic source.

Results: introduced by random noise in y(F) and y(D) averaged 5.0% and 4.7%, respectively, and just 3.3% in y*. Bias similarly reached 5.0% in y(F) and y(D), but averaged just 3.6% across all data points for y*. The largest respective contributors to variance and bias were W-value calibration and pulse pileup.

Conclusion: study establishes an error budget for microdosimetric carbon measurements by quantifying the uncertainty associated with TEPC measurements. Measuring microdosimetric values with reasonable accuracy is needed to verify clinical implementation of biological efficacy models and establish consistency in delivered dose.

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Microdosimetry, Heavy Ions, Monte Carlo


TH- External Beam- Particle/high LET therapy: Carbon ion therapy - calibration protocols and primary standards (including detector response simulation)

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