Room: AAPM ePoster Library
Purpose: Our research group previously estimated the radiobiological parameter values for cervical cancer cells by fitting in vitro experimental cell survival data with the modified Linear Quadratic model. Smaller a/ß (4.71-6.63 Gy) and larger T(1/2) (1.6-3.9 hours) values were determined than conventionally assumed (10 Gy and 1.5 hours, respectively). This research refines our analysis by incorporating the effects of identified experimental uncertainties.
Methods: Radiobiological parameters of three cervical cancer cells were determined by conducting clonogenic assays with radiation delivered in either a single acute fraction (HDR) or hourly pulses (PDR) using clinical Ir-192 afterloaders. Uncertainties were identified and quantified for the following: source positioning, source calibration, treatment planning software dose calculation, transit time effects, colony counting, and variations in cellular experimental conditions. Least chi-squared fitting of experimental survival data incorporating these uncertainties was used to refine a/ß and T(1/2).
Results: Uncertainties in the dose delivered and the cell survival fraction was estimated to be <1.5% and <4%, respectively. The largest potential uncertainties were in the source strength calibration and due to variations in cell uniformity across the tissue culture dishes; conversely, positional uncertainties and variations in cellular conditions (e.g. temperature, time between cell seeding, and dose delivery) were not significant contributors. The range of calculated a/ß, 3.64-8.03 Gy, and T(1/2), 1.28-5.13 hours, are nearly similar to our previous results not incorporating these experimental uncertainties. Based on these values, the assumed equivalency of conventional HDR and PDR brachytherapy could be impacted for at least one cell line, with PDR delivering >19 Gy EQD2 more than HDR (>30 Gy when considering the full range of uncertainties).
Conclusion: Analysis incorporating experimental uncertainties reinforces our previously reported trends in cervical cancer radiobiological parameter values and suggest PDR brachytherapy may deliver more radiobiological dose than conventionally equivalent clinical HDR boosts, potentially impacting patient outcome.