Distribution of Interfractional Dosimetric Variation of HRCTV in Cervix HDR Brachytherapy
B Washington*, M Randall, D Fabian, D Cheek, C Wang, W Luo, University of Kentucky, Lexington, KY
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PO-GePV-T-25 (Sunday, 7/10/2022) [Eastern Time (GMT-4)]
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Purpose: To evaluate and quantify the interfractional dosimetric variation (IDV) of the high-risk clinical target volume (HRCTV) from the prescribed dose in high dose rate (HDR) tandem and ovoid (T&O) brachytherapy.
Methods: Fifty patients diagnosed with cancers of the uterine cervix and treated with HDR T&O brachytherapy were retrospectively analyzed. The dose absorbed by 90% of the HRCTV (D_90) was the dosimetric parameter of interest. Each patient’s IDV was defined as the average dosimetric variation from the prescribed dose throughout their course of brachytherapy. The distribution of IDVs were assessed against eighty-nine probability density functions (PDF) to determine the best fit distribution via their respective residual sum of squares (RSS) score and Anderson Darlington (AD) test to determine statistical significance. This allowed the identification of trends in the data and the calculation of HRCTV under-dose (-5% variation from prescription) and overdose (+5% variation) probabilities via the corresponding cumulative distribution function (CDF). These calculated probabilities were compared to under and overdose probabilities calculated from a fitted normal distribution and the raw data.
Results: HRCTV IDVs from the prescribed dose are not normally distributed and are best described as a left skewed distribution to model the tendency to under-dose the HRCTV. In this study, the best fit distribution for IDVs was the Generalized-Extreme Value (GEV) distribution. The GEV yielded under and overdose probabilities of 30.0% and 23.3%, respectively. The fitted normal distribution yielded under and overdose probabilities of 41.2% and 18.7%, respectively.
Conclusion: HRCTV IDVs from the prescribed dose are not normally distributed and are best described by a left skewed distribution. The observed under-dose trend is due to the need to spare organs at risk. This can be modeled as a 30.0% under-dose and 23.3% overdose probability. Non-normality of IDVs must be considered when studying HDR T&O brachytherapy uncertainty and dosimetry.
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