Extracting Organ-Specific Radiobiological Model Parameters of Abdominal Organs from Clinical Outcome Data for Biological Radiation Treatment Planning

Abstract

<h3>Purpose/Objective(s)</h3> It is desirable to consider organ-specific treatment response in a variety of biological radiotherapy (RT) planning scenarios, e.g., adaptive RT with varying daily dose, reirradiation, multi-modality RT, varying fractionation. This study aims to extract organ-specific radiobiological model parameters for selected abdominal organs and to demonstrate their applications in determining OAR dose constraints using biologically effective dose (BED) and equivalent dose in 2Gy fractions (EQD2). <h3>Materials/Methods</h3> Available clinical studies from the literature reporting dose volume constraints for abdominal organs, e.g., duodenum, stomach, small bowel, and rectum, along with the compiled data in available sources such as TG101, QUANTEC, HyTEC, national and institutional trial protocols were analyzed. The data ranging from conventional fractionation RT (CFRT) to stereotactic body RT (SBRT) were analyzed to determine the maximum point doses (MPD) corresponding to no more than 5% grade ≥ 3 toxicity. The endpoints were selected according to toxicity scales (e.g., CTCAE (v3 or v4), RTOG). Iso-effect fittings (e.g., MPD vs. fractional dose) on the data collected for each organ were performed using both the linear quadratic (LQ) and the linear-quadratic-linear (LQL) models to extract the organ-specific radiobiological parameters. The dose constraint MPD in terms of BED and EQD2 for each OAR was calculated using the obtained organ-specific parameters. The use of the obtained MPD constraints was demonstrated in reirradiation planning. <h3>Results</h3> More than 50 studies were screened to find an average of 10 to 20 relevant studies for each OAR. The LQL and LQ models fit the clinical data from CFRT to SBRT reasonably well, although the LQL model showed a better fit compared to the LQ model. The table below shows the obtained LQ and LQL model parameters and the MPD constraints in terms of BED and EQD2 for selected abdominal organs, where χ²/df is the reduced chi-square. Using the data in the table, the MPD in the small bowel for a vaginal tumor case re-irradiated with 4 × 5Gy after one year from the initial RT of 10 × 3Gy was estimated as 50.2 Gy EQD2, below the constraint of 53.1 Gy. <h3>Conclusion</h3> We developed a robust method to successfully extract abdominal organ-specific radiobiological parameters from clinical data. These parameters can be used to convert OAR dose constraints from different RT fractionations/techniques for biologically optimized RT planning.