Modeling dosimetric benefits from daily adaptive RT for gynecological cancer patients with and without knowledge-based dose prediction

Abstract

Purpose: Daily online adaptive radiotherapy (ART) improves dose metrics for gynecological cancer patients, but the on-treatment process is resource-intensive requiring longer appointments and additional time from the entire adaptive team. To optimize resource allocation, we propose a model to identify high-priority patients.

Methods: For 49 retrospective cervical and endometrial cancer patients, we calculated two initial plans: the treated standard-of-care (Initial_SOC) and a reduced margin initial plan (Initial_ART) for adapting with the Ethos treatment planning system. Daily doses corresponding to standard and reduced margins (Daily_SOC and Daily_ART) were determined by re-segmenting the anatomy based on the treatment CBCT and calculating dose on a synthetic CT. These initial and daily doses were used to estimate the ART benefit ( $\Delta Daily$ = Daily_SOC-Daily_ART) versus initial plan differences ( $\Delta Initial$ = Initial_SOC-Initial_ART) via multivariate linear regression. Dosimetric benefits were modeled with initial plan differences ( $\Delta Initial$ ) of $Bowel{V}_{40Gy}$ (cc), $Bladder{D}_{50\%}$ (Gy), and $Rectum{D}_{50\%}$ (Gy). Anatomy (intact uterus or post-hysterectomy), DoseType (simultaneous integrated boost [SIB] vs. single dose), and/or prescription value. To establish a logistic model, we classified the top 10% in each metric as high-benefit patients. We then built a logistic model to predict these patients from the previous predictors. Leave-one-out validation and ROC analysis were used to evaluate the accuracy. To improve the clinical efficiency of this predictive process, we also created knowledge-based plans for the ΔInitial plans ( $\Delta Initia{l}_{RP}$ ) and repeated the analysis.

Results: In both $\Delta Initia{l}_{Orig}$ and $\Delta Initia{l}_{RP}$ our multivariate analysis showed low R² values 0.34-0.52 versus 0.14-0.38. The most significant predictor in each multivariate model was the corresponding ∆Initial metric (e.g., $\Delta Initial$ Bowel (V40 Gy), p < 1e-05). In the logistic model, the metrics with the strongest correlation to the high-benefit patients were $Bowel{V}_{40Gy}$ (cc), $Bladder{D}_{50\%}$ (Gy), $DoseType$ , and $SIBDose$ prescription. The models for original and knowledge-based plans had an AUC of 0.85 versus 0.78. The sensitivity and specificity were 0.92/0.72 and 0.69/0.80, respectively.

Conclusion: This methodology will allow clinics to prioritize patients for resource-intensive daily online ART.

Keywords: IGRT; adaptive radiotherapy; cervical cancer; cone‐beam CT; dosimetry; target margins.