Forecasting per-patient dosimetric benefit from daily online adaptive radiotherapy for cervical cancer

Adaptive Radiotherapy (ART) is an emerging technique for treating cancer patients which facilitates higher delivery accuracy and has the potential to reduce toxicity. However, ART is also resource-intensive, requiring extra human and machine time compared to standard treatment methods. In this analysis, we sought to predict the subset of node-negative cervical cancer patients who benefit the most from ART. CT images, initial plan data, and on-treatment Cone-Beam CT (CBCT) images for 20 retrospective cervical cancer patients were used to simulate doses from daily non-adaptive and adaptive techniques. We evaluated the correlation (R$^2$) between dose and volume metrics from initial treatment plans and the dosimetric benefits to the Bowel V$_{40Gy}$, Bowel V$_{45Gy}$, Bladder D$_{mean}$, and Rectum D$_{mean}$ from adaptive radiotherapy using reduced 3mm or 5mm CTV-to-PTV margins. The LASSO technique was used to identify the most predictive metrics for Bowel V$_{40Gy}$. The three highest performing metrics were used to build multivariate models with leave-one-out validation for Bowel V$_{40Gy}$. Patients with higher initial bowel doses were correlated with the largest decreases in Bowel V$_{40Gy}$ from daily adaptation (linear best fit R$^2$=0.77 for a 3mm PTV margin and R$^2$=0.8 for a 5mm PTV margin). Other metrics had intermediate or no correlation. Selected covariates for the multivariate model were differences in the initial Bowel V$_{40Gy}$. and Bladder D$_{mean}$ using standard versus reduced margins and the initial bladder volume. Leave-one-out validation had an R$^2$ of 0.66 between the predicted and true adaptive Bowel V$_{40Gy}$ benefits for both margins. This work could be used to prospectively triage cervical cancer patients, and presents a critical foundation for predicting benefits from daily adaptation that can be extended to other patient cohorts.