Evaluation of Relationship between Non-Adaptive Composite Dose and Adaptive Fraction Utilization
Abstract
Purpose
Online adaptive stereotactic body radiation therapy (SBRT) enables daily replanning to mitigate anatomic variation but increases treatment time and clinical workload. For left adrenal tumors, adaptive SBRT is frequently assigned due to the proximity of gastrointestinal (GI) organs at risk (OARs). This study retrospectively evaluates the relationship between composite OAR dose if no adaptations were to occur and adaptive fraction utilization to assess when daily adaptation meaningfully impacts cumulative dose relative to established clinical dose thresholds.
Methods
A retrospective cohort of patients treated with adaptive SBRT (50Gy in 5 fractions) for left adrenal tumors was analyzed. For each patient, per fraction dose to 0.5cc (D0,5cc) small bowel, large bowel, and stomach based on the simulation plan re-calculated on the daily anatomy were summed across the full treatment course. This generated a composite D0.5cc. The maximum OAR D0.5cc was subsequently plotted against the number of adapted fractions violating per fraction OAR constraints. Clinical dose thresholds used for treatment on MR and CT-guided platforms (33-36Gy) were overlaid to assess trends in fraction utilization in reference to dose in excess of these thresholds.
Results
Patients requiring a greater number of adaptive fractions demonstrated substantially increased composite OAR dose when evaluated against clinical thresholds. This reflects greater anatomic variability across fractions and the benefits of adaptation. In contrast, patients who received zero, one, or two adapted fractions largely did not exceed clinical thresholds for the GI-OARs with composite D0.5cc.
Conclusion
This composite dose analysis demonstrates that patients requiring limited adaptation may safely remain below established dose thresholds while those with greater anatomic variation derive clear benefit from adaptation. These findings suggest that a non-zero threshold of number of fractions requiring adaptation could be utilized to indicate a patient can forgo adaptive treatment for standard non-adaptive treatment.