KEY POINTS
- Two-part study of prostate cancer online adaptive radiotherapy using a cone-beam computed tomography-based adaptive platform with linear Boltzmann transport equation dose optimization.
- The simulation component included planning scans from 10 patients: 8 treated with an air-filled endorectal balloon and 2 with larger rectal air pockets.
- Water override within planning target volume and air overlap increased D1cc by 2.2% ± 0.7% for 2 mm posterior shift margins, 5.5% ± 0.7% for 5 mm, and 8.7% ± 1.0% for the entire rectal air cavity.
- Corresponding D1cc increases in the rectal wall were smaller: 1.4% ± 0.6%, 1.7% ± 0.9%, and 5.0% ± 0.5%, respectively.
- In the clinical component, 101 fractions from 15 patients treated with rectal balloons showed no clinically relevant systematic increase in rectal wall Dmax, D1, or D1cc; median intrafractional anterior rectal balloon surface motion was 0.7 mm, with 4.1 mm at the 90th percentile.
CLINICAL TAKEAWAY
This study is reassuring for prostate online adaptive radiotherapy when planning target volume and rectal air overlap remains limited and posterior soft-tissue shifts stay within 5 mm. Larger air overlap or larger posterior shifts can make linear Boltzmann transport equation-optimized plans more dose-sensitive, so rectal air remains a planning robustness issue rather than something to ignore. The evidence is dosimetric and retrospective, with no toxicity or outcome endpoint.