Prostate Intrafractional Motion In SBRT and PTV Margin Determination
Abstract
Purpose
Adaptive radiotherapy (ART) enables daily plan adjustment to account for anatomical changes. Given the high doses and tight margins in prostate SBRT, this study quantifies intrafractional prostate motion and its implications in the PTV margins.
Methods
Prostate SBRT treatments were delivered on a Varian Ethos ART platform. A total of 250 fractions from an institutional cohort were analyzed. For each fraction, an initial cone-beam CT (CBCT) was acquired for online adaptive planning. Following plan selection and approval, a second CBCT was obtained immediately prior to beam delivery to verify patient’s positioning. Translational prostate shifts were recorded, and three-dimensional displacement was calculated. The interval between the two CBCT acquisitions was recorded to evaluate any correlation between elapsed time and intrafractional motion. Shift distributions were assessed across all fractions.
Results
Intrafractional prostate shifts were predominantly small: 22% of fractions showed 0–1 mm shifts and 37% showed 1–2mm shifts. Shifts ≤3mm occurred in 77% of fractions, and ≤5 mm in 87%. Larger excursions were uncommon, with 12% exceeding 5mm and 2% exceeding 14 mm. The interval between the two CBCT scans ranged from 6.3 to 39 minutes across all fractions. No clear correlation between elapsed time and shift magnitude was observed (P = 0.399). For fractions with ~8-minute intervals, comparable to typical beam delivery time, the maximum observed shift was 3mm. Due to the dose gradient, ~3mm falloff exists between 100% and 90% isodose lines. Combined with the 3-mm PTV margin, prostate shifts up to ~6 mm would still maintain a minimal dose ≥90% of prescription dose, a condition met in 94% of fractions.
Conclusion
Intrafractional prostate motion during adaptive SBRT was generally small, with most shifts within 3mm. Occasional larger excursions were observed, indicating smaller PTV margins below 3mm are not supported by these data.