Prostate stereotactic body radiotherapy: quantifying intra-fraction motion and calculating margins using the new BIR geometric uncertainties in daily online IGRT recommendations

Abstract

Objectives: To measure the magnitude of intra fraction prostate motion (IFPM) during stereotactic radiotherapy (SBRT) delivered without intrafraction tracking. To assess if current margins adequately cover IFPM. To derive margins using new guidelines Methods: IFPM was determined in 20 patients receiving 36.25 Gy in 5 fractions using 97 pairs of pre and post treatment cone beam CT (CBCT) scans. Correlation of time between CBCT acquisitions and motion was determined. The magnitude of IFM was compared to current margins (6 mm isotropic, 3 mm posterior). Margins were calculated using IFM alone and updated guidelines. The averaged 3D root mean square IFPM was 2.5 mm (4.2 mm). Independent prostate motion was predominantly posterior (70%) and inferior (63%). There was weak correlation between posterior (ρ = 0.38) and inferior (ρ = 0.36) IFPM and time. IFPM greater than current margins occurred in eight of 97 fractions, six in the posterior direction. Margins were ≤3.5 mm using IFM alone and ≤3.3 mm Left3.5mm Right, 7.0 mm inferior, 3.7 mm superior, 4.4 mm anterior and 3.3 mm posterior using new guidelines, compensating for motion in 92% of fractions. Conclusions: Our current SBRT margins account for 92% of IFPM, predominantly posterior and inferior. Although updated guidelines suggest an increase in margins inferiorly, any increase must be balanced against the possibility of increased toxicity, particularly if biochemical control and side-effects are favourable with current practice. Advances in knowledge: The difference between current clinical margins and those determined using updated guidance is demonstrated. The implications must be considered against clinical outcomes.

Stereotactic body radiation therapy (SBRT) for prostate cancer: Improving treatment delivery efficiency and accuracy

“…Similar prostate displacements were found in other studies [34] , [35] , which highlighted that any reduction in overall treatment time could be beneficial for patients as reduces treatment uncertainties resulting from intrafractional motion. McNeice et al [35] showed that the PTV margins used in our work (6/3 mm) adequately account for most of intrafraction motion.…”

“…However, this would require the analysis of the contribution to margins of other sources of errors, e.g. delineation errors [35] , and will be investigated separately in a future study.…”

Treatment Optimization in Linac-Based SBRT for Localized Prostate Cancer: A Single-Arc versus Dual-Arc Plan Comparison

“…While slight target displacements during normo-fractionated treatments are generally tolerable because they occur randomly and average out during the treatment course, their impact becomes more critical in SBRT due to potential long-term effects and reduced statistical averaging of errors. Moreover, in high-dose-per-fraction treatments, it is important to balance any potential margins’ expansion against the risk of increased toxicity, especially if current practices are yielding favorable biochemical control and side effects [ 27 ]. Therefore, intrafraction motion management and real-time monitoring have emerged as strategies to ensure accurate radiation delivery and minimize adverse effects but with considerable and continuous investment of resources and time.…”