Session Item

Intra-fraction motion remains an important consideration in stereotactic body irradiation (SBRT) for prostate cancer because of the risk of serious adverse effects. With 10-year mortality less than 5% for low- and intermediate-risk patients, and a significant increase in the use of SBRT, robust techniques for reducing these effects are required. The aim of this study was to assess the potential of the RayPilot^® real-time motion management system for monitoring intra-fraction motion and consequently for margin reduction, which would lead to reduced side-effects.

Radiotherapy planning computed tomography (CT) images, treatment plans and cone beam CT (CBCT) images were collected at each fraction (N=5) of SBRT (36.25 Gy in 5 fractions) treatment for low- (N=7) to intermediate-risk (N=4) prostate cancer patients (Table 1). Inter-fraction motion displacements in the lateral, longitudinal and vertical planes were recorded every second by the RayPilot® system and used as input to the van Herk margin formula (VHMF) to establish a set of new planning target margins. All patients were replanned on the Varian Eclipse treatment planning system (TPS) using the original fractionation schedule and a new schedule (24 Gy in 3 fractions) based on the new margins.  Dose coverage was assessed for the clinical target volume (CTV), rectum and bladder.

Using this approach the planning margins were reduced from 1.0 cm to 0.67 cm in the lateral, 1.0cm to 0.67 cm in the longitudinal and 0.8 cm to 0.67 cm in the vertical directions, which were found to provide adequately dosimetric coverage of the CTV whilst accounting for inter- and intra-fraction motion on each patient. In addition there was a mean reduction in overall volume irradiated of 37.1% (min 32.4% and max 39.9%)(Table 2) . For both the original fractionation schedule and the new schedule organ-at-risk (OAR) toxicity was within tolerance.

This pilot study demonstrates that data using the RayPilot^® device it is possible monitor intra-fraction motion and as a result reduce treatment margins and subsequently SBRT fractions. However, analysis is required on a much larger cohort before any reduction in margins can be recommended.