Assessment of planning organ at risk volume margins in non-adaptive pancreatic stereotactic body radiotherapy.

Background and Purpose: Organ at risk (OAR) motion results in increased risk of radiotherapy treatment side effects due to the potential for greater dose than intended being delivered. This is of particular concern in stereotactic ablative body radiotherapy (SABR). The study aim was to assess the role of planning organ at risk volume (PRV) margins applied to OARs of the gastrointestinal (GI) tract over five fraction non-adaptive SABR for pancreatic cancer. Materials and Methods: The clinical adaptive magnetic resonance-guided radiotherapy baseline planning for 10 patients was combined with retrospective replanning respecting constraints to OAR plus a margin of 2-5 mm. Baseline planning/replanning was applied to OAR contouring at treatment fractions without plan adaptation. The impact of PRV margin was assessed in terms of the encompassing of OAR motion, effect on delivered dose, and degree of associated compromise in planned treatment volume (PTV) dose coverage. Results: Whilst variable across individual OARs, a 2-5 mm margin poorly encompassed motion in 40-70% of fractions. However, a 2-5 mm margin did effectively limit dose increases in most cases such that delivered biological effective dose (BED) did not exceed, on average, the baseline value. A margin of 2-3 mm resulted in minor PTV compromise (D Conclusions: A PRV margin of 2-5 mm for OARs limits the increase in delivered dose. Consideration of such a margin is relevant in optimizing non-adaptive treatment practice.