Toward a pre-clinical irradiator using clinical infrastructure

Purpose: Pre-clinical irradiation systems use kilovoltage x-ray systems to deliver small fields of radiation in static beam arrangements or arcs. The systems are costly and the radiobiological effectiveness of kilovoltage beams is known to differ from the megavoltage photon beams used clinically. This work used Developer mode on the Varian TrueBeam STx linear accelerator to create a pre-clinical irradiator capable of treating millimeter-sized targets. Materials and methods: A treatment field defined by a single opposed leaf pair was used to deliver arc-based treatments. Dynamic couch trajectories were used to create a shortened virtual isocentre. Initially, a pre-treatment imaging procedure was used to quantify target misalignment at control points along the arcs and determine appropriate couch positional corrections. This was followed by the treatment arcs in which the positional corrections were implemented. Monte Carlo simulations and radiochromic film were used to calculate and measure dose distributions. Results: A 1 mm leaf separation produced the optimal dose distributions. Couch position corrections up to 2.1 mm were required to maintain a target at virtual isocentre. Application of couch corrections reduced noncoplanar arc treatments dose profile by 1.2 mm at 30% of the maximum dose. Treatment of a 1 mm diameter target would result in falloff distances to the 80%, 50% and 25% of the 90% prescription line of 0.3 mm, 0.5 mm and 1.3 mm from the target edge respectively. Conclusions: This work has demonstrated that it is possible to deliver highly compact dose distributions using megavoltage photon beams from existing clinical infrastructure.