Room: Exhibit Hall
Purpose: Modern pre-clinical irradiation systems use kilovoltage x-rays to deliver small fields of radiation in static beams or arc-based treatments. The systems are costly and the RBE of kilovoltage beams is known to differ from the megavoltage photons used to treat patients 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 with rapid dose fall-off.
Methods: A treatment field defined by a single opposed leaf pair was used to deliver arc-based treatments. Single arc, coplanar deliveries and multiple arc, non-coplanar deliveries were studied. Developer mode was used to implement dynamic couch trajectories to create a shortened virtual isocenter. A two-step process was used in which a pre-treatment imaging procedure quantified target misalignment at control points along the arcs and determined appropriate couch positional corrections. This was followed by the treatment arc in which the positional corrections were implemented. Monte Carlo simulations and radiochromic film were used to calculate and measure dose distributions.
Results: A 1mm leaf separation produced as compact a dose distribution as a 0.2mm separation in the high dose region of the field but produced significantly better low dose profile width. The maximum magnitude of couch position correction was 2.1mm, and the maximum in any one direction (lateral,longitudinal or vertical) was 1.6mm. Application of couch position corrections reduced the non-coplanar arc treatments dose profile by 1.2mm at the level of 30% of maximum dose. Treatment of a 1mm diameter target prescribed to the 90% isodose line would produce falloff distances to the 80%, 50% and 25% of the 90% prescription line of 0.3, 0.5 and 1.3mm from the target edge respectively.
Conclusion: This work has demonstrated that it is possible to deliver highly compact dose distributions using megavoltage photon beams from existing clinical infrastructure.
Not Applicable / None Entered.