Abstract
Recent studies have indicated that radiotherapy treatments undertaken on a flattening filter-free linear accelerator have a number of advantages over treatments undertaken on a conventional linear accelerator. In addition, 4 MV photon beams may give improved isodose coverage for some treatment volumes at air/tissue interfaces, compared to when utilising the clinical standard of 6 MV photons. In order to investigate these benefits, flattening filter free beams were established on an Elekta Beam Modulator linear accelerator for 4 MV photons. Commissioning beam data were obtained for open and wedged fields. The measured data were then imported into a treatment planning system and a beam model was commissioned. The beam model was optimised to improve dose calculations at shallow, clinically relevant depths. Following verification, the beam model was utilised in a treatment planning study, including Volumetric Modulated Arc Therapy (VMAT), for a selection of lung, breast/chest wall and larynx patients. Increased dose rates of around 800 MU/min were recorded for open fields (relative to 320 MU/min for filtered open fields) and reduced head scatter was inferred from output factor measurements. Good agreement between planned and delivered dose was observed in verification of treatment plans. The planning study indicated that with a flattening filter-free beam, equivalent (and in some cases improved) isodose profiles could be achieved for small lung and larynx treatment volumes relative to 4 MV filtered treatments. Furthermore, flattening filter-free treatments with wedges could be replicated using open fields together with an 'effective wedge' technique and isocentre shift. Clinical feasibility of a flattening filter-free beam was therefore demonstrated, with beam modelling, treatment planning and verification being successfully accomplished.