UK SABR Consortium Lung Dosimetry Audit; absolute dosimetry results

Purpose/Objective: The UK SABR Lung Consortium dose audit was designed to assess the positional and dosimetric accuracy of SABR lung treatment delivery. The audit has been carried out in 21 radiotherapy centres between October 2013 and July 2014 in order to provide an independent check of safe implementation and to identify problems in the modelling and delivery of SABR lung treatment. Materials and Methods: A mail based audit using EBT3 GafChromic film and alanine dosimeters was designed. A CIRS Model 002LFC anthropomorphic thorax phantom which contained 9 adjacent alanine pellets in the tip of a Farmer chamber shaped insert was scanned, structure sets for the ITV and alanine pellets were pre-delineated, and was sent to radiotherapy centres to be loaded into their treatment planning system. Each centre used this CT scan set to create a SABR plan using their current planning protocol (including dose, fractionation and coverage) and technique. The centres used their own margin to create the PTV. A range of delivery techniques were used including conformal, VMAT and Cyberknife and calculated using local algorithms (AAA, Collapsed Cone, Monte Carlo and Pencil beam). The doses determined by the alanine dosimeters were compared to expected doses determined by treatment plan system (TPS) calculation, film and local ionisation chamber measurements. Results: The mean % difference between the alanine measured doses, the TPS calculated doses, and the local chamber measurements found to be within 2% (1 SD) as given in table 1. As shown, alanine findings were supported by the film results.