A national dosimetry audit for stereotactic ablative radiotherapy in lung

Background and purpose: A UK national dosimetry audit was carried out to assess the accuracy of Stereotactic Ablative Body Radiotherapy (SABR) lung treatment delivery. Methods and materials: This mail-based audit used an anthropomorphic thorax phantom containing nine alanine pellets positioned in the lung region for dosimetry, as well as EBT3 film in the axial plane for isodose comparison. Centres used their local planning protocol/technique, creating 27 SABR plans. A range of delivery techniques including conformal, volumetric modulated arc therapy (VMAT) and Cyberknife (CK) were used with six different calculation algorithms (collapsed cone, superposition, pencil-beam (PB), AAA, Acuros and Monte Carlo). Results: The mean difference between measured and calculated dose (excluding PB results) was 0.4 ± 1.4% for alanine and 1.4 ± 3.4% for film. PB differences were �6.1% and �12.9% respectively. The median of the absolute maximum isodose-to-isodose distances was 3 mm (�6 mm to 7 mm) and 5 mm (�10 mm to +19 mm) for the 100% and 50% isodose lines respectively. Conclusions: Alanine and film is an effective combination for verifying dosimetric and geometric accuracy. There were some differences across dose algorithms, and geometric accuracy was better for VMAT and CK compared with conformal techniques. The alanine dosimetry results showed that planned and delivered doses were within ±3.0% for 25/27 SABR plans