Evaluation of three presets for four-dimensional cone beam CT in lung radiotherapy verification by visual grading analysis

Objective: To evaluate three image acquisition presets for four-dimensional cone beam CT (CBCT) to identify an optimal preset for lung tumour image quality while minimizing dose and acquisition time. Methods: Nine patients undergoing radical conventionally fractionated radiotherapy for lung cancer had verification CBCTs acquired using three presets: Preset 1 on Day 1 (11mGy dose, 240s acquisition time), Preset 2 on Day 2 (9mGy dose, 133 s acquisition time) and Preset 3 on Day 3 (9mGy dose, 67s acquisition time). The clarity of the tumour and other thoracic structures, and the acceptability of the match, were retrospectively graded by visual grading analysis (VGA). Logistic regression was used to identify the most appropriate preset and any factors that might influence the result. Results: Presets 1 and 2 met a clinical requirement of 75% of structures to be rated "Clear" or above and 75% of matches to be rated "Acceptable" or above. Clarity is significantly affected by preset, patient, observer and structure. Match acceptability is significantly affected by preset. Conclusion: The application of VGA in this initial study enabled a provisional selection of an optimal preset (Preset 2) to be made. Advances in knowledge: This was the first application of VGA to the investigation of presets for CBCT.