Computerized tomography-based quality assurance tool for proton range compensators.

The authors have developed an automatic quality assurance method based on computerized tomography (CT) data for physically manufactured proton range compensators. Eight range compensators used for proton therapy at their institution were scanned using CT and their depth distributions were compared with data provided by a proton treatment planning system (TPS). The depth difference (DD), distance to agreement (DTA), and a composite analysis (CA) indicating regions that passed either of the tests were used for depth verification. When the tolerance limits were set at 3 mm for DD and 1 mm for DTA, the average percentages of points exceeding the acceptance criteria for DD, DTA, and CA for the eight compensators were 9.0%, 5.3% and 3.2%, respectively. In general, the percentages of points exceeding the acceptance criteria were much higher in DD than in DTA analysis and mismatch of depth was greater in regions with high depth gradients. This is considered to arise mainly because of systematic errors such as the physical size of the drill, slight bending or slipping of the drill bit near high gradient regions, the limit of CT resolution, and the uncertainty in CT number. When the tolerance limit of DTA was increased from 1 to 2 mm, the average percentage of points exceeding the acceptance criteria for DTA was reduced from 5.3% to 0.9%. Their results show that depth comparison of proton range compensators based on CT images can provide more systematic data than does the current method, in which a representative group of drill points is measured manually.