Quality control in radiotherapy: the reduction of field placement errors.

A 'blind' study using treatment verification films has been performed on two series of patients to assess the accuracy of placement of complex infradiaphragmatic fields planned to include para-aortic nodes. Sequential verification films (VF) for each field on all patients studied were compared to determine variations in field position relative to the anatomy, especially lymph nodes, and the simulator plan. Nodes included in the plan but partially or completely missed by one or more treatments were identified, as were the error types involved. In series I, 21% of 157 VF showed a nodal miss, and after changes in practice designed to minimize the error types responsible for those, the figure was reduced in series II to 5.5% of 194 VF. In series II a 50% reduction in magnitude of the average systematic lateral shift and rotation of posterior fields is attributed mainly to the discontinuation of the practice of treating posterior fields through the couch. Apart from a prescribed width of less than 9 cm, three quantitative key factors were derived: greater than 6 mm lateral shift, greater than 7 mm field narrowing, 2 degree or more rotation of fields. The correction of these errors if identified on verification films, should eliminate the subsequent occurrence of node misses. The value of routine clinical treatment verification films at the start of treatment to identify and correct systematic errors is emphasized, as is the importance of precise and standardized technical practice.