When preparing radiation treatment, the prescribed dose and irradiation geometry must be translated into physical machine parameters. An error in the calculations or machine settings can negatively affect the intended treatment outcome. Analysing incidents originating in the treatment preparation chain makes it possible to find weak links and prevent treatment errors. The aim of this work is to study the effectiveness of a multilayered error prevention system by analysing both near misses and actual treatment errors.

The system utilised in this centre has primary and secondary checking as two layers of independent calculation-checking. We studied near misses as well as the actual errors which were not picked up by these pre-treatment checking procedures over a year (5154 treatment plans). Furthermore, the primary checking was studied in more detail over three years (15,386 treatment plans) to increase the statistical accuracy.

For each reported actual treatment error originating in the treatment preparation chain, 13.8 near misses were found by primary and secondary checking and thereby prevented from becoming actual errors. The total frequency of near misses was 34.4 per 1000 treatment plans. The primary checkers reported 23 types of errors for manual treatment plans (without dose distribution) and 30 types of errors for computer plans. Computer plans also showed a near miss rate that was 42% higher than for manual plans. The high ratio of near misses per actual error demonstrates an effective error prevention system, independent of the quality of the initial treatment preparation. Complex treatment plans were shown to be particularly error-prone, thereby requiring extra vigilance when checking.