Accident prevention through treatment planning
Major cases of accidental exposures due to errors in treatment planning
» Erroneous use of treatment planning system (UK)
» Computer file not updated (USA)
» Errors in TPS data entry (Panama)
» Erroneous use of treatment planning system (UK)
Until 1982, a hospital in the United Kingdom relied on manual calculations for the correct dose to be delivered to the tumour. Treatments were generally performed at a standard source-to-skin distance (SSD) of 100 cm. Isocentric treatments were rarely performed because manual calculations of these treatments were cumbersome. A non-written procedure was in effect for treatments at non-standard SSD (including the few isocentric treatments). The technologists calculated a correction factor in these cases, based on the actual SSD used.
A computerized treatment planning system (TPS) was acquired in 1981, and after some preliminary testing, brought into clinical use in the autumn of 1982. The hospital began treating with isocentric techniques more frequently, partly because the new TPS simplified the calculation procedures. When the first isocentric TPS plan was ready, the technologists assumed that correction factors for non-standard SSD should be applied, and a hospital physicist approved this procedure. It was not recognized that the TPS already correctly applied an inverse-square correction for isocentric treatments. The technologists continued to apply the distance correction factor to all subsequent non-standard SSD calculations. As a result, a distance correction factor was applied twice for all patients treated isocentrically, or at non-standard SSD. This error caused patients to receive doses lower than prescribed.
In 1991 a new computer planning system was installed and a discrepancy was discovered between the new plans and those from the previous system. Further investigation revealed that the original TPS already contained within it the correction for calculations at non-standard SSD. A formal investigation was initiated, and the incorrect procedures were found to have been in place until 1991, or for approximately nine years. During the 9-year period, 6% of patients treated in the department were treated with isocentric technique; for many of these patients it formed only part of their treatment. Evaluation showed that out of 1045 patients whose calculations were affected by the incorrect procedures, 492 developed local recurrences that could be attributed to the error. Underdose varied between 5 and 35%.
Lessons learned for health professionals:
- Ensure that staff is properly trained in the operation of the equipment;
- Ensure that staff understands the operating procedures;
- Implement procedures to perform complete commissioning of treatment planning equipment before first use;
- Implement procedures for independent checking of individual patient treatment time calculations;
- Audit the treatment outcomes because underdose is difficult to assess, as it does not produce recognizable symptoms.
Read more:
- ASH, D., BATES, T., Report on the clinical effects of inadvertent radiation underdosage in 1045 patients, Clin. Oncol. 6 (1994) 214-225.
» Computer file not updated (USA)
An old Co-60 (60Co) source was exchanged for a new one in a hospital in Maryland, USA, in 1987. All corresponding data for treatment time calculations was updated by a consulting medical physicist, except data for treatment with cobalt beam trimmer bars. The oncologist stated to the consulting physicist that trimmer bars would not be used for treatment anymore, so the data file / program for this type of treatment was not updated by the physicist. Approximately half a year later, treatment with trimmer bars for whole brain treatment was initiated. The staff used the old computer file for calculation of treatment time with trimmer bars, but because the file contained the outdated source activity, patient treatment times were too long.
Therapy staff observed skin erythema on several patients during the time period September 1987 until October 1988. They occasionally expressed their concern to the hospital oncologist, but the reactions were judged as normal during radiotherapy. In October 1988 the consulting medical physicist was notified, and found a non-updated computer data file / program. Patients had been receiving doses 75% greater than prescribed. During this 13 month time period, 33 patients were treated with this particular technique and 20 patients were dead (either during the course of treatment or after conclusion of treatment) at the time of notification to the authorities.
Lessons learned for health professionals:
- Develop procedures for clearly distinguishing software that is commissioned for clinical use from software that has been removed from clinical service;
- Include in the Quality Assurance program:
- Procedures for verifying correct function of software for patient calculations;
- In-vivo dosimetry.
Read more:
- NUCLEAR REGULATORY COMMISSION, Report to Congress on Abnormal Occurrences, NUREG-0090, Volume 11, No. 4. USNRC, Washington DC (1988).
» Errors in TPS data entry (Panama)
A hospital in Panama, in the year 2000, was working to a busy schedule, treating 70 to 80 patients per day on a single cobalt unit. Many patients were treated in the evening with only a single therapist present, using multiple fields with an SSD set up technique including blocks and wedges. The treatment planning system (TPS) that was used in the hospital, allowed for a maximum of four shielding blocks to be entered in any field in order to calculate dose distribution and treatment time. In April 2000, one of the oncologists required one additional (fifth) block for some treatments in the pelvic region. To overcome the limitation of four blocks imposed by the TPS, a new way of entering data was tried (August 2000): entering several blocks "at once". The TPS accepted the data entry, without giving a warning, but calculated incorrect treatment times.
In November 2000, radiation oncologists observed unusual reactions in some patients (unusually prolonged diarrhoea). The physicists checked the patient charts but did not find any abnormality. However, the computer calculations were not questioned. The treatment time incorrectly calculated by the TPS was approximately twice the required for the correct dose. In total, 28 patients were affected by these incorrect calculations. A few months after discovery, at least five patients had died related to the overdose of radiation.
Lessons learned for health professionals:
- Include the TPS in the quality control programme, because it is a safety critical piece of equipment;
- Validate changes in treatment planning procedures before clinical use;
- Independently verify computer calculations (at least with manual calculations for one point);
- Instruct staff to be aware of unusual treatment parameters, such as longer than normal treatment time;
- Investigate unusual reactions and test dosimetry data.
Lessons learned for manufacturers:
- Avoid ambiguity in the instructions for TPS use;
- Perform thorough testing of software for both intended use and non-intended use;
- Guide users with warnings on the screen when data is incorrectly input;
- Be readily available for consultation, especially when a customer proposes to deviate from the published instructions for TPS use.
Read more: