Failure modes and effects analysis (FMEA) for Gamma Knife radiosurgery

Abstract Purpose Gamma Knife radiosurgery is a highly precise and accurate treatment technique for treating brain diseases with low risk of serious error that nevertheless could potentially be reduced. We applied the AAPM Task Group 100 recommended failure modes and effects analysis (FMEA) tool to develop a risk‐based quality management program for Gamma Knife radiosurgery. Methods A team consisting of medical physicists, radiation oncologists, neurosurgeons, radiation safety officers, nurses, operating room technologists, and schedulers at our institution and an external physicist expert on Gamma Knife was formed for the FMEA study. A process tree and a failure mode table were created for the Gamma Knife radiosurgery procedures using the Leksell Gamma Knife Perfexion and 4C units. Three scores for the probability of occurrence (O), the severity (S), and the probability of no detection for failure mode (D) were assigned to each failure mode by 8 professionals on a scale from 1 to 10. An overall risk priority number (RPN) for each failure mode was then calculated from the averaged O, S, and D scores. The coefficient of variation for each O, S, or D score was also calculated. The failure modes identified were prioritized in terms of both the RPN scores and the severity scores. Results The established process tree for Gamma Knife radiosurgery consists of 10 subprocesses and 53 steps, including a subprocess for frame placement and 11 steps that are directly related to the frame‐based nature of the Gamma Knife radiosurgery. Out of the 86 failure modes identified, 40 Gamma Knife specific failure modes were caused by the potential for inappropriate use of the radiosurgery head frame, the imaging fiducial boxes, the Gamma Knife helmets and plugs, the skull definition tools as well as other features of the GammaPlan treatment planning system. The other 46 failure modes are associated with the registration, imaging, image transfer, contouring processes that are common for all external beam radiation therapy techniques. The failure modes with the highest hazard scores are related to imperfect frame adaptor attachment, bad fiducial box assembly, unsecured plugs/inserts, overlooked target areas, and undetected machine mechanical failure during the morning QA process. Conclusions The implementation of the FMEA approach for Gamma Knife radiosurgery enabled deeper understanding of the overall process among all professionals involved in the care of the patient and helped identify potential weaknesses in the overall process. The results of the present study give us a basis for the development of a risk based quality management program for Gamma Knife radiosurgery.

plugs/inserts, overlooked target areas, and undetected machine mechanical failure during the morning QA process.
Conclusions: The implementation of the FMEA approach for Gamma Knife radiosurgery enabled deeper understanding of the overall process among all professionals involved in the care of the patient and helped identify potential weaknesses in the overall process. The results of the present study give us a basis for the development of a risk based quality management program for Gamma Knife radiosurgery. The FMEA is a reliability study tool for the analysis of the postulated component failures in a system and the resultant effects on the system operations. 7 It was initially developed by the US military 8 and has been extensively used in a variety of industries and health care services. [9][10][11] The importance of a risk based quality management program for radiation therapy has come to the attention of the medical physics community in recent years owing to two facts. First, with the technology advances in equipment manufacturing and the development of various quality assurance protocols, conventional device specific physics QA measurements can be done with much higher precision than before. Second, many reported radiation therapy incidences resulted from incorrect or inappropriate use of radiation treatment devices due to miscommunication or misunderstanding rather than device failures. As such, the development of quality management programs that focuses on the design and execution of various radiotherapy processes has become a subject of significant interest in recent years. [12][13][14][15][16][17][18][19][20] Following the recommendations from the AAPM task group report No. 100 (TG 100), the implementation of risk-based quality management for radiation therapy facilities may become a standard practice and a regulatory requirement in the future.
As the first step toward the development of a risk-based quality management program for Gamma Knife radiosurgery, we present in this work a FMEA study on the Gamma Knife radiosurgery process as performed at our institution following the methodologies described in the AAPM TG 100 report.

2.A.1 | Gamma Knife radiosurgery at our institution
More than 600 patients receive single fraction Gamma Knife radiosurgery treatments on a Leksell Gamma Knife Perfexion and a 4C at our institution annually. All the patients reported to the treatment suite at 5.30 a.m and were evaluated in one of the five exam rooms upon arrival. Each patient was cared for by a dedicated nurse during the entire treatment process. After the stereotactic coordinate frames were placed by the neurosurgical team shortly after 6.30 am, patients were transported to the radiology department for imaging.
Neurosurgical team members supervised the MR, CT, or Angiographic imaging appropriate for the pathology undergoing radiosurgery. Morning QA of the treatment machines and patient chart creation was performed by physicists starting at 6.30 a.m. The radiosurgical planning using GammaPlan workstation took place as soon as the first set of images was available, usually started by the neurosurgical team and joined by the radiation oncology team. Final approval of the plan and plan export were done by physicists. The written directive was signed by an authorized surgeon, a radiation XU ET AL. | 153 oncologist, and a medical physicist. The physicists also performed a secondary dose calculation check. The patients were then put on the treatment tables and docked into treatment positions. A patient identification check was performed in the presence of a physician, a physicist, and a nurse before the beam-on. The first treatment on each machines usually started around 8.00 a.m. During the treatment times, a radiation oncologist, a physicist, and a nurse were present in the close vicinity of each treatment console. Frame removal was performed by the surgical and nursing team immediately after the treatment was finished. Patients were usually discharged within an hour from the time of frame removal.

2.A.2 | Process tree and failure modes
The FMEA on the Gamma Knife radiosurgery was conducted following the methodology of the FMEA on IMRT as described in the AAPM TG 100 report. A team consisting of medical physicists, radiation oncologists, neurosurgeons, radiation safety officers, nurses, and schedulers at our institution and an external physicist expert on Gamma Knife was formed for the FMEA study. A preliminary process tree was prepared by the physics group first. Discussions about the details of each component of the treatment process between the physicists and other six professional groups were followed. The process tree was then revised and presented to all team members for further discussions and revision until a final version was agreed on.
The failure modes table was generated following the same path.
A template with an initial version of the failure mode table was prepared by the physics group using the Microsoft Excel and distributed to other professional groups for revision and addition. Some potential failures could be detected and prevented by the treatment delivery system and were not included in the failure modes table.
Examples of these failure modes include wrong collimator on the 4C, wrong Gamma angle on the Perfexion etc.

2.A.3 | Scoring and risk prioritizing
The scoring process involved the medical physicists, two radiation oncologists, and three neurosurgeons who have been routinely involved in the Gamma Knife treatments. Three scores for the probability of occurrence (O), the severity (S), and the probability of no detection for failure mode (D) were assigned to each failure mode by each professional on a scale from 1 to 10. The methodology of the FMEA, the content of the Gamma Knife failure mode table and the scoring guidelines were discussed in detail before the scoring process. The guidelines used for the O, S, D scores were exactly the same as described in the TG 100 report. Averaged O, S, D scores and an overall risk priority number (RPN) were then calculated for each failure mode. To analyze the variation of the O, S, D scores from different scorers, coefficient of variation (defined as the ratio of standard deviation and mean) for the eight sets of data points was also calculated for each score.
The identified failure modes were analyzed in term of the RPN scores and the severity scores for risk prioritizing. The five failure modes with the highest RPN scores and the five failure modes with the highest severity scores were sorted out as targets for improvement. Figure 1 shows the process tree for Gamma Knife radiosurgery as performed at our institution. A total of 10 subprocesses and 53 steps were identified for a Gamma Knife procedure starting from the diagnosis to the post-treatment follow-ups and chart filing. One subprocess (frame placement) and 11 steps are directly related to the frame-based nature of Gamma Knife radiosurgery.

| RESULTS
The neurosurgery team is essentially involved in all subprocesses of the process tree at our institution. The presence of the radiation oncology team during the frame placement and the imaging processes is not required. The role of the medical physicists starts from the treatment preparation subprocess.
Depending on the diagnosis and the location of the disease site, the course of a Gamma Knife treatment delivery process varies from patient to patient. On the Perfexion, Gamma angle change may be needed for some patients. On the 4C, helmet change, plug pattern, both the APS and the trunnion modes may be used for some patients. These details of the treatment delivery process were not documented in the process tree but were considered for the failure modes analysis. Table 1   61 "wrong plan exported", and No. 1 "incorrect patient ID data" respectively. The coefficients of variation for the severity scores are in general smaller than those for the occurrence and the detectability scores, indicating better agreement among the observers on the severity of the identified failure modes. Table 3 lists the five failure modes with the highest RPN scores.  Table 4 gives the 5 failures with the highest severity scores. Failure mode No. 72 "wrong plug pattern" is related to the inappropriate use of the plug pattern on the 4C. Failure mode No. 65 "frame adaptor not attached properly" is found on both the riskiest and severest lists and is the primary target of our risk-based quality assurance program.

| DISCUSSION
A single fraction Gamma Knife procedure is designed to be a fast and efficient radiation delivery process that can be completed (frame on to frame off) within a few hours. Comparing to other radiosurgery techniques, the use of the stereotactic head frame for patient positioning also helps to eliminate many uncertainties at the imaging, the target delineation, and the treatment setup stages. 22 Seventy six of the 86 failure modes found in this study are independent of the Gamma Knife treatment units and are common for the Perfexion and the 4C. Two failure modes (No. 13  No.
Step Potential failure modes   | 167 the basics of the MR, CT, and the Angio imaging techniques was given to all clinical staff. A policy was also developed to require at least one T1 weighted image series and one T2 weighted image series for any MR based treatments, so that targets and critical structures can be drawn on different image sets and cross-checked when necessary.
Several of the failure modes with the highest severities scores did not get high RPN scores because of their low scores in both the occurrence and the detectability. The reason was that many QA procedures had already been developed and implemented over the years to prevent these dangerous failure modes from happening.
These procedures include patient ID double check, plug pattern check, and image adaptor position check etc.
It should be pointed out that the present study was conducted for the Gamma Knife radiosurgery procedures as performed at our institution. The results of the risk prioritizing process, the details of the failure mode table, and even the layout of the process tree might be different for other institutions, even though the bulk part of the FMEA should be the same for all Gamma Knife facilities.

| CONCLUSIONS
We have performed a FMEA study on Gamma Knife radiosurgery based on our experience with the treatments of more than 600 patients annually. The implementation of the FMEA approach was first an important self-learning process that enabled deeper understanding of the radiosurgery procedure among all professionals involved in the care of the patient. The identified weaknesses in the overall process were the primary target areas for the development of a risk based quality management program for Gamma Knife radiosurgery.

CONF LICT OF I NTEREST
The authors declare no conflict of interest.