Application of Failure Mode and Effects Analysis to Intraoperative Radiation Therapy Using Mobile Electron Linear Accelerators

Purpose

Failure mode and effects analysis (FMEA) represents a prospective approach for risk assessment. A multidisciplinary working group of the Italian Association for Medical Physics applied FMEA to electron beam intraoperative radiation therapy (IORT) delivered using mobile linear accelerators, aiming at preventing accidental exposures to the patient.

Methods and Materials

FMEA was applied to the IORT process, for the stages of the treatment delivery and verification, and consisted of three steps: 1) identification of the involved subprocesses; 2) identification and ranking of the potential failure modes, together with their causes and effects, using the risk probability number (RPN) scoring system, based on the product of three parameters (severity, frequency of occurrence and detectability, each ranging from 1 to 10); 3) identification of additional safety measures to be proposed for process quality and safety improvement. RPN upper threshold for little concern of risk was set at 125.

Results

Twenty-four subprocesses were identified. Ten potential failure modes were found and scored, in terms of RPN, in the range of 42–216. The most critical failure modes consisted of internal shield misalignment, wrong Monitor Unit calculation and incorrect data entry at treatment console. Potential causes of failure included shield displacement, human errors, such as underestimation of CTV extension, mainly because of lack of adequate training and time pressures, failure in the communication between operators, and machine malfunctioning. The main effects of failure were represented by CTV underdose, wrong dose distribution and/or delivery, unintended normal tissue irradiation. As additional safety measures, the utilization of a dedicated staff for IORT, double-checking of MU calculation and data entry and finally implementation of in vivo dosimetry were suggested.

Conclusions

FMEA appeared as a useful tool for prospective evaluation of patient safety in radiotherapy. The application of this method to IORT lead to identify three safety measures for risk mitigation.

Introduction

Radiotherapy technologies to plan and conformally deliver the dose to the target significantly improved in the recent years, reaching a very high degree of complexity and sophistication. The rapidly increasing use of newer techniques, such as intensity-modulated radiation therapy (IMRT), arc therapy (IMAT), image-guided radiotherapy (IGRT), and stereotactic body radiotherapy (SBRT), places new demands on quality assurance (QA) programs as well as new attitudes and approaches for patient safety 1, 2, 3.

Accidents in conventional radiotherapy have been extensively investigated and the lessons learned from events with major consequences have been reported with the aim of prevention, following a retrospective approach 1, 4, 5. On one side, some general lessons can be applicable also to new technologies: in particular, the availability of a well-trained staff, the definition of written and comprehensive procedures and the identification of responsibilities are key elements for a successful treatment, independently of the applied technique 1, 6. However, to fully assess and manage the risks of accidental exposures deriving from the use of innovative radiotherapy methodologies, retrospective approaches are not adequate enough, because they have the intrinsic limitation of being confined to the reported experiences, thus leaving unreported events or latent risks unaddressed. Therefore, prospective approaches, widely applied in high-risk industry, have to be implemented to find out all the elements that could go wrong and identify, a priori, all the potential hazards that might occur during a radiotherapy treatment (1). Recently, the interest in using these methodologies, in particular the failure mode and effects analysis (FMEA), for safety assessment in complex medical practices such as radiotherapy is gaining importance and the literature on this topic is increasing 2, 7, 8, 9, 10, 11, 12.

On the basis of these new needs, at the beginning of 2010 a multidisciplinary working group (WG) has been established in the framework of the Italian Association for Medical Physics to promote the use of prospective approaches in the radiotherapy scientific community. Considering the large experience on electron beam intraoperative radiotherapy (IORT) gained by some of the components of the WG at the European Institute of Oncology (IEO), it was decided to first apply prospective techniques to that irradiation methodology. In particular, IORT treatments at IEO are mostly delivered in the context of the conservative treatment of early-stage breast cancer, using two different mobile linear accelerators 13, 14, 15, 16, 17, 18. The choice of IORT by the WG as a priority was also motivated by the lack of valuable lessons to be learnt from incidents already occurred, because only a case of accidental exposure involving IORT has been reported in the literature so far and regarded a very specific situation (beam calibration error in the phase of the commissioning of a 50 kV X-ray machine) (1).

As known, IORT is a treatment modality involving the delivery of a single-fraction large radiation dose to the exposed tumour or tumor bed at the time of surgery. For many years, IORT has been commonly performed using conventional linear accelerators located in the radiotherapy treatment vault 19, 20. More recently, dedicated and mobile linear accelerators, which only operate in electron mode up to 10–12 MeV and can work in almost any conventional operating room (OR), have been developed, avoiding the need of patient transportation outside the OR and thus facilitating the spread out of IORT 20, 21, 22.

In terms of risk of accidental radiation exposures to the patient, the main critical aspect in IORT seems to consist in its single-shot nature, unlike conventional radiation treatment modalities where fractionation can somehow permit dose compensation if a systematic error in the whole process is detected during treatment. Furthermore, in IORT so far no image-based treatment planning is normally performed before the dose delivery, treatment parameters (such as field size, beam energy, blocks, or bolus) being defined in the minutes immediately before the irradiation and Monitor Units consequently calculated by the medical physicist. In terms of added risk, it has also to be underlined that mobile linear accelerators still represent a relatively novel and scarcely diffused technology (the number of machines installed worldwide is far less than 100, including all the three models available on the market), as compared with the thousands of conventional treatment units working in radiotherapy.

The aim of this work was to present the results of the application of the FMEA prospective approach to IORT delivered by means of a mobile linear accelerator, starting from the identification of the process tree and potential failure modes, then through the assignment of a score for each failure mode using the risk probability number (RPN) and finally suggesting additional safety measures for process improvement and risk mitigation.