Table 2 Application of failure mode and effects analysis for the treatment planning stage in proton beam radiotherapy
Sub-process | N | Potential failure mode | Potential causes of failure | Potential effects of failure | O | S | D | RPN |
|---|---|---|---|---|---|---|---|---|
(I) Selection of the reference CT scan for planning | 1 | Error in selecting the CT scan (e.g. incorrect patient set up, outdated representation of the anatomy) in case of multiple CT scans | Human error, failure in the communication between operators | Wrong dose distribution/wrong dose delivery | 3 | 8 | 4 | 96 |
2 | Outdated representation of the anatomy (single CT scan) | Anatomical changes (related to time delay) | Wrong dose distribution/wrong dose delivery | 3 | 8 | 8 | 192 | |
(III) Manual correction of external contour | 3 | Incorrect external contour definition (body or patient mask countour underestimation, i.e. not fully included in the external contour) | Human error | Wrong dose distribution / wrong dose delivery | 4 | 5 | 4 | 80 |
| Â | 4 | Failure of object/region identification | Human error | Wrong dose distribution | 3 | 8 | 4 | 96 |
5 | Inaccurate delineation | Human error | Wrong dose distribution | 4 | 6 | 6 | 144 | |
(IV) Delineation of CT artefacts, altered structures, metal implants and manual assignment of specific HU numbers | ||||||||
6 | Incorrect HU number manual assignment | Human error or lack of documentation from the referring clinicians (e.g. surgeons) | Wrong dose distribution | 4 | 7 | 7 | 196 | |
| Â | 7 | Lack of couch origin of coordinates definition | Human error | Unintended normal tissue irradiated and CTV missing | 3 | 10 | 3 | 90 |
(IX) Couch origin of coordinates identification for absolute positioning | 8 | Wrong definition of couch origin of coordinates (large amount) | Human error | Unintended normal tissue irradiated and CTV missing | 3 | 10 | 3 | 90 |
9 | Wrong definition of couch origin of coordinates (small amount, in terms of 2–3 mm) | Human error | Unintended normal tissue irradiated and CTV missing | 4 | 5 | 6 | 120 | |
(XI) Target selection and dose prescription for each target (dose prescription type - point or volume, mean, median, minimum-, total dose, fractionation scheme) | 10 | Wrong setting of dose prescription type | Human error | Wrong dose delivery | 3 | 8 | 4 | 96 |
11 | Wrong dose fractionation setting | Human error and lack of verbal-written communication (patient chart) | Wrong dose delivery | 2 | 10 | 4 | 80 | |
| Â | 12 | Improper selection of gantry angle/couch rotation: beam passing through OARs, when otherwise avoidable | Inadequate operator skill | Sub-optimal treatment quality: increased treatment toxicity | 4 | 6 | 4 | 96 |
(XV - XVI) Selection of field direction (gantry angle and couch rotation) | 13 | Improper selection of gantry angle/couch rotation: beam stopping against OARs, when otherwise avoidable | Inadequate operator skill | Low plan robustness (range uncertainty) | 4 | 8 | 4 | 128 |
14 | Improper selection of gantry angle/couch rotation: beam passing through unstable tissues (such as bowel), when otherwise avoidable | Inadequate operator skill | Low plan robustness (range uncertainty) | 3 | 8 | 5 | 120 | |
(XVII) Setting of pencil beam parameters: | 15 | Improper selection of beam parameters: FWHM, scanning or energy step too large in relationship to PTV volume | Inadequate operator skill | Sub-optimal treatment quality: increased treatment toxicity or reduced TCP | 4 | 5 | 4 | 80 |
- FWHM | ||||||||
- scanning step in the transversal plane | ||||||||
- energy step | ||||||||
- passive elements | ||||||||
(XIX) Definition of dose calculation parameters: | 16 | Improper selection of physical beam model and/or calculation grid | Human error due to time pressure or inadequate skills | Wrong dose distribution | 4 | 7 | 5 | 140 |
- physical beam model | 17 | Improper selection of properties of the particles per spot matrix | Human error | Sub-optimal treatment quality | 4 | 4 | 5 | 80 |
- dose calculation grid | ||||||||
- properties of the particles per spot matrix | ||||||||
- dose calculation algorithm, nuclear correction, spot decomposition | ||||||||
(XX) Setting of the optimization modality (SFUD, IMPT) | 18 | Improper selection of IMPT modality | Inadequate operator skill | Low plan robustness: increased treatment toxicity or reduced TCP | 4 | 5 | 4 | 80 |
(XXII) Initial/iterative definition of target/OAR constraints and weights for dose optimization | 19 | Wrong or incomplete definition of one or more dose constraints | Inadequate operator skill or inattention | Sub-optimal treatment quality | 4 | 8 | 3 | 96 |
(XXV) Plan evaluation | 20 | Improper acceptance of results | Time pressure or inadequate operator skill | Sub-optimal treatment quality | 5 | 4 | 4 | 80 |
(XXVI) Production of competing plan | 21 | Lack of producing enough competing plans | Time pressure or inadequate operator skill | Sub-optimal treatment quality | 5 | 4 | 4 | 80 |
(XXVIII) Creation of set-up fields and calculation of DRRs | 22 | Wrong definition of field isocenter (small amount 2Â mm) | Human error | Wrong dose delivery | 3 | 5 | 8 | 120 |
(XXX) Plan approval | 23 | Approval of wrong plan | Human error, failure in the communication between operators | Wrong delivery | 3 | 8 | 5 | 120 |