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11.04.2012 STAMP/STPA Workshop - Massachusetts Institute of Technology - April 17-19, 2012 - (c) copyright by PSI, 2012 Page 1
Evaluation of STPA in the Safety Analysis of theGantry 2 Proton Radiation Therapy System
Martin Rejzek, Paul Scherrer Institute, Switzerland
11.04.2012 STAMP/STPA Workshop - Massachusetts Institute of Technology - April 17-19, 2012 - (c) copyright by PSI, 2012 Page 2
Agenda
• Proton Therapy at the Paul Scherrer Institute, Switzerland
• Evaluation of STPA for the Advanced Scanning Technique
– Scope of the Project
– Examples
• Conclusions
11.04.2012 STAMP/STPA Workshop - Massachusetts Institute of Technology - April 17-19, 2012 - (c) copyright by PSI, 2012 Page 3
Proton Therapy at thePaul Scherrer Institute, Switzerland
11.04.2012 STAMP/STPA Workshop - Massachusetts Institute of Technology - April 17-19, 2012 - (c) copyright by PSI, 2012 Page 4
PSI Center for Proton Therapy
• 250 MeV Proton accelerator (superconducting cyclotron)• Beamlines to 4 user areas• OPTIS• Gantry 1• Gantry 2• Experimental area
11.04.2012 STAMP/STPA Workshop - Massachusetts Institute of Technology - April 17-19, 2012 - (c) copyright by PSI, 2012 Page 5
Gantry 1
Sweeper magnet(1 dimension)
Dose monitoringRange shifter
Beam entersrotating Gantry
11.04.2012 STAMP/STPA Workshop - Massachusetts Institute of Technology - April 17-19, 2012 - (c) copyright by PSI, 2012 Page 6
Gantry 1 – Spot Scanning Technique
Elements of spot scanning:
• Beam on/off 50 µs• Sweeper magnet 5 ms/step• Range shifter 30 ms• Patient table 1 cm/s
•10‘000 spots to treat 1 liter volume
11.04.2012 STAMP/STPA Workshop - Massachusetts Institute of Technology - April 17-19, 2012 - (c) copyright by PSI, 2012 Page 7
Gantry 2
Sweeper magnets(2 dimensions) Dose monitoringBeam enters
rotating Gantry
11.04.2012 STAMP/STPA Workshop - Massachusetts Institute of Technology - April 17-19, 2012 - (c) copyright by PSI, 2012 Page 8
Gantry 2 – Advanced Scanning Technique
Advanced scanning:
• Increased speed• Increased flexibility• New treatment modalities
11.04.2012 STAMP/STPA Workshop - Massachusetts Institute of Technology - April 17-19, 2012 - (c) copyright by PSI, 2012 Page 9
Evaluation of STPA for theAdvanced Scanning Technique
11.04.2012 STAMP/STPA Workshop - Massachusetts Institute of Technology - April 17-19, 2012 - (c) copyright by PSI, 2012 Page 10
Scope of this Project
• Safety analysis done with classical methods• Evaluation of STPA as supplementary method for advanced scanning
– explored different approaches– considered different parts of whole facility
• Few examples:1) STPA during workshops2) How to model controllers that
can insert „veto“3) What is the reference for
inadequate timing and the„Thomas process“
Treatment Facility
Patient
Irradiation Treatment Operator
Nurse
11.04.2012 STAMP/STPA Workshop - Massachusetts Institute of Technology - April 17-19, 2012 - (c) copyright by PSI, 2012 Page 11
Evaluation of STPA for theAdvanced Scanning Technique
1) Performing STPA during workshops
11.04.2012 STAMP/STPA Workshop - Massachusetts Institute of Technology - April 17-19, 2012 - (c) copyright by PSI, 2012 Page 12
Recipe for a STPA Workshop – Step 1
• Performing STPA analysis during workshop with engineers–Preparation: Hierarchical control structure / high-level hazards–Table with guidewords
11.04.2012 STAMP/STPA Workshop - Massachusetts Institute of Technology - April 17-19, 2012 - (c) copyright by PSI, 2012 Page 13
Recipe for a STPA Workshop – Step 2
• Performing STPA analysis during workshop with engineers–Preparation: Hierarchical control structure / high-level hazards–Table with guidewords–Reduced process loop
Controller
(1) Control Algorithm(relevant part)
(3) Sensor 1
(3) Sensor 2
Param1Param2...
Param1Param2...
Var1Var2...
Var1Var2...Param1
Param2...
(2) Controller / Actuator Var1
Var2...
Influence1Influence2...
11.04.2012 STAMP/STPA Workshop - Massachusetts Institute of Technology - April 17-19, 2012 - (c) copyright by PSI, 2012 Page 14
Recipe for a STPA Workshop – Classification
Work in progress !
UCA = Unsafe Control Action
PV = Process Variable
11.04.2012 STAMP/STPA Workshop - Massachusetts Institute of Technology - April 17-19, 2012 - (c) copyright by PSI, 2012 Page 15
Evaluation of STPA for theAdvanced Scanning Technique
2) How to model controllers which can insert “veto”
11.04.2012 STAMP/STPA Workshop - Massachusetts Institute of Technology - April 17-19, 2012 - (c) copyright by PSI, 2012 Page 16
Controllers that can insert „Veto“
Typical Situation: • One Treatment Delivery System (TDS)
In Proton Therapy:• One source of beam for all treatment areas
Treatment Delivery System
Dose ControllerIntensity Controller
various actuators and sensors
Patient Treatment
11.04.2012 STAMP/STPA Workshop - Massachusetts Institute of Technology - April 17-19, 2012 - (c) copyright by PSI, 2012 Page 17
Controllers that can insert „Veto“
Treatment Delivery System
Dose ControllerIntensity Controller
Req. MasterSet Intensity
various actuators and sensors
Patient Treatment
Other Areas
Beam Allocator
Treatment Delivery System
Dose Controller
Intensity Controller
Set Intensity
various actuators and sensors
Patient Treatment
Beam Allocator
Other Areas
First approach: Second approach:
11.04.2012 STAMP/STPA Workshop - Massachusetts Institute of Technology - April 17-19, 2012 - (c) copyright by PSI, 2012 Page 18
Evaluation of STPA for theAdvanced Scanning Technique
3) What is the reference for inadequate timing
11.04.2012 STAMP/STPA Workshop - Massachusetts Institute of Technology - April 17-19, 2012 - (c) copyright by PSI, 2012 Page 19
Reference for Inadequate Timing
Personnel brings patientin room and installs him
Start
Pres. dose reached ?
Personnel leaves room
Turn beam on
Turn beam off
Personnel enters room
Personnel takes patient out
End
yes
Personnel brings patientin room and installs him
Start
Pres. dose reached ?
Personnel leaves room
Turn beam on
Turn beam off
Personnel enters room
Personnel takes patient out
End
yes
Dose Beam Patient Personnel
Personnel brings patientin room and installs him
Start
Pres. dose reached ?
Personnel leaves room
Turn beam on
Turn beam off
Personnel enters room
Personnel takes patient out
End
yes
Dose Beam Patient Personnel
null
null
null
accumulating
prescribed
prescribed
prescribed
off
off
off
on
off
off
off
out of room
in room
out of room
out of room
out of room
in room
out of room
out of room
in room
in room
in room
in room
in room
out of room
prescribeddose reached
11.04.2012 STAMP/STPA Workshop - Massachusetts Institute of Technology - April 17-19, 2012 - (c) copyright by PSI, 2012 Page 20
Reference for Inadequate Timing
Personnel brings patientin room and installs him
Start
Pres. dose reached ?
Personnel leaves room
Turn beam on
Turn beam off
Personnel enters room
Personnel takes patient out
End
yes
Dose Beam Patient Personnel
null
null
accumulating
accumulating
prescribed
prescribed
prescribed
off
off
on
on
off
off
off
out of room
in room
out of room
out of room
out of room
in room
out of room
out of room
in room
in room
in room
in room
in room
out of room
prescribeddose reached
Turn beam on
too early
too late
Xaccumulating on in roomin room
11.04.2012 STAMP/STPA Workshop - Massachusetts Institute of Technology - April 17-19, 2012 - (c) copyright by PSI, 2012 Page 21
Conclusions
• STPA is a very useful method–Results achievable in straitforward way and rather short time–Feasible to perform with non safety experts–Discussion points raised for components not yet developed
• Next steps–Finish this project–Compare Results with those of existing safety analysis–hopefully continue with STPA method
11.04.2012 STAMP/STPA Workshop - Massachusetts Institute of Technology - April 17-19, 2012 - (c) copyright by PSI, 2012 Page 22
Acknowledgments
Prof. Dr. Nancy LevesonBlandine AntoineMassachusetts Institute of Technology, US
Prof. Dr. Christian HilbesZurich University of Applied Sciences, CH
Dr. Martin GrossmannDr. David MeerCenter for Proton TherapyPaul Scherrer Institute, CH
Contacts:Dipl. el. Ing. FH Martin RejzekE-mail: martin.rejzek@psi.ch
Prof. Dr. Nancy LevesonE-mail: leveson@mit.edu
Blandine AntoineE-mail: blandine@mit.edu
Dr. Christian HilbesE-mail: christian.hilbes@zhaw.ch
Dr. Martin GrossmannE-mail: martin.grossmann@psi.ch
Dr. David MeerE-mail: david.meer@psi.ch