Diversity in Medical Applications: The Linac Example

Transcription

1 Diversity in Medical Applications: The Linac Example Brian Bibb 19 January 2005 Outline Describe linac Describe some safety systems Arrive at some conclusions Diversity in Medical Applications: The Linac Example - 2

2 Treatment on a linac Diversity in Medical Applications: The Linac Example - 3 Layout of Bunker Diversity in Medical Applications: The Linac Example - 4

3 Linac Diversity in Medical Applications: The Linac Example - 5 Linear accelerator Diversity in Medical Applications: The Linac Example - 6

4 Treatment head Diversity in Medical Applications: The Linac Example - 7 Radiation profiles Diversity in Medical Applications: The Linac Example - 8

5 Risks Accidents Incorrect dose Incorrect volume Collision Cause Equipment error Use error Risk Control is often diversity Diversity in Medical Applications: The Linac Example - 9 Control room equipment 1Control cabinet 2 Keyboard 3 Keypad 4 Mouse 5 Monitor Diversity in Medical Applications: The Linac Example - 10

6 GUI Diversity in Medical Applications: The Linac Example - 11 Diverse Elements Prescribed and Backup MU Set and Actual Diaphragm numerical and visual display Gantry and head angle - mechanical and GUI display Segment timer Diversity in Medical Applications: The Linac Example - 12

7 Turn off radiation Emergency OFF Supplies Hardware and software control HT PRF Magnetron Linear accelerator Diversity in Medical Applications: The Linac Example - 13 Diverse elements No HT no radiation No PRF no radiation No mains power no radiation Diversity in Medical Applications: The Linac Example - 14

8 Start and Terminate Key START Button Hardware interlock Software TERMINATE Button Hardware interlock Software Diversity in Medical Applications: The Linac Example - 15 Diverse elements Hardwired START button Regulatory requirement Software control of START Hardwired TERMINATE button Software can always TERMINATE A reset key-switch must be operated in the event of an abnormal termination Diversity in Medical Applications: The Linac Example - 16

9 System level checks Tolerance checking Drive circuits Supplies Set value Actual value Supplies Monitor circuits Diversity in Medical Applications: The Linac Example - 17 Diverse elements Separate Demand and Monitor circuits Separate display of SET and ACTUAL values Visual and Automatic comparison of SET and actual values Separate supplies for Demand and Monitor circuits Diversity in Medical Applications: The Linac Example - 18

10 Course and fine pots +V Course -V +V -V Fine Position is also on mechanical scale Diversity in Medical Applications: The Linac Example - 19 Diverse elements Mechanical and electronic scales Consistent offset for the 2 wipers on Fine pot Defined relationship for Fine pot wipers and Course pot wiper Diversity in Medical Applications: The Linac Example - 20

11 Position A or B detection Position A Transition Position B The switches are: -part of drive and decode circuits -decoded in both hardware and software Indicates a fault Diversity in Medical Applications: The Linac Example - 21 Diverse elements Set position separate from monitor Software checks movement time FPLA and SW Monitors position: both check consistency with linac state Diversity in Medical Applications: The Linac Example - 22

12 Beam monitoring Radiation beam Ion chamber 1 Ion chamber 2 Diversity in Medical Applications: The Linac Example - 23 Hardware counter Ch1 Software: Counts Ch 1 Compares counters Checks dose rate Times beam Hardware counter Ch2 Terminate beam Terminate beam Display on VDU Display on LCD Terminate beam Display on VDU Diverse elements Ion chambers have different gains HW counters have different scaling factors Hardware and Software Counters Timer check regulatory requirement Dose rate check Separate display on VDU and LCD Diversity in Medical Applications: The Linac Example - 24

13 Dose Integration 1MU Accumulated dose Σ = 1MU -1MU Duplicated for 1MU and 1/64 MU Diversity in Medical Applications: The Linac Example - 25 Discharge time Ideal Actual Missed dose Capacitor discharge time Diversity in Medical Applications: The Linac Example - 26

14 Self test Self test signals Ion chamber signals Digital to Analogue Analogue to Digital Signal processing Diversity in Medical Applications: The Linac Example - 27 Single pulse check Typical pulse Detect if a single pulse is too large Abnormal pulse Diversity in Medical Applications: The Linac Example - 28

15 Overflow check Ion chamber signal Analogue to digital Signal processing 1MU Reset Overflow Pulse detected Count Counter Diversity in Medical Applications: The Linac Example - 29 Other checks Watchdog on particular events PSU monitoring Diversity in Medical Applications: The Linac Example - 30

16 Free checks Free means no extra hardware required Is the gain correct? Do the interlocks work? Is the noise level (base level) reasonable? Is the noise level consistent? Diagnostic checks Diversity in Medical Applications: The Linac Example - 31 Some conclusions More complex increased potential for design errors More complex, better functionality e.g. less analogue distortions Safety is a system property System level checks can still be used Potential for additional free checks It is worth looking for non complex checks Diversity in Medical Applications: The Linac Example - 32