Real-time monitoring of linac performance using RT plans and logfiles NCCAAPM Wisconsin Dells 7 April 2017 Mark Wiesmeyer, PhD, DABR Technical Product Manager Technical talk with insights and results based on a commercial product.
Goals Check that a machine can deliver plans that you create*. Get real time feedback about plan delivery issues**. Create a longitudinal record of plan delivery quality. Determine if a machine may need mechanical calibration. * Pre treatment IMRT QA ** During treatment QA I hope that you will gain a further understanding of DICOM RT Plans and Logfiles.
Roadmap Testable Parameters Basic Information Flow RT Plans Control Points Logfiles Sampled Machine Data Records Mapping Logfile Records to RT Plan Control Points Some results
Testable Parameters Jaw Positions Carriage Positions MLC Leaf Positions Gantry and Collimator Angles Fraction of total MU Beam States (Hold, On/Off, etc.) Planned vs Delivered Fluence Table Positions and Angles
Basic Information Flow 1 3 RT Plans RT Plans Logfile Batch Folder Treatment Planning Computer RT Plan Batch Folder LinacView Computer Logfiles 2 Linac Console
RT Plans General Abridged Where the action is
RT Plans Beam Sequence Where the action is
RT Plan - Control Points (CP1) X Jaws or Carriages Y Jaws or Carriages MLC Positions Gantry Angle Collimator Angle Couch Angle Fractional MU s
Control Points (CPs 88,89,178) Gantry Angle Fractional MU s Gantry Angle Fractional MU s Gantry Angle Fractional MU s Quantities whose values do not change are not repeated in CP s.
Common Systems that Provide Logfiles Vendor Linacs Logfile Format Sampling (Hz) Comments Varian Clinic Series DynaLog 20 CSV (spec). TrueBeams Trajectory 50 Binary (spec). Elekta All Recent Linacs *.dat, *.xml 4 icom Vx API (spec). Agility 160 Leaf TRF Logfiles 25 Binary (no spec). Sampling rate the number of data records collected per second. Faster sampling is typically better. 4 Hz really only suitable for static analysis. Contents of DynaLog, Trajectory, and TRF files are comparable. Key problem and source of error is mapping logfile records to CP s.
Sample Logfile Specification DynaLog
Originally intended for maintenance QA 1 25000 mapping DynaLog Files 50 ms/row Notice: No explicit mapping to control points. Dose Fraction: 150 MU(Rx) * 2198/25000 = 13.2 MU.
Trajectory Files Much easier to match to RT Plan CP s Notice: Direct, fractional control point mapping. Direct, fractional MU s. 20 ms/row
DynaLog File Mapping RT Plan Control Points precede Logfile Samples Linear interpolation maps measurement records to control points. Worst case uncertainty is 25 ms Registration Error. For leaves this results in about: 2.5 cm/s * 0.025 s = 625mm additional error Max Leaf Speed.
Trajectory File Mapping Measurement records to map directly to fractional control points. Worst case uncertainty / registration error is negligible.
Clinac/DynaLog (Name 04) RT Plan Fluence Map Logfile Fluence Map Fluence maps are created by projecting MU weighted segments to the isocenter plane. Gamma index results 99.7%
TrueBeam/Trajectory(Name 18) RT Plan Fluence Map Logfile Fluence Map Gamma index results 100% Good results are not surprising. Compare simulated fluences not measurements. Comparison is as fine as needed. No finite spaced detector grids. No setup uncertainty.
Leaf Positions & MU s Clinac/DynaLog In addition to registration error, actual errors from MLC position may be caused by: Worst case per leaf error for this beam delivery. Faulty or loose T nuts Motor failures Encoder drift Dirty, sticky leaves MU delivery error range: 0.10 > 0.15 MU How much is Registration Error?
Leaf Positions & MU s TrueBeam/Trajectory Little or no registration error and a wellmaintained, somewhat more precise machine. MU delivery error range: 0.01 > 0.01 MU
What can Logfile analysis tell us? A. Your plan will deliver / is delivering correct dose to your patient. B. Your machine is capable of delivering the plan that you have created. C. Your machine is going out of mechanical calibration. Not dose D. All of the above. E. A & B only. F. B & C only. For dose tracking, patient setup and geometry changes are at least an order of magnitude larger than machine delivery errors which tend to be very small.