RapidArc ® is a sophisticated treatment technique (….which started in Denmark in 2008) RapidArc ® is a volumetric arc therapy that delivers a precisely sculpted 3D dose distribution with a single 360-degree rotation of the Linac Substantially decreases the treatment time
RapidArc ® – Basics Modulation of the dose distribution Varying doses per degree and dynamic MLC (DMLC) The variable dose per degree is achieved by changing both the dose rate or gantry speed MLC leaves are allowed to travel in and out + leaf Interdigitation capabilities The arc optimization algorithm, PRO (Progressive Resolution Optimizer), ensures the treatment precision. It optimizes leaf position, dose rate and gantry speed Demo..\RapidArc_Treatment_Timing.mp4..\RapidArc_Treatment_Timing.mp4
Technical description of RapidArc ® delivery on Clinac
Technical description RapidArc ® on Clinac Operation and Control Functioning of the Clinac control system during RapidArc ® delivery Synchronization of MLC, gantry and dose rate Constraints
Technical description RapidArc ® on Clinac A full arc is divided in simple segments defined by control points. Parameters to control? Gantry angle Dose delivered / dose rate MLC leaves positions Each control point specifies the gantry angle, cumulative fractional MU and MLC positions
Technical description RapidArc ® on Clinac Maximum 177 control points (at 5 th level of Progressive Resolution Optimizer -PRO) 1 segment every # 2 degrees for a full turn. For each single segment: Dose rate is constant Gantry speed is constant Starting and ending of MLC leaves are known Gantry, MLC and MUs are monitored every 50ms Dose rate or gantry speed are adjusted if needed
RapidArc ® – control points listing in Eclipse Cumulative MU Gantry Position Dose Rate Gantry Speed
Technical description RapidArc ® on Clinac The RapidArc ® plan is moded up through the 4DITC, and is then divided in two groups of control parameters The gantry angle as a function of cumulative MU is sent to the Clinac control system in the form of a segmented treatment table (STT) The MLC leaf positions as a function of gantry angle are sent to the MLC controller in the form of an arc dynamic beam The treatment delivery is controlled by the Clinac controller and the MLC controller
Animation of variable dose delivered to individual segments
Technical description RapidArc ® on Clinac MLC controller
Technical description RapidArc ® on Clinac Gantry speed must slow down so that the MLC leaves can catch up to the specified leaf positions Maximum treatment time depends on complexity of the treatment plan Gantry must slows down to deliver lots of doses or MUs or increase MU rate To maximize treatment time, use lower prescribed dose or maximum MU rate
Technical description RapidArc ® on Clinac Variable gantry speed 0.5 – 4.8 degree/sec Variable dose rate 0 – 600 MU/min Variable dose per degree 0.2 – 20 MU/degree Variable MLC speed 0 – 2.5 cm/s (5 mm/degree) Note : It depends on energies / DR
Technical description RapidArc ® on Clinac Arcs / plan* Total arc / plan* Min arc* Segments may be avoided Max dose : 7200 MU 9999 MU for 6X SRS (7.9+) *Note : It depends on SW releases
LOG Files During the treatment, the system can generate Logs: Clinac console (Communication/Log): Generation of a Dynamic beam delivery Log 4DITC WS : Generation of a MLC log (2 files Carriages A & B)
Dynamic MLC Arc log / Clinac Console OOPS!! January 2008!!! It is Not a RapidArc or VMAT Log!!!
MLC Log – Dynalog File Viewer Dynamic leaf deviation Log Activation : Because DynaLog files are typically large, Varian recommends that this feature should be turned OFF, and only activated when there is a specific need for DynaLog files to be saved. DiagAutoDynalogs 2,1 for SW 6.8 DiagAutoDynalogs 1 for SW 7.x How to read?
Control of the Dose rate Fast beam-on / beam-off control. On high energy Clinac : the key is the triode gridded gun (On Unique system : magnetron frequency)
Control of the Dose rate On high Energy Clinac the gridded electron gun allows Instantaneous dose rate control The cathode is heated to excite emissions of electrons. The injection is controlled by the grid.
Gridded GUN - HE Clinac The grid of the gun is used as an On/Off trigger which allows us to control the output electron emissions. A negative voltage is used to inhibit electron emissions and an approximately +160VDC pulse is used to allow the electrons to be released from the gun and into the guide. Gun is pulsed continuously for constant temperature and emission This gives us a very precise control so we can terminate or start the gun’s electron flow as required
Gun controller PULSE CONTROL GUN CONTROLLER KLY Current signal Used as the pulse Reference Constant time relationship to RF power in the guide.
MLC The MLC workstation integrated in the 4DITC, sends data for entire treatment for all leaves, including dose versus position information, to MLC Controller via serial link MLC Controller will set MLC leaves in place Upon start of treatment, MLC Controller sends commands to MLC head via optical link to move leaves MLC Controller compares planned and actual position of each leaf obtained from feedback system with the treatment plan
MLC - Primary Readout System Leaf Position is determined using 2 independent sources Primary Readout utilizes encoder mechanically attached to motor’s shaft for both carriages and leafs Carriage 540 counts = 1 mm of linear shift at isocenter Leaf 707 counts = 1 mm linear shift for full width leafs 512 counts = 1 mm linear shift for half width leafs at isocenter
MLC - Secondary Readout System / Interlock Secondary Feedback verifies, that motor count really represents actual motion Carriage – Mylar strip with fine black lines at the side of carriage path is read by optical pair Leaf Secondary Feedback – Ceramic Ball (Wiper) on each leaf arm provides contact pressure on a “Soft Potentiometer” Interlock PRO/SPRO :The MLC Controller compares the Secondary position of the leaf (Soft Pot) to its primary position (Motor Encoder Counts)
Gantry rotation control Prerequisite : From clutch drive to direct-drive Configuration - Clinac console SW 7.8 - 7.9 - 7.11 &+ Clutchless drive & Velocity check enable Chain tightness Speed : Aerotech Motor Control board. It drives the motor with a Pulse width modulated 100VDC (60s -0/+3s)
RapidArc® delivery of the TrueBeam® accelerator. What’s different?
RapidArc® delivery of the TrueBeam accelerator compared to Clinac) A TrueBeam system can deliver treatments up to 50% faster with a dose delivery rate of up to 2400 MU/min The TrueBeam delivers 'gated' RapidArc radiotherapy, which compensates for tumor motion by synchronizing imaging with dose delivery during a continuous rotation around the patient.
TrueBeam Overview The Truebeam is a full integrated sytem
Thank you for your interest and attention Questions Discussion
Optimize RapidArc delivery – Technical aspect Why a specific Machine QA ? During RapidArc: MLC leaves are moving Gantry is rotating Dose rate is changing
Many questions? Precision of the dose rate during gantry rotation ? Accuracy of gantry speed control ? Ability to accurately vary the MLC leaves speed ? Accuracy of the MLC leaves positions ? Gravity effect / gantry angle?
VMAT / RapidArc tests Sweeping gap ratio – Leaf gap Offset / dosimetry gap Picket fence (PF) Accuracy of DMLC position Static & during RapidArc DRGS : Ability to vary dose-rate and gantry speed 7 combinations of dose rate, gantry range and speed to give equal dose to each strip DRMLC : Ability to accurately vary MLC leaf speed and dose rate. 4 combinations of leaf speed and dose rate to give equal dose to each strip
Technical approach / optimization In a technical point of view what can we check ? The Mechanical performance (gantry, MLC) The Beam performance try to make a diagnostic in case of anomaly (beam or leaf related?) It is a separate approach, except for the absolute dosimetry calibration : Leaf Gap offset
The Mechanical performance (Clinac) Gantry Isocenter check (Basic system QA) Chain correctly tightened (V7.11 & +). Check at few angles.
Gantry (Clinac) Bearing lubrification - every 2 years Gantry well balanced? Check current (GAN MOTI - check the gain 6.45) Check “Auto Go” function (no stop short, no oscillation). AMC, 4 adjustments (current limit, loop back gain, offset, speed) 60s -0/+3s in both directions. If difference, check HCP / console Readout (PRO/SPRO). Replace the potmeters in case of frequent HWFA failures or “velocity check” errors. No backlash. Try to get the better “PRO calibration deviation” - Calibration
Calibration is performed at the time of installation and after any discrepancies in leaf positioning are found An Alignment tool (10 mm metal bar) is attached to the Collimator to perform Calibration of the Leaf Banks An Infrared Optical beam in front of each carriage (when retracted) creates reference position for each leaf Encoder Counts (information representing linear move of the leaf) are referenced to the optical beam Calibration process. Differences SW version 6.8 / 7.x Mlcxcal.txt file (sw 6.8) diagAdjustSysOffsets command (sw 7.2+)
MLC Calibration Skew Centerline Leaf Gap For rounded MLC leaf ends WARNING : don’t touch these parameters without knowing the consequences LightRadiation Leaf Gap
MLC Calibration Leaf Gap error / offset This parameter is directly linked to the Dosimetry Leaf Gap (TPS). Round leaf edge introduces additional transmission through the tip of the leaves. In the TPS, it is considered as an apparent gap between 2 closed leaves with non rounded edge.
Initialization gap 0.09mm A1 : Leading photodetector A2 : Trailing photodetector When the signal of the leading detector is 50% of the signal of the trailing detector, the optical receiver asserts the Leaf-at-cal signal. At this point the MLC controller monitors (counts) the motor encoder signals. If the trailing photodetector is not receiving light from the infrared emitter, something is blocking the beam and the optical receiver asserts a Leaf-in-field signal. Emitter From Jiri Bocanek / Varian
MLC Optical beam WARNING : If adjustment or replacement of the emitter and/or the receiver are required, MLC leaf calibration, and therefore, MLC dose delivery could be affected Measurement of the optical beam – Opto Receiver TP1 to TP2 = A1 minus A2 TP2 to TP3 = A1 plus A2
MLC Optical beam CTB-ML-570 Varian recommendations Use the detector value (TP2-TP3) alone to determine when to change the emitter If the voltage (TP2-TP3) is less than 65mV, need emitter replacement and alignment. Always perform a full alignment procedure whenever any IR beam component is moved for any reason
MLC Dynalog File Viewer Use the Dynalog files as reference to monitor the wear of components. Check RMS data (average/Maximum/Histogram) ie : Use DLV, for the leaf speed test (acceptance of the RapidArc) or during other specific tests (QC).
Carriages : Varian recommendation (CTB-ML-422) Annually - Carriage Backlash Test - Hyperterminal Command: ws 2 for SW6.8, ws for SW7.x) Collimator 0° Gantry rotation from 180° every 90° Collect Carriage A & B secondary readout from controller (1/100 th of mm) Difference between High and low values # 10 Higher values indicate wear, or looseness of the primary bearing
Diagnostic commands Using the Hyperterminal Version 6.8 SoftwareVersion 7.x Software diagPosILShow 2diagPosILShow diagSecStatsShow 2diagSecStatsShow diagMaxDeltaPriSecShow 2diagMaxDeltaPriSecShow diagPriStallStatsShow 2
Leaf speed Version 6.8 SoftwareVersion 7.x Software InitializeCTRL+xwr PWM commanddiagLeafPWMTestMlc 2diagLeafPWMTestMlc Each leaf speed should be similar to each other with a slight variation between the wider leaves and the thinner leaves Every leaf must meet a minimum speed of 2.5 cm/sec at isocenter (1,275 cm/sec at leaf plane) Initialize the MLC and run the PWM test
Leaf speed - PWM Test If PWM < 12 for full leaves or PWM < 22 for half leaves, then no action is required If PWM > 35 for full or half leaves, then cleaning or other service is required If PWM is in between these values, then particular attention should be paid to leaves with significantly different values than the average.
Leaf speed - PWM Test High PWM values typically require leaf cleaning and/or leaf train component replacement. Adjacent pairs of leaves Dirty leaves Individual leave Drive train component pb (excessive wear of the motor, drive screw…etc) Note : Higher PWM values (up to 2x) may be seen with CLL motors (1105360-09 and 1105182-07). Grey color cables, instead of black. Try to save all these diagnostic tests data for future comparison
Leaf Backlash test Version 6.8 SoftwareVersion 7.2 Software diagLeafBacklashAll 2,1 (carriage A) diagLeafBacklashAll 1 (carriage A) diagLeafBacklashAll 2,2 (carriage B) diagLeafBacklashAll 2 (carriage B) Position the gantry in the head-down position In Hyperterminal, run the following applicable diagnostic commands
Leaf Backlash Analyze the backlash values in the first column after the leaf numbers. Any values >0.200 require service. This typically requires leaf T-nut replacement
Leaf Touch Test – SW 7.x After a successful initialization, observe the Touch Test results at the bottom of the Hyper Terminal screen. Verify leaf gap values are less than +/- 0.20 mm. If any of the leaf gap values are >+/-0.20 mm, the MLC system requires a realignment. Leaf gap errors >0.30 mm will result in initialization failures.
Beam performance Machine QC: In case of an anomaly with DRGS, or DRMLC tests All leaves? Beam or carriage? Beam quality – main parameters to control High voltage (PFN / HVPSI) PFN servo Gun emission (GunI) Hyper Frequency power (RF driver and AFC) AFC servo (no limitation on AFC, good system temperature) FWRPW / KLYI / Target current - flat shapes
Beam performance A good STARTUP of the beam is important. Dose rate stability +15-18% with and w/o the PFN servo Dosimetry / POS & ANG servos No difference of DR with close and open loops. Field light / Xray beam correct with closed loops Gantry rotation : No loss of DR (DR servo open)
Thank you for your interest and attention Questions Discussion