Dose Rate Linac QA by HC120314134629

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									QA for the Radiotherapy
Salih Arican, M.Sc.
Quality Assurance
•Why do we need (IMRT) QA?
•Do I really need to do QA for each
IMRT patient?
•If I use an independent Monitor Unit
calculation program do I still need QA
for each Patient?
•Will I still need do IMRT QA after
we’ve treated 500 patients?
•If I expand my monthly machine QA
can I eliminate IMRT QA for each
patient?
What’s the Worst that Could Happen?

 Worst
         •Patient Death
         •Severe Complication
         •Bad administration
         •Major Treatment Deviation
         •Minor Treatment Deviation
         •Litigation
         •Lost Revenue

 Least
FDA Adverse Event Report
(06/16/2004):
Patient Overdosed by 13.8%

Patient subsequently died as a
result of complications related
to the mistreatment
FDA Adverse Event Report
(04/07/2005) :
   •Medical center reported that
   between 2004 and 2005 77 pts
   received radiation approx 52% in
   excess of their prescribed dose
   •The excess radiation was a result of
   a calculation error by the medical
   center physicist during calibration
   •This incident has been
   recognized/identified as "human
   error"
FDA Adverse Event Report
(04/22/2005)

 •Prostate IMRT patient
 treated to a higher dose
 than prescribed

 •Reported as Medical
 Physics user error
The overall accuracy
of (IMRT) treatment
depends on …
Reasons for errors

 Delivery errors
 TPS commissioning

 TPS algorithm weaknesses

 Organ Motion

 Patient Positioning
Mechanical accuracy of LINAC




• Gantry
• Collimator
• isocenter
Explanations for Failures
                             Explanation                                Minimum # of occurrences


                    incorrect output factors in TPS                                1


                        incorrect PDD in TPS                                       1

                            Software error                                         1


 inadequacies in beam modeling at leaf ends (Cadman, et al; PMB 2002)             14


  not adjusting MU to account for dose differences measured with ion
                                                                                   3
                               chamber

            errors in couch indexing with Peacock system                           3

                 2 mm tolerence on MLC leaf position                               1

                             setup errors                                          7


                          target malfunction                                       1
What is the Optimal Tool?
                                   Reliable and
                               Cost-effective QA




                        provide
                        3-D data
QA for IMRT: 4 Levels

                  •   Pre-Clinical verification of IMRT
                      treatment (patient related)
        4         •   Verification of fluence maps,
                      individual IMRT fields on water
                      phantom
        3         •   IMRT delivery specific QA


        2         •   Basic QA (LINAC, MLC)


        1
(IMRT) – QA Plan

                                         IMRT-QA Plan



 Comissioning and testing of the
                                           Routine QA of               Patient-Specific validation
      treatment planning
                                         the delivery system               of treatment plans
      and delivery system




         Accuracy of relative                                      Transmission characteristics
          MLC leaf position                                           (leakage) of the leafs


         The flatness and symmetry
                                                                Penumbra of the leaf ends
                 of the beam



                 Dose-per-MU constancy                         Speed of each leaf
Routine QA of the delivery system


• Does the radiation delivered have:

    The correct energy?
    The correct place?
    The correct dose?
    The correct intensity?
    The correct time?
Beam Stability: Flatness,Symmetry

 Stability of flatness and
 symmetry affects dose rate for
 small fields directed off the
 central axis.
Beam Stability: Dose Rate

  With IMRT delivery, there is the
  potential for short irradiation times
  (MUs).

  Dose rate stability influences the
  treatment precision.
Linac-QA: Dose Rate
Linac-QA: Dose / Pulse
Linac-QA: Beam Start-Up
Linac-QA:
Beam Position  Stabilization Time
LINAC-QA: Dose delivery
               8)               6)      15)     18)
             286,7            355,2    85,0    10,0




                       7)       5)
                     335,1    392,0




               4)      3)       2)      13)     16)
             423,9   459,3    515,3    124,5   47,6




                      11)       9)
                     196,9    257,0




          12)                  10)      14)     17)
          147,4               216,1    79,3     7,7




    Planned dose value pattern (18 steps, dose values in cGy)
+                           +     =



   Multiple Beam ‘Segments’        Resultant IMRT
Each with a Different MLC Shape   Beam Intensity Map
Measured   Calculated
MLC QA - check the influence of gravity
MLC Delivery Error at Gantry 90 deg
       Gantry 0 deg




                                                Individual segments


      Gantry 90 deg




                               After error analysis & correction
   leaf positioning failure!
Error in jaw position:




       Plan                measured       difference




                                         1.8 mm




                                                  Y1   jaw displaced by   1.8 mm
      Profiles   __ plan   __ measured
Leaf position uncertainties


  Beam widths of 1 cm,
  uncertainties of a few tenths
  of a millimeter in leaf position
  can cause dose uncertainties
  of several percent.
        e.g. 0.5mm  >5%
MLC QA: Accuracy of relative MLC leaf
position

 Leaf positioning accuracy:




MLC pairs form a narrow slot moving across the field,
stopping and reaccelerating at predefined positions
(garden fence technique)
Regular Pattern
   (golden standard)
Regular Pattern   Measured Pattern
1.0 mm
         1.0 mm
0.9 mm
0.8 mm
0.7 mm
0.6 mm
0.5 mm
0.4 mm
         0.5 mm
0.3 mm
0.2 mm
0.1 mm
Leaf speed accuracy


 The accuracy of dynamic MLC
 delivery depends on the
 accuracy with which the
 speed of each leaf is
 controlled.
MLC QA – Leaf Speed Test




 Leaf pairs form gaps moving with different speed




   Delivery with beam
   interrupts
Leaf transmission characteristic

  The transmission
  characteristics (leakage) of
  the MLC are important for
  IMRT because the leaves
  shadow the treatment area
  for a large fraction of the
  delivered MU.
                                                        Interleaf
                                                      Transmission


                                                         Leaf End
                                                       Transmission

All Leaves Closed         Radiation Leaks through between
   Completely                 Leaves and Across Ends


                                                        Interleaf
                                                      Transmission


                                                            Treatment
                                                              Field
   Collimator Jaw

Collimator Covers Field   Leaks between Sides Reduced with
 Up to Outermost Leaf            Backup Collimator
Transmission (Leakage) Check
Patient-specific Verification ?
• What is missing :


    Does the plan give correct dose distribution ?
    Does it fulfill the therapeutic requirements ?
    What is the influence of inter-fraction variation ?
    In case of 2D verification
       – What is the influence of revealed discrepancies on the dose distribution?
Pre-Treatment Verification

       Field oriented                     Plan oriented


   Gantry =0°
                        Rotating Gantry

                 X-


                                               X-
Comparison of predicted and measured
MLC-Shapes
                                                           MC-2


                         MC-SW

                                                                                  Deliveredfluence




    RTPS:Desired                                                                     Leaf- & Gantry
 3D-Dose- Distribution        RTPS:Desired Fluence-              Leaf Sequencer        sequence
                                      Map




             Inverse      Back-Projection                                         Delivered 3D-Dose-
                                                                                      Distribution
                                                      ArcCHECK
Patient-Specific validation of treatment plans




     Inverse
  Back-Projection


Treatment      MLC         Delivery    2D-Array
 Planning   Segmentation   System      /3D-Array



                           Delivered
                            Fluence
             Measured fluence map




comparison




                                              Predicted: --------
                                              Measured:

                      Predicted fluence map
                          DICOM_RT DOSE plan:




Beam1: G=210 C=180 segments=20
Beam2: G=260 C=180 segments=12
Beam3: G=310 C=180 segments=18
Beam4: G=0 C=180 segments=18
Beam5: G=50 C=180 segments=22
Beam6: G=100 C=180 segments=10
Beam7: G=150 C=180 segments=16
Plan oriented verification with 2D-Array
Beam 1: Gantry 210 degree   Beam 2: Gantry 260 degree     Beam 3: Gantry 310 degree        Beam 4: Gantry 0 degree




                                          Beam 5: Gantry 50 degree   Beam 6: Gantry 100 degree     Beam 7: Gantry 150 degree
Measured (composite) Beam 1 … Beam 7:
      Measured                     Calculated




IMRT-Composite field verification (MC-SW): Pass-rate: 97.5 %
Plan oriented verification with
ArcCHECK
ArcCHECK in action
VARIAN RapidArc Inselspital Bern-Switzerland




               Arc-1: Pass-Rate: 98%; Gamma: 3mm/3%
RD-Oxford Cancer Center H&N.dcm converted in AC_PLAN.txt   RD-Oxford Cancer Center H&N.dcm converted in AC_PLAN.txt imported
                                                           as 2D composite plan




                    The difference is clear: Cold-spot value at the
                    gantry angle x1 degree might be balanced
                    with hot-spot value at the gantry angle
                    degree x2. That effect can't be seen in
                    composite analysis result but with ArcCHECK
                    measured and unrolled fields!
 Film dosimetry: Plan oriented workflow




1. Planning of IMRT cycle for         2. Planning of same
      patient with RTPS               IMRT cycle but now
                                      with Body Phantom
                                                            3. Exposure of film in Body
                                                            Phantom to IMRT cycle



                                  5. Import of planned
                                  and measured data
                                  in analysis SW
                                                                  4. Development
                                                                  and digitization of
  6. Comparison of planned versus measured dose                   exposed film
Film



 The choise of film is very important.
 But even more important is the
 calibration of the film and the stability
 of the film processing environment
 and chemistry
Quantity               Calculation                    Measurement

3D-Dose Distribution   Apply Plan to Phantom.         Put Films in the Phantom. Process,
                       Calculate 3D-Dose              Scan, Calibrate Films. Compose 3D-
                       Distribution                   Dose Distribution




2D-Dose/Fluence        Calculate Fluence Pattern or   Film, 2D-Array, 3D-Array
                       2-D Dose Distribution


Leaf Positions         Leaf Positions from TPS        Film, 2D-Array, 3D-Array,
MLC QA


MU/Dose Check          Dose in a reference Point      Ion-Chamber/Electrometer


Penumbra               Needed for TPS                 Small Ion Chamber or Diode (SFD) in
measurement            Set-up                         3D-Phantom
Conclusions


 Quality assurance reduces
 uncertainties and errors in dosimetry,
 treatment planning, equipment
 performance, treatment delivery,
 etc., thereby improving dosimetric
 and geometric accuracy and the
 precision of dose delivery.
Conclusions

Quality assurance not only reduces the
likelihood of accidents and errors
occurring, it also increases the
probability that they will be
recognized and rectified sooner if they
do occur, thereby reducing their
consequences for patient treatment.
Conclusions


 Quality assurance allows a reliable
 comparison of results among
 different radiotherapy centers,
 ensuring a more uniform and
 accurate dosimetry and treatment
 delivery.
Conclusions


   Improved technology and
   more complex treatments in
   modern radiotherapy
   can only be fully exploited if a
   high level of accuracy and
   consistency is achieved.
Thank you,
 Questions?

								
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