Dosimetric Verification of IMRT , AnCheng Shiau Department of Radiation Therapy and Oncology, China

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Dosimetric Verification of IMRT ???, An-Cheng
Shiau?????????? ?????Department of Radiation
Therapy and Oncology, China Medical University
Hospital
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What is Intensity Modulated Radiation Therapy?
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Biological Rational for IMRT
Tumor Control Probability (TCP)
Normal Tissue Complication Probability (NTCP)
Conformal Avoidance
Dose Escalation under clinical trial
Conventional Technique
DOSE
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• Errors in dose distributions calculated by TPS
• the lack of lateral charged particle equilibrium
  in the small subfields,
• interleaf leakage of the multileaf collimator
  (MLC) is not accurately accounted for.
• Errors in the transfer of MLC leaf sequence files
  from TPS to the RV system
• Errors in the mechanical accuracy of the MLC leaf
  movements

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• IMRT treatment plans are complex and must be
  validated before use

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• Methods for IMRT QA
• Ion chamber ( point dose, low spatial resolution,
  chamber volume effect .. )
• TLD ( in vivo, stability ? time consuming ..)
• Film ( 2-D dose distribution, high spatial
  resolution, uncertainty ? time consuming ..)
• Gel dosimetry ( 3-D information, low spatial
  resolution, high uncertainty, time consuming )
• EPID ( 2-D, 3-D ? in vivo ? image or dose ?
  efficient ? real time dose verification ? )
• ? ( 3-D, in vivo, real time, efficient, accuracy
  .. )

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• Point dose verification
• Hybrid patient plan to solidwater phantom
• Irradiate according to this plan
• Measure the absolute dose of the selected point
  by using an pin point ion chamber

CC01 , 0.01 cm3
Farmer , 0.6 cm3
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• Point dose verification
• From 2003-8 2004-2 at CMUH

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• Point dose verification
• GR200F ,LiF (Mg, Cu, P), 0.1mm thickness , 5 mm
  diameter
• 95 confidence level (1.96s),
• CVlt5

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• Clinical dose measurement

Int. J. Radiation Oncology Biol. Phys., Vol. 53,
No. 3, pp. 630637, 2002
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• 2-D dose distribution check
• Film sensitometric curve
• Kodak XV2 film or EDR2 film
• or GafChromic film
• Kodak LS75 laser film digitizer

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• 2-D dose distribution check

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• Electronic Portal Imaging Device ( EPID )
• Mount on the linac
• Real-time, digital feedback to the user

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• EPID

• amorphous silicon based system
• less excess dose to be delivered to the patient
  per portal image and yet yielding a superior
  image quality
• aSi imaging device aS500 Rel. 6, Varian Medical
  systems
• Image detection unit ( IDU )
• Image acquisition unit ( IAS2 )
• Workstation ( PortalVision )

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• EPID aS500 system

• Image detection unit ( IDU )
• matrix of 512 x 384 pixels
• resolution of 0.784 x 0.784 mm2
• total sensitive area of 40 x 30 cm2.
• total water-equivalent thickness of the
  construction materials in front of the
  photodiodes is 8 mm

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• EPID aS500 system

• Image detection unit ( IDU )
• Each pixel consists of a light sensitive
  photodiode and a thin film transistor (TFT) to
  enable readout.
• The electric charge generated by the incident
  photons is accumulated in the photodiode until
  the signal is read out and digitized through an
  analogue to digital converter.
• Overlying the array is a scintillating layer
  (gadolinium oxysulphide) and a copper plate (of
  1 mm thickness), making the portal imager an
  indirect detection system.

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• EPID aS500 system

• Image acquisition unit ( IAS2 )
• During dose delivery, a continuous acquisition of
  frames is obtained.
• The ACPU contains a 14 bit A/D converter and is
  capable of adding 64 frames in its 20 bit
  hardware adder. Therefore, a transfer of the
  frame buffer to the CPU is mandatory after every
  64th frame. This introduces a readout interrupt
  of ,0.164s.
• Charge accumulation in the photodiode is not
  interfered with and it will thus not affect the
  final image accuracy, provided the accumulated
  charge between two subsequent readouts does not
  drive the 14 bit A/D converter into saturation.

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• EPID aS500 system

• Indirect flat panel imager designed for detecting
  MeV photons
• Photons incident on the detector
• interact with a 1 mm Cu plate and a
  scintillation screen 340 mm gadolinium
  oxy-sulfide phosphor (Gd2O2STb) , deposit
  energy into the screen.
• The fraction of energy deposited in the screen is
  converted to optical photons, which are then
  detected by an array of photodiodes

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• EPID aS500 2-D IMRT dose verification

• The signal from photodiodes can be quickly read
  this signal, once processed, forms the raw EPID
  image, EPIDraw

• EPIDrad averaged image
• EPIDflood flood field image
• EPIDdark dark current

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• EPID aS500 2-D IMRT dose verification

• The degradation of spatial resolution in the
  aS500
• x-ray scatter in the copper plate and screen,
• Optical scattering (or glare) in the screen.
• As a result, EPIDrad does not directly represent
  the incident photon fluence on the EPID.
• EPID image must be performed to measure the true
  spatial distribution of photon fluence required
  in IMRT verification.

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• EPID aS500 2-D IMRT dose verification

• Convolution method and scatter kernels
• linearity of the aS500s dose response
• each pixel value is proportional to the optical
  energy incident on that pixel
• the 2-D signal S(x,y) is essentially a
  convolution of the primary photon fluence,
  ?p(x,y), with a spreading kernel

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• EPID aS500 2-D IMRT dose verification

• Spreading kernel
• Kdose(x,y), dose deposition in the Gd2O2STb
  screen.
• generated using EGSnrc Monte Carlo software using
  the phantom geometry ( 30304.7 cm3 )
• Kglare(x,y), the optical photon spreading from
  the screen to the photo-diode layer
• fitted to satisfy the measured incident fluence
  in open fields using a detector
• Parameters C1 , C2 , and C3 were determined by
  fitting the tail region of fluence profiles
  derived from aS500 EPID images

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• EPID aS500 2-D IMRT dose verification

• Correction of raw EPID profile distortions caused
  by the flood-field correction
• Ideally, a flood-field image should be generated
  using a perfectly uniform fluence incident on the
  EPID

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• EPID aS500 2-D IMRT dose verification

• The incident photon fluence ?p(x,y)
• Conversion to dose in a solid water phantom

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• EPID aS500 2-D IMRT dose verification

• Summary of the steps

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• EPID aS500 2-D IMRT dose verification

• Can be performed with EPID-based method
• Errors ???
• Systemic ( TPS dose calculated . )
• Random ( MLC leaf movements, pt setup, organ
  motion . )
• Error elimination
• Monte Carlo based dose calculation algorithm
  using the photon fluence measured by EPID
• Modify the fluence map of TPS to deliver a
  correct dose distribution to the patient

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• EPID aS500 3-D IMRT dose verification

• limitation of 2-D dose verification
• how the errors quantified in a 2-D dose at a
  single depth in a water phantom relate to the
  cumulative errors in a 3-D dose distribution in
  the patient from all beams in the IMRT plan
• errors that appear small in the 2D dose might
  become additive in the 3D patient dose
  distribution
• or vice versa, errors that appear high in a
  single field may not be relevant in the total 3D
  plan

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• EPID aS500 3-D IMRT dose verification

• IMRT verification using 3D dose distribution on a
  patients CT anatomy

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• EPID aS500 3-D IMRT dose verification

• measured 2D fluence modulation
• for each field is re-sampled on a larger
  0.15?0.15 cm2 grid using linear interpolation,
  and then formatted appropriately for import as a
  compensator file into the TPS
• the measured replaces the
  optimized by the TPS for each field in the IMRT
  plan

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• EPID aS500 3-D IMRT dose verification

• Fluence profiles from the aS500 EPID and diamond
  detector scans

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• EPID aS500 3-D IMRT dose verification

• 3-D IMRT verification of clinical IMRT treatment
  plans

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• EPID aS500 3-D IMRT dose verification

• DISCUSSION
• The advantage of 3D IMRT verification is that
  dosimetric uncertainties can be quantified
  directly
• Limitation
• Calculation of our EPID-based 3D doses relies on
  the TPS itself
• errors introduced in this step by the TPS will
  not be identified by our verification procedure

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• DISCUSSION
• EPID dosimetry method could potentially be
  extended to in vivo verification
• the position and geometry of the patient should
  also be known.
• by cone beam CT ?
• So far no inhomogeneity corrections are
  implemented in the model
• 3D verification ?
• On-line verification ?

Cone beam image from Varian system
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Radiographic Mode (kV-MV)
Clinically practical, fast and automated use
FDA 510(k) Movie courtesy Hirslanden Klinik
Aarau, Switzerland