Greetings from Philadelphia!

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Greetings from Philadelphia!
Independance Hall
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Quality Assurance of CT Simulatorin Modern
Radiation TherapyAndrew Wu, PhD,
FAAPMDepartment of Radiologic SciencesThomas
Jefferson UniversityPhiladelphia,
PennsylvaniaU.S.A.

• AAPM, Task Group Reports - 39 (CT scan), 40(Rad
  Onc), 53(Tx planning), and 66 (CT simulator).

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What are the differences in QA between CT
scanner and CT simulator?

• References
• AAPM Report 39
• "Specification and Acceptance Testing for
  Computed Tomography Scanners".
• AAPM Report 66
• "Quality Assurance for Computed Tomography
  Simulators and Computed Tomography Simulation
  Process".

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The CT-simulator IS a CT scanner equipped with

• 1) A flat table-top,
• 2) A large bore opening (donut hole)
• with a large field-of-view (FOV),
• 3) External patient positioning lasers, and
• 4) A specialized 3D software for treatment
  planning (Virtual simulation).

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What is Virtual simulation ?

• Using specialized CT and planning software in an
  advanced computer, the planning of radiation
  therapy treatment has the ability
• 1) to delineate tumors and adjacent normal
  structures in 3 dimensions, and
• 2) to accurately place radiation beams for the
  patients to be treated.

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QA of CT-simulator includes

• (I) Geometric accuracy
• Positioning lasers
• Movements of mechanical components.
• (II) Imaging performance
• CT number accuracy
• Image noise
• In plane spatial integrity
• field uniformity
• Spatial resolution
• Contrast resolution
• (III) Safety of patients
• Radiation dose to patient

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• (I) Geometric accuracy
• Positioning lasers

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• (I) Geometric accuracy
• Positioning lasers

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• (I) Geometric accuracy
• Positioning lasers

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• (I) Geometric accuracy
• Positioning lasers

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(I) Geometric accuracy - Movements of mechanical
components.
The registration device allows the patient
immobilization device to be moved from the
CT-scanner to a treatment machine in a
reproducible manner,
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• (I) Geometric accuracy -
• Movements of mechanical components.

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• (I) Geometric accuracy -
• Movements of mechanical components.

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(II) Image Quality QA Tests

1. CT number accuracy
2. Image noise
3. In plane spatial integrity
4. field uniformity
5. Spatial resolution
6. Contrast resolution

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Image performance evaluation phantom Image
noise and field uniformity
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Image performance evaluation phantom In plane
spatial integrity
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Image performance evaluation phantom spatial
resolution (line-pair/cm)
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Image performance evaluation phantom contrast
resolution.
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• (III) Safety of patients - Radiation dose to
  patient - CTDI

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Liberty Bell
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Hair loss in patients who received CT radiation
overdoses
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• (III) Verify safe dose delivered from CT
• CTDI is Computerized Tomography Dose Index
• represents the integrated dose, along the z axis,
  from one axial CT-scan (one rotation of the x-ray
  tube)

CT slide
CTDI
z
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TDCI .
Z

• Theoretically, the CTDI should be measured along
  z-axis from plus to minus infinite, i.e., CTDIFDA
  or CTDI8.
• In practice, the ion chamber to measure CTDI is
  typically 100 mm long, the IEC has specifically
  defined CTDI100.
• CTDI100 is defferent from CTDI 8 .

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-50
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Definition of CTDI100
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A body and head phantom for measurement of dose
from CT-scans. Pencil ionization chamber is
inserted in the center of the body phantom.
32 cm dia
16 cm dia
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where
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and
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What are you measuring?

• If you just measured CTDI100 for QA,
• CTDI100 the dose in air at the center of a
  single slide of an axial scan integrating over a
  length of 100 mm in the clinical setting with an
  ionization chamber.

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CT Dose Index and Patient Dose They Are Not the
Same Thing ?

• By Cynthia H. McCollough, PhD, Shuai Leng, PhD,
  Lifeng Yu, PhD, Dianna D. Cody, PhD, John M.
  Boone, PhD and Michael F. McNitt-Gray, PhD
• Radiology

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Radiation exposure from CT scans in childhood and
subsequent risk of leukaemia and brain tumours a
retrospective cohort study

• Published by The Lancet, Early Online
  Publication, 7 June 2012
• doi10.1016/S0140-6736(12)60815-0Cite or Link
  Using DOI
• Dr Mark S Pearce PhD a , Jane A Salotti PhD a,
  Mark P Little PhD c, Kieran McHugh FRCR d,
  Choonsik Lee PhD c, Kwang Pyo Kim PhD e, Nicola L
  Howe MSc a, Cecile M Ronckers PhD c f, Preetha
  Rajaraman PhD c, Alan W Craft MD b, Louise Parker
  PhD g, Amy Berrington de González DPhil c

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TIME magazine reported

• Between 1996 and 2010, the use of CT scans
  climbed nearly 8 annually.
• That translated into a doubling of radiation
  exposure per capita among the study participants,
  from 1.2 mSv to 2.3 mSv.
• Also, a doubling of the portion of patients who
  ended up receiving high (gt20-50 mSv) or very high
  (gt50 mSv) doses of radiation from the
  scans.Read more http//healthland.time.com/2012
  /06/13/too-many-scans-use-of-ct-scans-triples-stud
  y-finds/ixzz1ykHNz5vt

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Findings

• During follow-up,
• 74 of 178 604 patients were diagnosed with
  leukaemia and 135 of 176 587 patients were
  diagnosed with brain tumors.
• It was noted a positive association between
  radiation dose from CT scans received in
  childhood and leukaemia (p00097) and brain
  tumours (plt00001).

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Findings (continue)

• Compared with patients who received a dose of
  less than 0.5 cGy,
• the relative risk of leukaemia for patients who
  received a cumulative dose of at least 3 cGy
  (mean dose 5.1-1.3 cGy) was 318 and
• the relative risk of brain cancer for patients
  who received a cumulative dose of 5-7.4 cGy (mean
  dose 6.0 cGy) was 282.

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Radiation exposure from CT scans in childhood and
subsequent risk of leukaemia and brain tumours a
retrospective cohort studyMark S Pearce, Jane A
Salotti, Mark P Little, Kieran McHugh, Choonsik
Lee, Kwang Pyo Kim, Nicola L Howe, Cecile M
Ronckers, Preetha Rajaraman, Sir Alan W Craft,
Louise Parker, Amy Berrington de
Gonzálezwww.thelancet.com Published online June
7, 2012 DOI10.1016/S0140-6736(12)60815-0
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Radiation exposure from CT scans in childhood and
subsequent risk of leukaemia and brain tumours a
retrospective cohort studyMark S Pearce, Jane A
Salotti, Mark P Little, Kieran McHugh, Choonsik
Lee, Kwang Pyo Kim, Nicola L Howe, Cecile M
Ronckers, Preetha Rajaraman, Sir Alan W Craft,
Louise Parker, Amy Berrington de
Gonzálezwww.thelancet.com Published online June
7, 2012 DOI10.1016/S0140-6736(12)60815-0
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Interpretation .

• Use of CT scans in children to deliver cumulative
  doses of about 5 cGy might almost triple the risk
  of leukaemia and doses of about 6 cGy might
  triple the risk of brain cancer.

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To verify patient dose for medical imaging
examinations or procedures such as CT Simulation
in current radiation therapy practices today, it
is important to revisit the concept of CTDI.
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Therefore, CTDIave CTDIw

• For an axial image of body CT, the CTDI is
  typically a factor or two higher at the surface
  than at the center of the field-of-view.
• The average CTDI across the field-of-view (x-y
  plane) is given by the weighted CTDI (CTDIw),
• where CTDIw 2/3 CTDI(surface)
• 1/3 CTDI(center)

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Equipment typically used to measure CTDI100
includes an integrating electrometer (black
arrow), a 100-mm-long CTDI ionization chamber
(white arrow), and a CTDI phantom made of
polymethylmethacrylate (arrowhead).
CTDIave CTDIw
McCollough C H et al. Radiology 2011259311-316
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(a) Radiation dose profile along a line
perpendicular to the scan plane shows a peak dose
level at the center of the primary beam and long
dose tails caused by scattered radiation.
McCollough C H et al. Radiology 2011259311-316
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(a) Radiation dose profile along a line
perpendicular to the scan plane shows a peak dose
level at the center of the primary beam and long
dose tails caused by scattered radiation.
McCollough C H et al. Radiology 2011259311-316
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Multiple Scan Average Dose (MSAD)

• In a volumetric scan with multiple slides, N,
• MSAD SUM of (individual scan dose profiles)
• As the number of individual scans increases,
• the multiple scans average dose (MSAD) also
  increases and reaches a limiting value.

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• where
• DN,I(z) is the dose as a function of position for
  a multiple scan dose profile consisting of N
  scans separated by a constant distance between
  scans equal to I.

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Multiple scan dose profiles (MSDP)Using the
CTDI100 definition, volume CTDIw (CTDIvol),
which is equivalent to MSAD.
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Spiral CT

• the ratio of the table travel per rotation (I) to
  the total nominal beam width (N.T) is referred to
  as pitch.
• Therefore,

• where
• CTDIw represents the average radiation dose
  over the
• x and y directions and
• CTDIvol represents the average radiation
  dose over the
• x, y, and z directions.

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Dose-Length Product (DLP)

• Defined as the total energy absorbed by a scan
  volume from a given clinical protocol.
• DLP (mGy-cm)CTDIvol (mGy) x Scan length(cm)
• While two scan protocols may have the same
  CTDIvol, their DLP value may be substantially
  difference in scan volume length.

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• Real CT Doses - pt size-specific dose (PSSD)
• For a patient with an anteroposterior dimension
  of 30 cm and a lateral dimension of 40 cm, the
  anteroposterior lateral value would be 70 cm
  and the mean patient dose in the center of the
  scan range would be approximately equivalent to
  the CTDIvol value reported on the console.
• CTDIvol measurements made on the basis of 32-cm
  phantoms.
• It would require a different scale factor if the
  measurements made on the basis of 16-cm phantoms .

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Graph shows relative dose (mean patient dose per
1 mGy of scanner output, CTDIvol) for an
abdominal CT scan and different patient sizes
(here represented by the sum of anteroposterior
A/P and lateral dimensions).
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McCollough C H et al. Radiology 2011259311-316
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Real CT Doses

• For a neonate having both anteroposterior and
  lateral dimensions of 10 cm, the anteroposterior
  lateral value would be 20 cm and the mean
  patient dose in the center of an abdomen scan
  would be about 2.3 times the displayed CTDI
  value.
• CTDIvol measurements made on the basis of 32-cm
  phantoms.
• It would require a different scale factor if the
  measurements made on the basis of 16-cm phantoms
  .

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Graph shows relative dose (mean patient dose per
1 mGy of scanner output, CTDIvol) for an
abdominal CT scan and different patient sizes
(here represented by the sum of anteroposterior
A/P and lateral dimensions).
2.3
70
20
McCollough C H et al. Radiology 2011259311-316
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CT Dose Index and Patient Dose They Are Not the
Same Thing ?
NO, the real patient dose is HIGHER than CTDI !
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FDA proposed

• A label that would include a cautionary statement
  saying the device is not for use on patients
  under a certain size.
• The label, along with other aspects of the draft
  guidance and FDA's approach to pediatric imaging
  devices,
• were debated during a public meeting on Monday,
  July 16, 2012.

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Thank you!