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Quality Assurance of CT Simulator in Modern Radiation Therapy* Andrew Wu, PhD, FAAPM Department of Radiologic Sciences Thomas Jefferson University Philadelphia, Pennsylvania U.S.A. * AAPM, Task Group Reports - 39 (CT scan), 40(Rad Onc), 53(Tx planning), and 66 (CT simulator).

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".

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).

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.

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

(I) Geometric accuracy Positioning lasers

(I) Geometric accuracy Positioning lasers

(I) Geometric accuracy Positioning lasers

(I) Geometric accuracy Positioning lasers

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

(I) Geometric accuracy - Movements of mechanical component s.

(I) Geometric accuracy - Movements of mechanical component s.

(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

Image performance evaluation phantom: Image noise and field uniformity

Image performance evaluation phantom: In plane spatial integrity

Image performance evaluation phantom: spatial resolution (line-pair/cm)

Image performance evaluation phantom: contrast resolution.

(III) Safety of patients - Radiation dose to patient - CTDI

Liberty Bell

Hair loss in patients who received CT radiation overdoses

(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) z CT slide CTDI

TDCI …. Theoretically, the CTDI should be measured along z-axis from plus to minus infinite, i.e., CTDI FDA  or CTDI ∞. In practice, the ion chamber to measure CTDI is typically 100 mm long, the IEC has specifically defined CTDI 100. CTDI 100 is defferent from CTDI ∞  Z

Definition of CTDI

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

where

and

What are you measuring? If you just measured CTDI 100 for QA, CTDI 100 = 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.

CT Dose Index and Patient Dose: They Are Not the Same Thing ? By Cynthia H. McColloughBy Cynthia H. McCollough, PhD, Shuai Leng, PhD, Lifeng Yu, PhD, Dianna D. Cody, PhD, John M. Boone, PhD and Michael F. McNitt-Gray, PhDShuai LengLifeng YuDianna D. CodyJohn M. BooneMichael F. McNitt-Gray Radiology

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 doi: /S (12) Cite or Link Using DOICite 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 cMark S PearceaJane A SalottiaMark P LittlecKieran McHughd Choonsik LeecKwang Pyo KimeNicola L HoweaCecile M Ronckerscf Preetha RajaramancAlan W CraftbLouise ParkergAmy Berrington de Gonzálezc August 23,

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 (>20-50 mSv) or very high (>50 mSv) doses of radiation from the scans. Read more: study-finds/#ixzz1ykHNz5vthttp://healthland.time.com/2012/06/13/too-many-scans-use-of-ct-scans-triples- study-finds/#ixzz1ykHNz5vt

Findings During follow-up, 74 of patients were diagnosed with leukaemia and 135 of patients were diagnosed with brain tumors. It was noted a positive association between radiation dose from CT scans received in childhood and leukaemia (p=0·0097) and brain tumours (p<0·0001).

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 cGy) was 3·18 and the relative risk of brain cancer for patients who received a cumulative dose of cGy (mean dose 6.0 cGy) was 2·82.

Radiation exposure from CT scans in childhood and subsequent risk of leukaemia and brain tumours: a retrospective cohort study Mark 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 González Published online June 7, 2012 DOI: /S (12)

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.

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.

Therefore, CTDI ave = CTDI w 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)

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). McCollough C H et al. Radiology 2011;259: CTDIave = CTDIw

(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 2011;259:

(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 2011;259:

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.

where D N,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.

Multiple scan dose profiles (MSDP) Using the CTDI 100 definition, volume CTDIw (CTDIvol), which is equivalent to MSAD.

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.

Dose-Length Product (DLP) Defined as the total energy absorbed by a scan volume from a given clinical protocol. DLP (mGy-cm)=CTDI vol (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.

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.

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). McCollough C H et al. Radiology 2011;259:

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.

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). McCollough C H et al. Radiology 2011;259:

NO, the real patient dose is HIGHER than CTDI ! CT Dose Index and Patient Dose: They Are Not the Same Thing ?

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.

Thank you!