Download Accounting for Imaging Dose: Impact of California Regulation

4/3/2011
High Profile Over-exposures Lead to Growing Concern
Accounting for Imaging Dose
FDA issues warning in October 2009
- 209 patients exposed to 8 times typical dose for CT brain
perfusion scan (3-4 Gy)
- Some patients experienced erythema and hair loss
- No hardware malfunction was found
NYT Series “The Radiation Boom”
Josephine Chen, PhD
UCSF Radiation Oncology Update
April 1-3, 2011
How Can Large Over-Exposures Happen?
CT images are reconstructed from raw data
- noise level may change with patient exposure, but
reconstructed image will never look over-exposed
Operator can manually change scan parameters affecting
CT output.
Some scanners have automatic controls that adjust output.
- highlights over-exposures and errors in diagnostic imaging
and radiotherapy
CA Legislature Responds with New Regulations
CA Senate Bill 1237 passed Sep 2010
Section 1 (effective July 1, 2012)
• Record CT dose parameters for each CT scan
• Annual verification of the CT dose displayed by scanner
Section 2 (effective July 1, 2013)
Mandatory CT accreditation
Section 3 (effective Jan 1, 2011, SB 38 July 1, 2012)
• Report CT over-exposures exceeding certain dose limits
• Report CT or RT over-doses to fetus or incorrect site, etc
www.legislature.ca.gov
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Radiation Oncology Exemption
Understanding Patient Dose from CT
CA Dept of Public Health Information Notice, Jan 14, 2011
• Factors Determining Patient Dose
“By definition, CTs used for radiation therapy planning is
defined as radiation therapy simulation systems, and so
are not subject to this new reporting requirements.”
• Quantities Used to Describe CT Dose
Rad Onc depts are not required to report CT doses/incidents.
• Recommendations and Sources of Additional Information
• Typical and Recommended CT Dose Values
www.cdph.ca.gov/programs/pages/radiologichealthbranch.aspx
Parameters that Affect Patient Dose
CT
mAs: xray tube current × time for one rotation
Effective mAs: mAs / slice (spiral scans)
pitch: table travel (for 1 rotation) / total beam width
kVp: tube acceleration potential
beam filtration, beam collimation, ring geometry
scan length
Patient
size/weight
New CT Technology Also Affects Patient Dose
MDCT (multiple-detector-row CT):
multiple slices acquired in one rotation
AEC (automatic exposure control) or
ATCM (automatic tube current modulation):
automatic adjustment to output to compensate for variation
in patient size/attenuation
- small vs. large patient
- neck vs. abdomen
AEC can both decrease and increase patient dose
organ size/location
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How Do We Measure CT Dose
Standard measures of CT output
Finding CTDIvol and DLP
CT study protocol page
CTDIvol (Volume CT Dose Index) [mGy]
Ion chamber measurement in standardized phantoms
32 cm dia cylinder (body), 16 cm diameter cylinder (head)
Ave dose to center slice of phantom for 10 cm scan
DLP (Dose Length Product) [mGy cm]
CTDIvol × (Length of scan)
- increases with scan range
- Although these parameters refer to “dose”, the dose is
for a specific phantom. Dose to the patient depends on
patient shape/size, length/range of scan, organ position.
- Most CT scanners report CTDIvol and DLP
How Does CTDIvol Relate to Organ Doses
Dose Quantities Used for Non-therapeutic Exposures
Focus is potential stochastic effects (cancer induction)
Use Monte Carlo or measurements to find organ doses.
Organ
Dose Equivalent (Sv):
dose multiplied by radiation weighting factor
(1 for photons/electrons)
Brain
Adult
Dose/CTDI
0.84a
Thyroid
1.84b
2.45b
Breast
1.03b
1.98b
Effective Dose (Sv):
weighted sum of dose equivalent × tissue weighting factors
effective uniform whole body dose
- sensitivity of each tissue to stochastic effects
- averaged over gender and age
Lung
1.53b
2.25b
Liver
1.14b
1.94b
Stomach
1.22b
1.11b
Skin
2.18c
Absorbed Dose (Gy)
aPhys
Child (6 yrs)
Dose/CTDI
Med Biol 53:N9; bBr J Radiol 82:1010; cAJR 190:W100
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How Does DLP Relate to Effective Dose
Empirical relationship proposed by the NRPB (UK)
Typical CT Doses
Effective Doses
Examination
Effective Dose = k × DLP
k [mSv mGy-1 cm-1]
Region of Body
Head
Neck
Chest
Abdomen/Pelvis
Adult
0.0021*
0.0059*
0.014
0.015
5-year old
0.004*
0.011*
0.018*
0.020*
Head
Neck
Chest
Abdomen
Average Effective
Dose (mSv)
2*
3*
7*
8*
Pelvis
4D CT Lung
6*
15
(annual background dose ~ 3 mSv)
* CTDI measured in head phantom
Organ Doses: 15-30* mGy
ICRP 102
CT Dose Compared to RT Dose
Inside treatment volume: ~ 0.1% of RT dose
Exit beam: ~ 1% of RT dose
In treatment plane, No direct beam: ~ 2% of RT dose
Peripheral (10-30 cm outside field): 10-100% of RT dose
*Radiology 2008 248:254
Recommended CTDIvol Limits
2008 ACR Guidelines
Adult Abdomen < 25 mGy
Adult Head < 75 mGy
Image Gently Recommendations for Reduction in mAs/slice
5 yr-old Abdomen - 41% reduction
5 yr-old Head – 7% reduction
Alliance for Radiation Safety in Pediatric Imaging
www.imagegently.org
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Recommended Actions
Additional Information
• CT operator training for specific scanner
AAPM Report 96 (TG 23): CTDIvol and DLP
• Periodic evaluation of CT protocols
remove unused protocols, create new protocols
ICRP Pub 102: CT technology and dose
• Check for accidental changes in protocol presets
Image Wisely and Image Gently Campaigns
www.imagewisely.org
www.imagegently.org
• Record the CTDIvol and DLP for each scan (SB 1237)
2011 AAPM CT Dose Summit, Oct 2011
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