When Dosimetry Goes Wrong in Therapy and Imaging · When Dosimetry Goes Wrong in Therapy and...
Transcript of When Dosimetry Goes Wrong in Therapy and Imaging · When Dosimetry Goes Wrong in Therapy and...
When Dosimetry Goes Wrong in Therapy and Imaging
Keith FaulknerOla Holmberg
Joanna IzewskaTommy Knöös
Pedro Ortiz-LopezGeoffrey S. Ibbott
IDOS, Nov. 9, 2010
Categories of errors• Errors made at commissioning: new machine or procedure
• Errors made through QA, such as patient-specific dosimetry, or QA/dosimetry wasn’t done
• Errors introduced through calculation (incorrect dosimetry data, incorrect application of data, inadequate procedures)
• Dose error resulting from equipment malfunction or introduced by incorrect or unrecognized service work
IDOS, Nov. 9, 2010
Errors in Radiology and Nuclear Medicine
• Determinations of fetal dose
• Unknown pregnancy, errors in determining dose
• Unwanted exposure from incorrect imaging parameters
• CT angiography events in the US recently
• pediatric patient receiving repeated exposures
• Dose to members of the public from I-131 patients
• Use of public transportation or hotel rooms
IDOS, Nov. 9, 2010
Errors in Radiation Therapy
• Experience from TLD audits showing variations in machine calibration.
• Experience from RPC phantoms showing errors in IMRT and 3D dose delivery.
• Published reports of events including:
• Errors in basic machine calibration
• Errors in output factors (small fields, half-blocked fields)
• Errors in applying calculation parameters
IDOS, Nov. 9, 2010
Distribution of TLD resultsPhotons beams
within 7%Number of beams: 3051
Avg. RPC/Inst.: 0.999Stdev.: 1.6%
Electrons beamswithin 7%
Number of beams 4310Avg. RPC/Inst: 0.998
Stdev.: 1.7%
IAEA, July 6, 2010
Frac
tion
of in
stitu
tions
Institutions with One or More Unacceptable TLD Measurements
Year
7
75
80
85
90
95
100
AF AM EM EU SE WP
world region
% a
ccep
tabl
e re
sults
1st participation after f-up
IAEA/WHO TLD audit results within the 5% limit
In 2000-2009, 4440 beams were checked with 511 TLD results followed-up, of these ~50% in Eastern Europe
8
Reasons for discrepancies in TLD results
no data42%
calc. error16%
set-up error32%
other10%
Analysis of 511
discrepancies occurring in 2000 2009
9
No physicist, no dosimetry equipment?
1029 empty or incomplete TLD data sheets (no data on ion chamber
dosimetry)
3411 correctly filled-out TLD data sheets
Results of the IAEA/WHO TLD results, 2000-2009
75%
4%9% 12%
Within 5%>20%10%-20%5%-10%
93%
4%1% 2%
10
IAEA TLD audits: use of dosimetry Codes of Practice
TLD results vs. dosimetry Codes of Practice Years 2000-2009; N=4440, m=1.007, SD=3.8%
CoP N Mean DTLD /Dstat
SD (%)
NDw 1875 (13)* 1.003 2.6
NK 1162 (13) 1.005 3.3
NX 340 (7) 1.020 4.7
Ion chamber details only 129 (3) 1.020 5.4
No dosimetry data 858 (39) 1.011 5.1
*75 deviations beyond 20% excluded, in brackets
NDw42%
Nk27%
Nx8%
Unknown23%
110
10
20
30
40
50
60
70
80
90
perc
ent
Farmer 0.6 cc Small volume 0.1 - 0.3 cc Local make
2002-2003 2004-2005 2006-2007 2008-2009
1 4
IAEA TLD results: use of ionization chambers
(75 deviations beyond 20% excluded)
Chamber type N Mean DTLD /Dstat
SD (%)
Farmer 0.6 cc 2807 1.005 2.9Small volume 0.1 - 0.3 cc 316 1.008 3.9
Local make 229 1.016 5.1
No chamber details 1013 1.011 5.1
TLD results 2000-2009
12
94%
1%4%
(within 5 %)
(dev. 10 - 20 %)
(dev. > 20%)
6%
3 %
82%(within 5 %)
(dev. >20%)
Medical accelerators Co-60 units
IAEA/WHO TLD results (2000-2009)
2427 high-energy X-ray beams:148 deviations
beyond 5 % level
2013 Co-60 beams:363 deviations
beyond 5 % level
misadministration?
accident?
1%
(dev. 5-10%)tolerance
9%
tolerance(dev. 10 - 20 %)
misadministration?
(dev. 5-10%) accident?
IDOS, Nov. 9, 2010 13
RPC visits: the only completely independent comprehensive radiotherapy quality audit in North AmericaIAEA QUATRO visit is similar
On-Site Dosimetry Review Visit
Identify errors in dosimetry and QA and suggest improvements.Collect and verify dosimetry data for chart review.Improve quality of patient care.
IAEA, July 6, 2010
Reference Beam Calibration
Percent of Beams out of Criteria
(since 2000)
Photons ElectronsTLD (±5%) 3-5% 5-8%
Visits (±3%) 2-4% 3-14%
Reference Beam Calibration Percent of Inst. with ≥ 1 beam out of
criteria (since 2000)
Photons ElectronsTLD (±5%) 7-11% 6-12%
Visits (±3%) ~13% ~15%
On-Site Dosimetry Review Visit
IDOS, Nov. 9, 2010 15
Errors Regarding Number of Institutions (%)Review QA Program 127 (77%)
*Wedge Transmission 53 (32%)*Photon FSD (small fields) 46 (28%)Off-Axis, Beam Symmetry 42 (25%)
*Photon Depth Dose 34 (21%)*Electron Calibration 25 (15%)*Photon Calibration 22 (13%)
*Electron Depth Dose 19 (12%)
Selected discrepancies discovered 2004 – 2008
On-Site Dosimetry ReviewOn-Site Dosimetry Review
*70% of institutions received at least one of the significant dosimetry recommendations.
IDOS, Nov. 9, 2010
RPC Treatment Record Review
• Independent calculation of tumor dose
Agree within 5% (15% for implants)
• Verify dose, time, fractionation per protocol
• Notify institution if major deviation seen during review to prevent further deviations
IDOS, Nov. 9, 2010
Examples of Systematic Errors > 5% (>15%)
Error MagnitudeTPS used wrong depth when head frame used 27%
TPS did heterogeneity corrections incorrectly 8.5%
Institution ignored effects when >50% of the field was blocked 5%
Point of calculation near edge of field 6-7%
Non-measured output with average TLD > 5% 7%
Lung correction used, not allowed on protocol 9-13%
TPS wedge factor differs from clinical wedge factor 9%
AAPM/ASTRO Patient Safety Conference, June 24, 2010
RPC Phantoms
Pelvis (10)
Thorax (13)
Liver (2)H&N (31)SRS Head (4)
Spine (3)
IAEA, July 6, 2010
Phantom Results
Comparison between institution’s plan and delivered dose.
Phantom H&N Prostate Spine Lung LiverIrradiations 752 174 19 174 23
Pass 585 143 13 124 12Pass % 78% 82% 68% 71% 52%
Criteria 7%/4mm 7%/4mm 5%/3mm 5%/5mm 7%/4mm
Year introduced 2001 2004 2009 2004 2005
IAEA, July 6, 2010
Explanations for Failures
Explanation Minimum # of occurrences
incorrect output factors in TPS 1
incorrect PDD in TPS 1
IMRT Technique 3
Software error 1inadequacies in beam modeling at leaf
ends (Cadman, et al; PMB 2002) 14
QA procedures 3errors in couch indexing with Peacock
system 3
equipment performance 2
setup errors 7
IDOS, Nov. 9, 2010
Apparent Causes of Error
• Inadequate training
• Insufficient staffing
• Inadequate procedures • Inconsistent use of dosimetry protocols
• Poor equipment design• Obsolete equipment (cobalt units, dosimetry
equipment)• Cluttered accelerator consoles• Cumbersome and inaccurate methods of beam data
entry into TPSs
IDOS, Nov. 9, 2010
Lessons Learned – Conventional Equipment
• Independent check of calibration of new beams
• Independent verification of basic data entered into TPS, or of model developed to represent data
• Independent check of TPS calculations, including regular checks and removal of obsolete data and files
• Written policies and procedures
• Policy requiring verification by a physicist of all equipment repairs before treatment is resumed
• Patient monitoring upon completion of brachytherapy procedures
IDOS, Nov. 9, 2010
Lessons Learned – Advanced Technologies
• Commissioning procedures for conventional equipment are applicable to advanced technologies
• Implementing a new technology should be based on an evaluation of the expected benefit. A step-by-step approach should be followed, for example from 2D to 3D conformal therapy and from there to IMRT
• Proper training should always be included with new technologies
• Data integrity should be checked after any computer malfunction
• Dosimetry protocols for non-standard radiation beams are necessary
• Increased attention to image identification and understanding is critical
IDOS, Nov. 9, 2010
ASTRO Six-Point Response
• Database of errors
• Practice accreditation program
• Improved educational programs on QA and safety
• Education for patients and caregivers
• Improving data transfer
• New and expanded federal initiatives