Transcript IPEM 2014

Cone Beam CT Protocol
Optimisation for Prostate
Imaging with the Varian
Radiotherapy OBI imaging
system
Dr Craig Moore & Dr Tim Wood
Background
• With the increasing use of CBCT imaging alongside
complex radiotherapy treatment regimes, it is becoming
more important to understand the implications of current
practice
– On board CBCT daily imaging for verification of patient
position is now common practice across the UK
– It is not acceptable to simply dismiss these concomitant
exposures as negligible in comparison with the
radiotherapy treatment dose
• Currently, all of our CBCT systems operate using Varian
default settings
– A single set of exposure factors for all patients is clearly
not optimised!
• Vital we have an idea of patient doses so that we
can develop optimisation strategies
kV tube
Treatment head
– MV beams
generated here
kV detector
Aims
• This talk will focus on:
– Development of a computational method to
estimate dose and risk for CBCT prostate
imaging
– Development of a strategy for patient sized
protocol optimisation for CBCT prostate
imaging
The first step…
• The first phase of this project is to gain an understanding
of the doses involved in CBCT imaging
• Given the context of these procedures (i.e. as part of a
RT treatment), simple risk estimates based on the
effective dose are probably not sufficient in isolation
– We need to start thinking about organ-at-risk tolerances and
other healthy tissues that are not involved in the actual treatment
• Hence, we need to develop a broader understanding of
where the dose is being deposited, i.e. organ doses
• What is the best way to do this?
– TLDs?
– A computational model?
– A bit of both?
Developing a CBCT dose model
with PCXMC
• We have commercially available software (PCXMC) that
is widely used for performing dose assessments for
radiological examinations, etc
• Allows you to rotate around a reference point within a
mathematical (Christy) phantom (ideal for modelling RT
imaging)
– Only for simple uniform X-ray spectra
PCXMC
Only
uniform
beams
Half-fan bow-tie
filter = nonuniform beam
Can we account
this nonuniformity to
make it ‘fit’ with
PCXMC
The PCXMC model
• To model the Varian CBCT
system, 8 projections around
the patient were used (at 45°
intervals), with equal weighting
for the final dosimetry
• Each projection was split into 4
‘slithers’ to account for nonuniformity of the x-ray beam
• Treat each slither
independently for each
projection
– PCXMC requires the
correct air kerma and
4 slithers used to correct for
filtration for each slither to beam non-uniformity – treat
perform its calculation –
independently for each
need some beam
projection
profiling!!!!
CBCT beam profiling
• Air-Kerma and tube
filtration profiles were
measured with the Unfors
Xi chamber at the
isocentre, and using the
bed to step in 1 cm
increments across the full
width of the bow-tie
profile
• Air kerma taken directly
from the Unfors Xi,
filtration a little more
tricky!!
2.0
1.5
S2
S1
S3
S4
1.0
0.5
0.0
-5
0
5
10
15
20
25
Position relative to centre of field (cm)
35
Use this info to plug into
PCXMC to calculate
patient dose per slither
Total filtration (mm Al) .
Air Kerma (mGy/20 mAs @ 125 kVp) .
2.5
30
25
20
S1
15
S2
S3
5
10
S4
10
5
0
-5
0
15
Position relative to centre of field (cm)
20
25
Model validation
•
Performed TLD dosimetry on two
linear accelerators (RT treatment
machines), with Rando phantom
loaded with 80 TLD-100H chips in the
positions of the various important
organs in and around the scan volume
–
–
–
–
•
Liver & stomach were most superior
organs measured (well outside the
primary beam)
Uterus & ovaries – I know prostate
patients don’t have these, but it was
useful for validation purposes!
Bladder, prostate & testes – these were
all fully irradiated by the primary beam
Small and large intestine – partially
irradiated by the primary beam
Rando was positioned with the
‘prostate’ at the isocentre, and three
CBCT ‘Pelvis’ scans performed
Model validation – TLD dosimetry
Mean Organ Dose* (mGy)
Organ
RT1
RT2
Mean
Liver
0.5
0.5
0.5
Stomach
0.5
0.5
0.5
Uterus
15.9
15.3
15.6
Ovaries
8.6
8.3
8.4
Bladder
33.9
33.3
33.6
Prostate
30.2
30.0
30.1
Testicles
39.8
35.1
37.4
Small Intestine
3.0
3.0
3.0
Large Intestine
9.5
9.8
9.6
* Measured Air Kerma corrected for ratio of (μen/ρ)ICRU soft tissue/(μen/ρ)air
Model validation – The comparison
• So how do these compare with the PCXMC model?
Mean Organ Dose (mGy)
Organ
Mean TLD
PCXMC
% diff.
Liver
0.5
0.1
-80.0
Stomach
0.5
0.2
-60.0
Uterus
15.6
14.6
-6.4
Ovaries
8.4
9.1
8.3
Bladder
33.6
31.0
-7.7
Prostate
30.1
29.6
-1.7
Testicles
37.4
38.5
2.9
Small Intestine
3.0
2.6
-13.3
Large Intestine
9.6
7.9
-17.7
Large
distance
from beam
Not bad
given the
inherent
errors
associated
with TLD
dosimetry
Effective dose?
• Using PCXMC to calculate the effective dose, taking out
contribution to ovaries and uterus (not applicable to our
prostate patients!), and the prostate (which is the target
of the RT treatment, so probably should not be included
in the calculation)
– Effective dose = 6.0 mSv per scan
• Using TLD dosimetry with Rando
– Effective dose = 5.9 mSv per scan
• Good agreement!!
• For daily prostate imaging we get up to 222 mSv for a
37 fraction treatment regime , risk of fatal cancer:
– 1 in 150 for a healthy 60 year old male (using organ specific risk
factors)
– 1 in 90 using generic 5% per Sv
• Not insignificant!!!
Organ doses?
• Total individual organ doses for daily imaging with 37 fractions
(ignoring prostate);
– Bladder > 1.2 Gy
– Testicles > 1.4 Gy
– Large Intestine > 0.3 Gy
• These don’t feel insignificant!
Mean Organ Dose for 37# (Gy)
Organ
RT Treatment
Pelvic CBCT
CBCT as %
of RT
Gonads
0.8
>1.4
>175
Bladder
51.8
>1.2
>2.3
Colon
1.2
>0.3
>25
Rectum
40.0
>0.9
>2.3
Size specific CBCT
• Currently all Pelvis exposures use the same factors
(125 kVp/80mA/13ms/650 projections ~ 680 total
mAs)
• No compensation for patient size means the
organ/effective dose reduces as the patient gets
bigger
• But, we should probably be increasing exposure
factors for the biggest patients to ensure we get
acceptable images
– We have it on good authority that these patients are
difficult to image
• Equally, smaller patients should have a lower dose
protocol
Protocol Optimisation
•
•
•
•
•
•
Have started looking at patient size specific exposure protocols
We have used the CT AEC phantom Tim discussed in his talk earlier today
Scanned this at the default exposure setting
– 125 kVp, 80 mA, 13 ms per projection, 650 projections,
– Total of 680 mAs
Decreased the mA to assess the effect on image noise:
– 60
– 40
– 20
– 10
Wanted to increase mA as well but 80 mA is its upper limit!!!
Also scanned with increased/decreased ms:
– 7
– 13
– 14
– 15
– 16
– 17
– 20
– 23
– 26
Protocol Optimisation – Effect of mA (dose)
Noise with mA
120
100
80mA, 13ms
80
Noise (SD)
60mA, 13ms
40mA, 13ms
60
20mA, 13ms
40
20
Patient thickness
0
0
10
20
30
CT slice
40
50
60
Protocol Optimisation – Effect of
mA (dose)
• As expected decrease in noise as the mA (dose)
increases, for a given patient thickness
• Also, increase in noise as the patient gets
thicker, for a given mA (dose)
• There is definitely scope to optimise the mA for
average and thinner patients
– Possibly as low as 40 mA for the very thin ones??
• Even scope to decrease mA for thicker patients
– 60 mA is not too different in terms of noise compared
to 80 mA
Protocol Optimisation – Effect of ms
Noise with ms
70
60
80mA, 26ms
50
80mA, 23ms
Noise (SD)
80mA, 20ms
40
80mA, 13ms
80mA, 7ms
30
Large patients
20
Small patients
10
0
0
10
20
30
CT slice
40
50
60
Protocol Optimisation – Effect of
ms
• As patient size increases noise increases
– Less obvious with thinner patients
• As ms increases noise decreases
• May be able to decrease to 7ms for very thin
patients (with 80 mA)
• Given that we have been told larger patient
images can be poor, and that we can’t increase
the mA (max 80mA which is the default), it may
be possible to increase the ms for larger
patients to improve image quality.
– Probably go to 26ms for same noise as average
patient
Hot off the press!!!
• Very large patient
scanned with default
settings led to images
that were not usable
• We recommended
they use 26 ms and
image quality had
improved such that
images are now
acceptable for the
clinical intent
Summary
• Developed a PCXMC model that simulates CBCT organ
doses for pelvic (prostate) imaging
• Organ doses are not insignificant for daily CBCT
imaging!!!
• Developing size specific protocols should be possible
– Increase/decrease in mA
– Increase decrease in ms
• Future work will include
– Adopt size specific protocols into clinical practice
– Looking in more detail at the organ specific risks of cancer
induction
– Create some written justification protocols for the use of CBCT
with dose information for size specific scans
– Looking at other anatomical sites