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Cone Beam CT Protocol Optimisation for Prostate Imaging with the Varian Radiotherapy OBI imaging system. Dr Craig Moore & Dr Tim Wood. Background. - PowerPoint PPT Presentation
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Dr Craig Moore & Dr Tim Wood
Cone Beam CT Protocol Optimisation for Prostate Imaging with the Varian
Radiotherapy OBI imaging system
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
Treatment head – MV beams
generated here
kV tube
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 = non-
uniform beam
Can we account this non-
uniformity 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 non-uniformity of the x-ray beam
• Treat each slither independently for each projection– PCXMC requires the correct
air kerma and filtration for each slither to perform its calculation – need some beam profiling!!!!
4 slithers used to correct for beam non-uniformity – treat
independently for each projection
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!!
0.0
0.5
1.0
1.5
2.0
2.5
-5 0 5 10 15 20 25
Position relative to centre of field (cm)
Air
Ke
rma
(m
Gy/
20
mA
s @
12
5 k
Vp
) .
0
5
10
15
20
25
30
35
-5 0 5 10 15 20 25
Position relative to centre of field (cm)
To
tal f
iltra
tion
(m
m A
l) .
S1 S2 S3 S4
Use this info to plug into PCXMC to calculate
patient dose per slitherS1 S2 S3 S4
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
* Measured Air Kerma corrected for ratio of (μen/ρ)ICRU soft tissue/(μen/ρ)air
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
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)
CBCT as % of RTOrgan RT Treatment Pelvic CBCT
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
Noise with mA
0
20
40
60
80
100
120
0 10 20 30 40 50 60
CT slice
No
ise (
SD
)
80mA, 13ms
60mA, 13ms
40mA, 13ms
20mA, 13ms
Protocol Optimisation – Effect of mA (dose)
Patient thickness
• 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 mA (dose)
Protocol Optimisation – Effect of msNoise with ms
0
10
20
30
40
50
60
70
0 10 20 30 40 50 60
CT slice
No
ise
(S
D)
80mA, 26ms
80mA, 23ms
80mA, 20ms
80mA, 13ms
80mA, 7ms
Large patients
Small patients
• 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
Protocol Optimisation – Effect of ms
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
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