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

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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

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30

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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