kV imaging dose in RT: evaluation and protocol optimization
G. Bolard, Medical Physicist SSRPM
FOPH Radiation Protection Day RT
09/21/2012
Imaging in RT
♦ Highly conformal treatments are
becoming the standard of care
– IMRT/VMAT/TOMO
♦ Conformal plans are more
sensitive to patient
misalignment/organ position
♦ kV or MV imaging is the main
modality used in linacs for pretreatment positioning and
evaluating organ
movement/deformation
♦ With the introduction of adaptive
radiotherapy (ART), the use of
volumetric imaging (CBCT) in RT
treatments is increasing
FOPH Radiation Protection Day RT
Bern, 09/21/12
Status
♦ Daily imaging is now the standard in RT
– Planar imaging or kV/MV CBCT on linacs/TOMO
♦ Imaging dose depends on the frequency of use
and tumor location
♦ Treatment machines are not equal regarding the
compromise dose/image quality
FOPH Radiation Protection Day RT
Bern, 09/21/12
Context
♦ Volumetric imaging (MV/kV CBCT) is the
most irradiating modality
♦ In some cases, daily CBCT may lead to
doses of the same level as out-of-field
radiation dose
♦ Dose summation with RT dose is not
straightforward
%
– kV dose distribution differs from typical MV
distribution: kV is characterized by a high
surface dose and rapid attenuation in the
body
– Gy can be added to Gy but does it make
sense from a biological point of view?
♦ Important for the RT staff (technologists
and doctors) to know the level of dose
delivered per imaging procedure
FOPH Radiation Protection Day RT
cm
Bern, 09/21/12
Context
♦ ALARA principle is applicable
♦ Recommendations exist to
manage additional imaging dose
– AAPM TG75 2007
♦ Technologists, doctors and
physicists must all be involved in
imaging dose optimization
♦ Manufacturers begin to indicate
dose indices
– CTDI for CBCT
– Air kerma for radiography
FOPH Radiation Protection Day RT
Bern, 09/21/12
CBCT
♦ Estimation of dose delivered to
the patient
– Derived from CTDI measurements
– TLD measurements in
anthropomorphic phantoms
– Direct measurements
– Prediction (computation)
♦ Measurements
– TLDs in Rando pelvic phantom
– CTDI with pencil and Farmer
ionization chamber (calibrated in
RQA9)
– Dosimeters supplied by Institute
of Radiophysics (IRA/CHUV)
FOPH Radiation Protection Day RT
Bern, 09/21/12
CBCT: CTDIw Results for Varian OBI
Protocol
Measurement (mGy)
Manufacturer Data (mGy)
Standard Dose Head
4.8
3.9
Pelvis
20.0
17.7
Pelvis Spot Light
25.5
19.4
Low Dose Thorax
5.8
4.7
♦ Keep in mind that CTDI is not patient dose
– Mean dose in a slice for 32cm diameter perspex phantom
– For thinner patients, the same exposure parameters
(kV,mAs) result in a greater dose to the patient
– AAPM TG 204 2011 (Size-Specific Dose Estimates in
Pediatric and Adult Body CT Examinations)
FOPH Radiation Protection Day RT
Bern, 09/21/12
CBCT: TLD results
OBI: Pelvis
OBI: Pelvis Spot Light
FOPH Radiation Protection Day RT
Bern, 09/21/12
CBCT: Daily use scenario
♦ 39 fractions (prostate ttt, 78Gy, 2Gy/fr)
Organ
Absorbed dose (Gy)
Pelvis spot light
Pelvis
Bladder
0.6
0.5
Prostate
1
0.9
Rectum
1.5
1
Left femoral head
0.6
0.7
Right femoral head
0.3
0.9
Skin
3.6 (post & lat)
1.7
Should this dose be taken into account for retrospective clinical analyses?
FOPH Radiation Protection Day RT
Bern, 09/21/12
CBCT Modeling
♦ Convolution/Superposition algorithms may be used for
computation on actual patient anatomy
– Modeling of OBI in our TPS Philips Pinnacle3
– Monokernels with energy less than 100 keV and initial beam model
supplied by Dr. Parham Alaei, University of Minnesota, Minneapolis
FOPH Radiation Protection Day RT
Bern, 09/21/12
CBCT Modeling
Daily use of CBCT in pelvic ttts
PTV & OAR mean doses are increased by 1Gy
2-3 Gy for skin and bones
Inhomogeneous dose in PTV
FOPH Radiation Protection Day RT
Bern, 09/21/12
Volumetric Imaging: Typical doses
Imaging dose and
image quality are
manufacturer
dependent
FOPH Radiation Protection Day RT
Bern, 09/21/12
Volumetric Imaging: Image quality
Contrast to Noise Ratio (CNR)
and resolution (lp/mm)
The best CNR is not from the
equipment that delivers the
highest dose
FOPH Radiation Protection Day RT
Bern, 09/21/12
CBCT optimization
♦ Patient size-specific protocols
– Adapt mAs to account for patient size
– Axial collimation
♦ Consider image requirements for the specific task
– Soft contrast detectability, resolution, HU accuracy
– Effect on matching
♦ Also
– Reconstruction parameters (filter, voxel size)
– Amount of scatter (FOV)
– Geometric calibration of the system
♦ → Image quality/dose relationship may be complex
FOPH Radiation Protection Day RT
Bern, 09/21/12
kV planar imaging
♦ Dose is mainly deposited superficially and is
characterized by air kerma or entrance skin dose
♦ Dose at depth is much less than for MV images
♦ Greater possibilities to reduce dose
–
–
–
–
Only bony anatomy/marker visualization needed
Collimation (blades)
mAs & kV
Incidence
♦ kV imaging is not always necessary
– Orthogonal imaging should not be default procedure
– MV imaging of treatment fields where MU subtraction is
possible can be better (breast, palliative)
♦ Technologists’ role is essential → training
FOPH Radiation Protection Day RT
Bern, 09/21/12
Measurements : Varian OBI
♦ With pancake 60cc chamber, Air kerma @ 85cm (mGy)
Body site
Default protocol
Manufacturer Display
In-house measurement
AP Med
0.7
0.8
Lat Med
10.7
13.1
AP Large
1.3
1.5
Lat Large
21.6
26.8
Head AP
0.98
1.2
Head Lat
0.30
0.4
Thorax AP
0.34
0.5
Thorax Lat
4.4
6.8
Abdomen AP
2.5
3.9
Abdomen Lat
3.5
5.5
Pelvis
Head
Thorax
Abdomen
FOPH Radiation Protection Day RT
Bern, 09/21/12
Planar imaging optimization
Body site
Protocol
Original settings
New settings
Air kerma divided by
AP Med
75 kV, 10 mAs
70 kV, 5 mAs
2.3
Lat Med
105 kV, 80 mAs
90 kV, 12.6 mAs
8.6
AP Large
80 kV, 16 mAs
90 kV, 10 mAs
1.3
Lat Large
120 kV, 126 mAs
105 kV, 16 mAs
10.3
Head AP
100 kV, 8 mAs
70 kV, 4 mAs
4.1
Head Lat
70 kV, 5 mAs
70 kV, 4 mAs
1.2
Thorax AP
75 kV, 5 mAs
70 kV, 5 mAs
1.1
Thorax Lat
95 kV, 40 mAs
85 kV, 6.4 mAs
7.8
Abdomen AP
80 kV, 32 mAs
80 kV, 5 mAs
6.4
Abdomen Lat
85 kV, 40 mAs
90 kV, 12.6 mAs
2.8
Pelvis
Head
Thorax
Abdomen
Varian OBI 1.5
FOPH Radiation Protection Day RT
Bern, 09/21/12
Conclusion
♦ Important to know the level of dose delivered by
imaging for the patient treatment course
– For multiple fractions and daily 3D imaging, the additional
dose can represent 1-2% of the prescribed dose
♦ Optimization of manufacturer protocols is possible especially for planar kV imaging
♦ Technology must progress for CBCT
– Improve the image quality/dose ratio
FOPH Radiation Protection Day RT
Bern, 09/21/12
Thank you for your attention!
FOPH Radiation Protection Day RT
Bern, 09/21/12