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The Royal Marsden
kV Dosimetry in the Clinic 20.01.2016 1
Kilovoltage dosimetry in the clinic
Michael Thomas
The Royal Marsden kV Dosimetry in the Clinic 20.01.20162
Outline
– How to calibrate a treatment machine based on NPL calibration certificate
– How to calculate monitor units for a calibrated machine in a simple case
– Complex issues
The Royal Marsden kV Dosimetry in the Clinic 20.01.20163
Codes of Practice
– 1996 code: – The IPEMB code of practice for the determination of absorbed
dose for x-rays below 300 kV generating potential (0.035 mm Al–4 mm Cu HVL; 10–300 kV generating potential) Phys. Med. Biol. 41 2605–25
Energy kV HVL (mm Al)
HVL (mm Cu)
Very low 8-50 0.035-1.0
Low 50-160 1.0-8.0
Medium 160-300 0.5-4.0
– 2005 addendum (R J Aukett et al):– Addendum to the IPEMB code of practice for the determination
of absorbed dose for x-rays below 300 kV generating potential (0.035 mm Al-4 mm Cu HVL) Phys. Med. Biol. 50 2739–48
---- 300 ---- 20 0.5-4.0
The Royal Marsden kV Dosimetry in the Clinic 20.01.20165
Calibration of a treatment unit
ISL
airairw
enwfKff FBNMD
/
,
tf
ssKfK M
MNN
,,
ISL
airairw
enw
tf
ssKff FB
MM
NMD
/
,
– Key equation:
– From code of practice:
The Royal Marsden kV Dosimetry in the Clinic 20.01.20167
Measuring half value layer
– Beam with minimal scatter
– Narrow beam with additional collimation
– Position attenuators well away from chamber
– Use a horizontal setup well away from walls
source
attenuator
collimation
chamber
film
The Royal Marsden kV Dosimetry in the Clinic 20.01.20168
Measuring half value layer
– Ensure chamber is fully in beam with film
– Use chamber appropriate for energy
– Advice in 1996 code among others going back to Trout’s Am. J. Roentgenology paper of 1960.
– conflicting
The Royal Marsden kV Dosimetry in the Clinic 20.01.201610
Measuring half value layer
– Rates of change of dosimetric factors with HVL
Quality Nk,s μen Bw Total
60 kV
1.2 mm Al
-2/mm Al -0.6/mm Al +8/mm Al +5.4%/mm Al
+0.06%/error %
100 kV
2.8 mm Al
0 +0.2/mm Al +4%/mm Al 4.2%/mm Al
+0.1%/error %
220 kV
1.8 mm Cu
+0.2%/mm Cu +0.4/mm Cu -4%/mm Cu -3.4%/mm Cu
-0.06%/error %
The Royal Marsden kV Dosimetry in the Clinic 20.01.201611
The Nk,s graph from NPL
2.3 %
HVL is 1.5 mm Al
9 cm
5.1log10log
cm 9 x
x
1.5
cm 1.6x
The Royal Marsden kV Dosimetry in the Clinic 20.01.201613
Cross-calibration
Attenuation in anode
– Similar to MV cross-calibrations
– Side-by-side technique
– Heel effect
– Varies with energy
– Orient chamber perpendicular to cathode-anode direction
The Royal Marsden kV Dosimetry in the Clinic 20.01.201615
Measuring inverse-square law
– Because of scatter, effective source is not at the real source
– Measure in empty water phantom
– Assume a virtual source position r0
00
2
0
0
)(
rD
Dr
rr
DD
The Royal Marsden kV Dosimetry in the Clinic 20.01.201616
Measuring the inverse-square law
0
1
2
3
4
5
6
50 51 52 53 54 55 56
Distance (cm)
Sig
nal
(a.u
.)
Farmer
Markus
The Royal Marsden kV Dosimetry in the Clinic 20.01.201617
Measuring the inverse-square law
50
51
52
53
54
55
56
0.4 0.5 0.6 0.7 0.8 0.9 1
Signal-1/2 (a.u.)
Dis
tan
ce (
cm
)
Farmer
Markus
The Royal Marsden kV Dosimetry in the Clinic 20.01.201618
Measuring the inverse-square law
0
10
20
30
40
50
60
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Signal-1/2 (a.u.)
Dis
tan
ce (
cm
)
Farmer
Markus
r0
The Royal Marsden kV Dosimetry in the Clinic 20.01.201619
Measuring inverse-square law
– So increase dose measured by FISL to account for offset between the measured position rmeas and the calibration position rref (at the applicator end)
2
0
0
rr
rrF
ref
measISL
The Royal Marsden kV Dosimetry in the Clinic 20.01.201620
ISL
airairw
enw
tf
ssKff FB
MM
NMD
/
,
Calibration done!
The Royal Marsden kV Dosimetry in the Clinic 20.01.201621
Calculating Monitor Units for a calibrated treatment unit
The Royal Marsden kV Dosimetry in the Clinic 20.01.201622
Monitor Unit calculations
ISL
pat
w
ref
w
pat
ref
pat
ref
ISL
pat
airairw
enpat
w
tf
ssKpatpat
ref
airairw
enref
w
tf
ssKrefref
FB
B
M
M
D
D
FBM
MNMD
BM
MNMD
1111
/
,
/
,
–In patient treatments, we might use
– a non-reference applicator
– additional shielding with lead sheet
– a non-reference SSD
The Royal Marsden kV Dosimetry in the Clinic 20.01.201623
Inverse-square law
–Corrects for non-reference SSD
– Usually due to patient shape
–Already done
–Probably independent of field size
– Worth checking once
ISLF
The Royal Marsden kV Dosimetry in the Clinic 20.01.201624
In-air dose ratio (applicator factor)
–Corrects for non-reference applicator
–Measure at one standard source to chamber distance
–For each applicator and energy
–Depends on internal corrections to monitor chamber as well as physics
pat
ref
M
M
The Royal Marsden kV Dosimetry in the Clinic 20.01.201625
Backscatter factor ratio
–Applicator factor depends on the applicator (and energy)
–Backscatter factor depends on the field size irradiated (and energy)
Field diameter (cm)1 2 3 4 5 6 8 10 15
1.056 1.113 1.160 1.204 1.242 1.272 1.324 1.363 1.428
Backscatter factor
pat
w
ref
w
B
B
The Royal Marsden kV Dosimetry in the Clinic 20.01.201627
Complex issues
Caused by patients who do not always resemble water phantoms
The Royal Marsden kV Dosimetry in the Clinic 20.01.201628
Irregular contours
At 30 cm SSD, 1 cm of standoff changes the dose by 7%
The Royal Marsden kV Dosimetry in the Clinic 20.01.201629
Internal shielding
Effective shielding for underlying structures
The Royal Marsden kV Dosimetry in the Clinic 20.01.201630
But at a cost of dosimetric uncertainty
M.S. Huq et al, IJROBP, 24, 171-175 (1992)
See also Yu and Butson, Phys. Med. Biol. 58 N95–N102 (2013)
Corrections should be applied
The Royal Marsden kV Dosimetry in the Clinic 20.01.201631
Overlap
Close enough to treat in one field: avoids risk of overlap but with increased stand-off.
Separate fields but with care taken to avoid overlap at depth
The Royal Marsden kV Dosimetry in the Clinic 20.01.201632
Matching for large fields
Long keloid scars
Minimise stand-off Minimise overlap
Compromise normally required
The Royal Marsden kV Dosimetry in the Clinic 20.01.201633
Summary
– Calibration based on NPL calibration certificate
– Monitor unit calculations in simple cases
– Complexities relating to patients
The Royal Marsden kV Dosimetry in the Clinic 20.01.201634
Acknowledgements
Michael [email protected]
020 8661 3475
Contact details
Liz Fernandez for images
Patients who consented to have their images used for teaching
Royal Marsden radiotherapy physics