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 20.01.20164

Calibrating a kilovoltage treatment unit

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 20.01.20166

ISL

airairw

enw

tf

ssKff FB

MM

NMD

/

,

HVL

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 20.01.20169

ISL

airairw

enw

tf

ssKff FB

MM

NMD

/

,

HVL

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 20.01.201612

ISL

airairw

enw

tf

ssKff FB

MM

NMD

/

,

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 20.01.201614

ISL

airairw

enw

tf

ssKff FB

MM

NMD

/

,

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 20.01.201626

Complex issues

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 Thomasmichael.thomas@rmh.nhs.uk

020 8661 3475

Contact details

Liz Fernandez for images

Patients who consented to have their images used for teaching

Royal Marsden radiotherapy physics