Small fields dosimetry

in the commissioning of medical linacs

Kinga Marta Jakub Agnieszka Aneta

Polaczek-Grelik Stefańczyk Derus Giłka Kawa-Iwanicka

Department of Medical Physics University of Silesia

Katowice, Poland

Cancer Diagnosis and Treatment Center Katowice, Poland

Small-field issues

Physical aspects

Occlusion of primary radiation source

Overlapping penumbra regions (dose profile broadening as a consequence)

No plateau in the field center

Lateral charged particles disequilibrium

Technical problems

Detector perturbation of photon fluence

Volume-averaging effect

• Underestimation of the dose in the center of field – output factors affected

• Washed out the penumbra region – profile shape affected

No enough space for a reference detector

In-air measurements instead of water

IPEM Report 103

Das et al. Med. Phys 35(1), 2008.

Introduction

Linac commissioning

• Field sizes: 10x10, 30x30 [cm2]

• In-plane and cross-plane scans

• Dmax on the base of PDD curves

• Open fields measurements

• Particular detector type not stated

Oncentra External Beam modelling

• Field sizes: 5x5, 10x10, 15x15, 20x20, 30x30, max x max [cm2]

• In-plane and cross-plane scans

• Output factors in water and in air

• PDD curves

• Diagonal and star profiles

• Open and wedge fields measurements

• As small detector as available, preferably diamond or diode detector

The aim: Investigation of the influence of detector type and orientation on profile parameters examined during linac commissioning and important for TPS modelling.

Material: Three Elekta Precise linear accelerator with Agility system Photon beams: 6 MV, 10 MV and 15 MV

Radiation Detectors

Ionization chambers

less quantum noise

more tissue-equivalent materials

decrease of response in high dose rates

Semiconductor diodes

energy dependence of signal in low-energy range

more uncertain for large fields and depths

overresponse in high dose rates

Feature CC13 CC01 SFD3G PFD3G

Outer electrode C-552 C-552 ― ―

Inner electrode C-552 Steel ― ―

Active material air air pSi pSi

Enclosure material ― ― ABS ABS

Stem material PEEK PEEK Steel Steel

Diameter [mm] 6.0 2.0 0.6 2.0

Length [mm] 5.8 3.6 0.5 0.5

Active volume [mm3] 0.13 0.01 0.0004 0.001

Sensitivity [nC/Gy] ~3.8 ~0.4 ~4 ~100

Detector orientation

Ionization chambers

upward – parallel to CAX

perpendicular to the beam axis

Semiconductor diodes

upward – parallel to CAX

Geometrical conditions

Field sizes at isocentre

10x10 cm2, 5x5 cm2, 3x3 cm2

Depth of measurements

1.5 cm, 5 cm, 10 cm, 20 cm

SSD = 90 cm

Method

Air-filled ionisation chambers

CC01 ion chamber reproduces regions of field edges as good as diodes. Whereas dose gradient regions seen by CC13 are washed out.

Outside the field of irradiation CC01 indications are higher than other investigated detectors, therefore the field size based on its readings is larger than for other detectors (from 0.6% to 3%)

TPS based on CC01 scan profiles might give overestimated doses outside the PTV. This is important for combination of several adjacent fields in radiotherapy process.

Results & Conclusions

Semiconductor diodes

Unshielded photon diode tends to overresponse outside the field of radiation.

This process is energy-dependent and field size-dependent, as the contribution of scattered radiation increases.

3x3 cm2 beam proflies as seen by stereotactic and shielded photon diodes at isocentre in water phantom

Results & Conclusions

6MV

15MV

CC13 vs. PFD

Profile scans for 5x5cm2 field size at different depths and for various beam energies.

6 MV 15 MV

Results & Conclusions

Transversal scans (limited by MLC)

Longitudinal scans (limited by solid jaws)

Profile parameters

Field size Beam energy CC13 CC01 SFD3G PFD3G

10 x 10 cm2 6 MV 100.3 100.3

100.9 101.4

100.6 100.3

10 MV 100.4 100.5

101.1 101.5

100.4 100.3

15 MV 100.6 100.6

101.1 101.3

100.6 100.7

5 x 5 cm2 6 MV 100.4 100.4

101.0 100.6

100.7 100.2

10 MV 100.5 100.5

101.2 101.9

100.7 100.3

15 MV 100.6 100.7

102.9 102.6

100.9 100.4

3 x 3 cm2 6 MV 100.1 100.2

100.6 101.2

100.7 100.5

10 MV 100.4 100.5

100.4 100.9

100.5 100.6

15 MV 100.6 100.6

100.5 101.3

100.2 100.3

Symmetry parameter for in-line (Y direction) profile scans by various detectors in upward orientation. Data for orthogonal location of ion chambers for comparison purposes are shown.

Depth Beam Energy CC13 S F

PFD S F

1.5 cm 6 MV 100.2 100.8 100.1 100.3

10 MV 100.6 100.8 100.6 101.1

15 MV 100.2 101.2 100.5 100.9

5.0 cm 6 MV 100.2 101.4 100.2 100.9

10 MV 100.4 101.8 100.4 100.7

15 MV 100.3 102.8 100.6 101.8

10.0 cm 6 MV 100.4 102.0 100.3 101.5

10 MV 100.5 102.3 100.3 101.2

15 MV 100.3 103.5 100.5 102.3

20.0 cm 6 MV 100.5 102.8 100.4 102.0

10 MV 100.7 103.1 100.4 101.9

15 MV 100.5 104.6 100.7 102.9

Symmetry and flatness parameters for in-line (Y direction) profile scans of 5x5cm2 field as seen by ionisation chamber and shielded photon diode.

Results & Conclusions

Penumbra region

- Diodes show good reproducibility in small fields (below 3x3cm2).

- Shielded diode could be used even for standard field sizes, whereas stereotactic diode is not appropriate for this purpose

- Small ion chambers (CC01) has an advantage over CC13 for fields below 5x5cm2, but its orientation has to be carefuly chosen.

- For large/standard fields ion chambers of various dimensions give similar results

- CC13 ion chamber overestimates the penumbra size of about 55% regardless of the measurement depth .

Penumbra regon as a function of in-water depth for the field 5x5 cm2 at isocentre for 10 MV beam.

10 x 10 5 x 5 3 x 3

CC13 parallel 7.4 6.8 6.3

CC13 orthogonal 7.5 6.8 6.4

CC01 parallel 7.8 6.4 5.6

CC01 orthogonal 6.8 6.0 5.4

PFD 4.8 4.4 4.1

SFD 5.6 4.7 4.2

Penumbra region as seen by various detectors as a function of field size. Scans in Y direction.

Depth [cm] PFD CC13

1.5 3.6 5.7

5.0 4.1 6.3

10.0 4.4 6.8

20.0 4.9 7.6

Results & Conclusions

Penumbra region

CC13 CC01

PFD

SFD

10 x 10 cm2

5 x 5 cm2

3 x 3 cm2

Results & Conclusions

- Diodes show good reproducibility in small fields (below 3x3cm2).

- Shielded diode could be used even for standard field sizes, whereas stereotactic diode is not appropriate for this purpose

- Small ion chambers (CC01) has an advantage over CC13 for fields below 5x5cm2, but its orientation has to be carefuly chosen.

- For large/standard fields ion chambers of various dimensions give similar results

- CC13 ion chamber overestimates the penumbra size of about 55% regardless of the measurement depth .

Thank you for your attention

kinga.polaczek-grelik@cdito.pl