Cone Beam CT Dosimetry

Medisinsk Fysikk – RadiumhospitaletMaria Sastre, Ellinor Bilet, Wibeke Nordhøy

Strålevern – 18 Nov 2008

Definition of Cone Beam CTCT Modalities

Cone Beam CT

area detector, 2D field

Fan CT

linear detector, 1D filed

CBCT mounted on a linacFlat panel

imager

X Ray source

FilterCollimator

Imaging modalities

Planar View kV planar image

Motion view kV fluoroscopy

Volume view ConeBeam CT

CBCT_video.mpg

Volume View results

The clinical use of the CBCTCBCT taken before treatment, 9 mm lateral shift

The clinical use of the CBCTafter repositioning the patient with CBCT software

But, how much CBCT dose does the patient recieve?

Is it worth it?

Protocols for VolumeView

AREA gantry rotation Energy kV mA, msProstateM10 F1

360 120 64,40

PelvisM20 F1

360 120 40,40

Pelvis BigM10 F1

360 120 64,40

ChestM20 F1

360 120 40,40

Head and NeckS20 F1

200 100 10,10

• Detector: CT-SDD2

calibration according to TRS277, accuracy < ±3%

• Barracuda X-ray multimeter and associated software

• Two standard body CTDI phantoms

Dosimetry equipment2x2 mm2 pin diode

32 cm diameter

30 cm long

Sampling of the pulses

mGy/sec (6 pulses in 1 sec)

Calculation of isocenter dose

Time of a whole rotationt = 360 deg / 3.18 deg/sec = 113 sec

Isocenter Dose =

0.2 mGy/sec * 113 sec =

22.6 mGy

Probe in the center

Table absorption

Acumulated dose

Probe at surfaceprobe at 12-18 o'clock

probe at 15-21 o'clock

Dose at 12 = 38.26 mGy

Dose at 18 = 34.81 mGy

Dose at 15 = 38.95 mGy

Dose at 21 = 38.95 mGy

How to add the CT dose??

ionization chamber 10 cm long

Dosimetry on a CT scanner:

the chamber measurement includes phantom scatter

1 cm field

2x2 mm2 pin diode

Dosimetry on a CBCT:

The diode measures on a central point

The previous study showed surface doses of 25 mGy for helical CT and

44 mGy for CBCT, measured witha farmer io

nization chamber (0.6cc) (1) .

CBCT dose versus others in RT

• One MV CBCT ~ 60 mGy center

• One MV planar view ~ 23 mGy center

•One CT scan1 ~ 25 mGy surface

2 Comparison of peripherial dose from CBCT to IMRT. J. Perks. UC Davis Medical Center USA. Radiotherapy and Oncology (2008). In press

Measured with TLDs 25 cm from the central axis.

• One CBCT (kV) ~ 23 One CBCT (kV) ~ 23 mGymGy centercenter and 38 and 38 mGymGy onon surfacesurface

• One fraction IMRT 25 cm from cax2 ~ 4 mGy

• One CBCT 25 cm from caxOne CBCT 25 cm from cax22 ~ 2 ~ 2 mGymGy

Patient’s fractionated treatments• Pelvis and prostata (25-35 x 2 Gy)

– CBCT the first 4 sessions and then once a week (~5-10 CBCTs)

• Prostata with gold seeds implants (25 x 2Gy)– Every session (25 CBCTs)

• Stereotactic lung (3 x 15Gy)– Every session, 3 CBCT per session (9 CBCTs)

CBCT total Dose for a gold seeds prostate patient

Isocenter Doseone CBCT gives 23 mGy (from literature1 28 mGy)

25 CBCTs x 23 mGy = 575 mGy ~ 0.6 Gy

Surface Doseone CBCT gives 38 mGy (from literature1 44 mGy)

25 CBCTs x 38 mGy = 950 mGy ~ 1 Gy

1 Cone-Beam CT guided radiation therapy: technical implementation. D. Létourneau. University of Toronto Canada. Radiotherapy and Oncology 75 (2005)

Measured with CTDI phantom and farmer chamber, probably without bowtie filter

Total dose to a gold seeds prostate patient

25 Gy1.5 Gy

50 Gy

Radiotherapy

CBCT

0.6 Gy

1.0 Gy1.0 Gy

Worst case patient, 25 CBCT during treatment

Intensity Modulated MRT will smear the surface dose

CT scan doses will add also to these ones

Risk of second primary cancer in the contralateral breast (2003)3

3 Xiang Gao et al. Int J Radiation Oncology Biol. Phys. Vol 56, No. 4, pp 1038-1045

134.501 patients

37.379 RT 97.122 no RT

5 year2.9% CBC

10 years6.5% CBC

15 years10.2% CBC

20 years13.4% CBC

5 year3.0% CBC

10 years6% CBC

15 years8.9 %

20 years11.8% CBCCancer patients have already a higher risk of a secondary cancer.

The risk-increase after radiotherapy is low...?

Conclusions

The benefits of being able to hit the target and reduce the margins might justify the additional imaging dose to the patient

CBCT doses will add to the surface and peripheral RT doses and to the CT scan doses

For a prostata patient with gold seeds, 25 CBCTs, the surface dose will increase ~1 Gy

PARDOX:

• CBCT dose will reduce RT dose via margin reduction..

• CBCT dose asures that we hit the target ...