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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 ...
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