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Dose Delivered to Patients for Megavoltage Cone-Beam CT Imaging
Olivier Morin1,2, Je Bellerose1, Clayton Akazawa1, Amy Gillis1, Martina Descovich1, Michèle Aubin1, Josephine Chen1, Hong Chen1,
Jean-François Aubry1, Ping Xia1,2 and Jean Pouliot1,2
PURPOSE
UCSF
University of California San Francisco
1600 Divisadero Street, Suite H1031
San Francisco CA 94143-1708
Email: morin@radonc17.ucsf.edu
WebSite: http://www.ucsf.edu/jpouliot/
AAPM 2006 SU-FF-I-13
1- Department of Radiation Oncology, UCSF Comprehensive Cancer Center
2- UCSF / UC Berkeley Joint Graduate Group in Bioengineering
Megavoltage Cone-Beam CT (MVCBCT) uses a
conventional treatment unit equipped with a at
panel detector to obtain a 3D representation of the
patient in treatment position. MVCBCT has been
used for 2 years in our clinic for anatomy verication
and to improve patient alignment
prior to dose delivery. Depending
on the soft-tissue information
required for setup we are currently
using a total exposure ranging
between 2-10 monitor units (MU).
CT MVCBCT (5MU)
The objectives of this work are:
To evaluate the dose delivered to patients for MVCBCT acquisition.
To develop a simple plan modication receipe to compensate for
the dose received by daily MVCBCT.
METHODS & MATERIALS
The MVCBCT dose calculated by our treatment planning system
(Phillips, Pinnacle) was compared to measurements.
Typical MVCBCT acquisition:
6 MV beam, 27.4 x 27.4 cm2 eld size, 145 cm source-detector distance, Arc: 270o to 110o Experimental setup to
measure MVCBCT dose
IMRT QA Cylinder
MOSFETS and an
ion chamber
Dose (cGy/MVCBCT MU)
1.12
1.05
-2.1 % 1.7 %
2.3 %
0.95
0.3 %
-1.1 %
1.4 % 0.2 %
0.85
2.8 % 1.4 %
0.75
-0.9 %
2.6 %
16 cm
Percentage dierence
between the dose
calculated by Pinnacle
and the measurements
ALL WITHIN 3 %
MVCBCT acquisition
simulated in Pinnacle
Simulation of an arc
treatment with
MVCBCT setting
Pinnacle can be used to simulate the
MVCBCT dose received by the patients.
Normalized Volume
RESULTS
100
90
80
70
60
50
40
30
20
10
MVCBCT Portal Imaging
or Film
Tx (solid) with Tx+MVCBCT (dashed)
Rectum
Right Femoral
Spinal Cord
Penis Bulb
cGy per MU
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.4
Bladder
0
0 10 20 30 40 50 60 70 80
CONCLUSION
Seminal Vesicles
Prostate
90
80
70
60
50
40
30
20
10
Dose (Gy)
MVCBCT dose evaluated in the treatment planning system
Table 1. Dose per fraction delivered to prostate
patients at UCSF for daily alignment verication.
MVCBCT (9 MU) and portal imaging (4 x 2 MU = 8 MU).
Simple method to compensate for daily MVCBCT
Prostate (daily 9MU MVCBCT, 40 fractions)
Head and Neck (daily 5 MU MVCBCT, 33 fractions)
Tx Alone Tx + MVCBCT Compensated Tx
+ MVCBCT
A compensation factor (CBCF) was introduced
to keep the target mean dose the same.
Tx Alone Tx + MVCBCT Compensated Tx
+ MVCBCT
77 Gy
70 Gy
55 Gy
45 Gy
35 Gy
28 Gy
15 Gy
Prostate
Seminal Ves.
Nodes
Rectum
Bladder
Penis Bulb
Small Bowel
Spinal Cord
MVCBCT [cGy] Portal Imaging [cGy]
Min Mean Max Min Mean Max
6.4
6.3
5.2
5.3
6.7
6.5
2.7
0.9
6.9
6.5
6.8
5.9
7.8
6.9
8.3
3.9
Tx (solid) with CBCF. Tx+MVCBCT (dashed)
100
CBCF = 96%
0
0 10 20 30 40 50 60 70 80
7.6
6.8
10.1
6.8
9.5
7.6
11.2
5.2
5.5
5.4
4.4
4.8
5.8
5.8
3.0
0.8
20
Brain
20
10
Right Eye
10
0
0 10 20 30 40 50 60 70 80
0
0 10 20 30 40 50 60 70 80
Dose (Gy)
Patient CT scans can be imported in Pinnacle to evaluate the dose delivered by MVCBCT. For a typical MVCBCT, the delivered dose forms
a small anterior-posterior gradient roughly ranging from 0.6 to 1.2 cGy per MVCBCT MU. A MVCBCT acquisition of 9 MU in the pelvis area
delivers slightly more dose than what is currently delivered by portal imaging at UCSF. Daily 9 MU MVCBCT remains a small dose addition
compared to treatment dose (