24.08.01 Erik Korevaar PhD, Department of Radiation Oncology, University Hospital Zürich, 8091 Zürich,
Switzerland. Email: erik.korevaar@dmr.usz.ch tel: +41 1 255 3249, fax: +41 1 255 4547
Influence of the detector size used for beam data acquisition on the shape of the
pencil beam kernel in Cadplan
At the University Hospital Zürich a Cadplan/Helios system is being commissioned for IMRT
treatments. The first step was the measurement of beam data to configure the single pencil
beam model in Cadplan. Beam data includes depth dose curves and dose profiles of open
fields. Measurements were done in a water phantom using a 0.125 cc thimble type ionization
chamber (PTW nr. 31002). The agreement between Cadplan calculated and measured depth
dose curves and dose profiles of open fields was satisfactory. However, for IMRT fields
discrepancies between Cadplan calculations and film measurements of up to 8% were found.
The calculated dose profiles showed less steep dose gradients than the measurements. The
accuracy of film measurements was confirmed by measurements in a water phantom using a
diamond detector and a PTW LA48 detector.
An explanation for the low accuracy of the Cadplan calculation is an inaccurate pencil beam
kernel. If the pencil beam kernel is too broad, calculated dose profiles show too low dose
gradients. In principle, it is possible to modify the pencil beam kernel data 'by hand' to create
a sharper kernel and hence steeper dose gradients in calculated dose profiles. This was
suggested by Varian. However, this is 'tricky' since the consistency in the model is lost; even
if the accuracy for some IMRT fields improve, it is uncertain what the consequences are for
other fields.
It was found that the origin of the inaccurate pencil beam kernel was the relatively wide
penumbra's of the measured dose profiles used for the configuration. The inner radius of 5.5
mm of the thimble chamber used for the measurements is large compared to the steep dose
gradients in the penumbra region. Measurements of dose profiles with a diamond detector
with a thickness in one dimension of about 0.25 mm showed more narrow penumbra's (20%80% penumbra width at 10 cm depth in water 4.5 mm instead of 7 mm, see figure 1). The
measurements of the dose profiles needed for the configuration of the single pencil beam
model were performed again with the diamond detector. Indeed, the pencil beam kernel that
was derived from the diamond detector measurements was sharper than the original kernel
(figure 2). The agreement between calculated and measured dose profiles of IMRT fields
improved to within 2 mm/2% (figure 3). Furthermore, the agreement between calculations
and measurements for depth dose curves and dose profiles of open fields improved,
although differences were already small after the first configuration.
Conclusion: to use Cadplan/Helios for IMRT treatments it is important to measure the dose
profiles used for the configuration with a detector with a small effective volume. An ionization
chamber with an inner radius of 5.5 mm is unsuitable for a high accuracy.
Comparison diamond, LA48, Ion. chamber detectors
2
Clinac 6EX, Open field 10 x 10 cm , SSD 90 cm, depth 10 cm
100
Diamond
LA48
Thimble PTW 00033
90
Film (after shift)
80
70
D (%)
60
50
40
30
20
10
0
30
35
40
45
50
55
60
X (mm)
Figure 1. Dose profiles of a 6 MV photon beam, measured with various detectors in a water phantom. The diamond detector and film
measurements show steep penumbras, the ionization chamber measurements (circles) result in a too broad penubra. The profile
measured with a PTW LA48 detector (ionization chamber array) is close to the diamond and film measurements.
Cadplan single pencil beam kernels
Depths: Dmax, 5, 10, 20 and 30 cm
1.0E+01
Original configuration
Based on diamond measurement
1.0E+00
Dose (a.u.)
1.0E-01
1.0E-02
1.0E-03
1.0E-04
0
1
2
3
4
5
6
X (cm)
Figure 2. Pencil beam kernels from Cadplan 6.2.7., based on measurements done with an ionization chamber (original configuration),
and a diamond detector. In the original configuration the pencil beam kernel is too broad and the maxumum value (at x = 0) is too low
compared to the --more accurate-- pencil beam kernel derived from diamond detector measurements
Dose profiles IMRT field - Influence of dose kernel
Stair (#8) Clinac 6EX
Dosimetric leaf separation = 2.5 mm, transmission = 1.5%
Film (2.5mm/1.5%)
100
Cadplan original configuration
New config based on diamond measurement
80
D (%)
60
40
20
0
-100
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
10
20
30
40
50
60
70
80
90
X (mm)
Figure 3. The calculated dose profile in the original configuration shows too shallow dose gradients (squares). This is improved in
the calculated dose profile obtained after the new configuration based on diamond detector measurements (circles).
100