Pencil Beam and Collapsed Cone Algorithm Calculations for a
Lung-type Volume Using CT and the OMP Treatment Planning System
Methods
Measurements have been carried out in both phantom and a specifically designed phantom
which simulated human lung volume. Samples were taken from the Lung Planning CT
images for 15 patients using the Oncentra Masterplan OMP Treatment Planning System.
The X-axis was, following convention, taken to be horizontal, and the Y-axis to be vertical;
accordingly, abscissa and ordinate distances to the skin, heart and the lungs were measured
(see figure 8). Figures 4 and 5 show typical CT images for a patient’s lungs, while Tables 1
and 2 give the beam information and dose information for typical patients. The X-ray
images were taken using CT-SIM: Philips Brilliance Big Bore. A print out of the planning
CT images was produced by the Oncentra Masterplan OMP treatment Planning system (see
section 3.3).
1 |
Figure 4: Lungs Image for Patient by CT-SIM: Philips Brilliance Big Bore
Beam InformationBeam 1ANT 2RPO 3RPO 4ARO
Nom. Acc. Pot.(MV or MeV) 6 6 6 6FX (cm) 8.2 8.2 8.6 8.6FY (cm) 9.4 10.6 8.2 8.6SSD (cm) 87.2 86.8 85.6 84.7
Gantry (degrees) 0 223 267 320Wedge Angle(degrees) 60/60 60/33 60/25
Dose Information: Absolute dose 5500 cGy (275 cGy / fraction)Number of Fraction 20 20 20 20
MU or min / FractionIN = 424.77
OUT = 0IN = 185.78OUT = 85.22
IN = 92.85OUT = 69.34
16.39
Table 1: Beam Information and Dose Information for Patient
2 |
Figure 5: Lungs Image for Patient by CT-SIM: Philips Brilliance Big Bore
Beam Information
Beam3LPO
THORAX4ANT 5MINI-ANT 6LAO
Nom. Acc. Pot.(MV or MeV)
6 6 6 6
FX (cm) 10.6 10.1 9.7 14.2FY (cm) 9.7 14.3 11 10.1SSD (cm) 86.3 87.2 87.2 82.4
Gantry (degrees) 120 0 0 60Wedge Angle(degrees) 60/25 60/28 60/60 60/9
Dose Information: Absolute dose 4000 cGy (267 cGy / fraction)Number of Fraction 15 15 15 15
MU or min / FractionIN = 2291.07
OUT = 1711.09IN = 1277.72OUT = 793.65
IN = 1974.08OUT = 0
IN = 476.08OUT = 1301.50
3 |
Table 2: Beam Information and Dose Information for Patient
Design of multi-block chest phantoms
The first phantom was introduced to the experiment as shown in figure 7, in order to reduce
the uncertainty within the results and to increase the accuracy all that because of the very
inhomogeneous lung region that may led to poor dose distribution.
Figure 7: for Design 1 of the Multiblock phantom (first phantom)
The specially designed phantom
Using measurements taken from 15 patients, who had previously been scheduled for lung
radiotherapy, a second phantom consisting of multi-block components was designed. A
multi-block phantom is essentially a phantom containing a number of blocks with different
shapes and materials used to form an approximate cross-section of the patient. This
facilitates taking measurements on the phantom volume to confirm the prescribed dose. A
plan for the phantom was designed using similar field parameters, for example collimator
settings, beam weightings, wedge fractions, and gantry angles as the clinical plan. The two
lungs are presented in a lateral position, as shown in Figure 8 the heart is represented in the
4 |
middle to reflect the correct anatomy and the lighter color in both Figures 7 and 8 represent
the lungs.
Figure 8: Design 2 for the Multiblock phantom (second phantom), where (S-L) is skin and lung, and
(L-H) is lung and heart.
Table 3 shows the average distance between the skin, lungs and heart of the patient from
the X-ray for the X and Y axes. The table also illustrates the maximum and minimum
values for the X and Y axes, as well as the range of maximum and minimum values. Figure
8 illustrates the distance for X and Y axes between the skin, lungs and heart in the Multi-
Block Phantom. The phantom blocks designed for the experiments were 30 cm in length,
having square or right angled triangular cross-sections and 4 cm sides. They were made of
an epoxy resin-based tissue-equivalent material to represent water (WT1, density=1.02
5 |
gcm-3), lung (LN10 density=0.27 gcm-3) and bone (IB7, density=1.13 gcm-3). Some of the
square blocks of WT1 were drilled to accommodate a 0.6 cm3 graphite ionisation chamber.
The phantoms were composed by putting the epoxy blocks within an adjustable wooden
frame in desired configurations. The frame was held together using a series of small
wooden pins with diameters of 5 mm.
Skin to Lung (cm)
Lung to heart (cm)
Heart (cm)
Heart to Lung (cm)
Lung to Skin (cm)
X,Y axis
Average for patient(Lateral)
2.8 4.2 4.2 3.3 3.1 17.4
Average for patient (Ant-post)
2.3 1.9 4.4 1.3 2.6 11.1
Average for Phantom(X)
4 8 4 8 4 28
Average for Phantom(Y)
4 4 6 2 4 20
MAX in X axis for patients (Lateral) 19MIN in X axis for patients (Lateral) 15MAX in Y axis for patients (Ant-post) 13.7MIN in Y axis for patients (Ant-post) 9.6RANGE in X axis for patients (Lateral) 4RANGE in Y axis for patients (Ant-post) 4.1Table 3: The area for Lateral and Ant-post in 15 Patients (average and range for 15 patients) and
average for Multiblock phantom
Figures 9 and 10 below depict various stages in the construction of the thorax phantom
within its frame. Expanded polystyrene spacer elements with triangular cross sections
stabilised the slanted surfaces.
6 |
Import and Plan
After scanning the multi-blocks phantom using CT - SIM: Philips Brilliance Big- Bore, the
planning CT images were sent to the Oncentra Masterplan (OMP) Treatment Planning
System. The Oncology Management System: Impac, MOSAIQ was used to transfer the
data from the OMP treatment planning system to the Linac before running the Linac to
determine the points’ ISO center, Beam Information and Dose Information, as shown in
figure 14, 15, 16 and 17 for the first and second phantom.
The phantoms were positioned on the Elekta Precise linac, isocentre and aligned with
lasers, and the ion chamber was placed at each dose point, for example Iso, DP1, DP2, DP3
and DP4 (see figure 12 and 13). Doses were measured for the dosimeters and chambers.
The field size and gantry angles chosen are typical of clinical plans for the same 15 patients
as used to design phantom 2. A field size of 10 x 10cm, was used for all fields. Gantry
angles of 00-3150-2700 and 00-600-1200 were used for phantom 1 and 2 respectively. Tables
4 and 5 show beam information for the first and second phantoms, respectively. The energy
7 |
used for the plans was 6MV because lung cancer is treated clinically with 6MV in HOF
Hospital 10 MV beam is not used because considered very high energy and risky to the
lungs. Wedges were used for beam one and three- the angle of the wedge is 60/60 for each
beam. Figure 12 and 13 show the plan for phantoms 1 and 2, with the isocentre and dose
points measured.
For the first phantom was generated using three 6 MV photon beams, all with a 10 x 10
cm2 field size, as shown in fig A.
Figure A. The plan used for first phantom.
The plan was isocentric and included an ANT beam with a 60º wedge and a right RAO
beam with no wedge. The third field was a right LAT oblique beam with a 60º wedge. The
first phantom was outlined and the total dose prescribed to the isocentre was 5492.8 cGy
(274.6 cGy / fraction).
8 |
For the second phantom was generated using three 6 MV photon beams, all with a 10 x 10
cm2 field size, as shown in fig B.
Figure B. The plan used for second phantom.
The plan was isocentric and included an ANT beam with a 60º wedge and a left LAO beam
with no wedge. The third field was a left LPO oblique beam with a 60º wedge. The second
phantom was outlined and the total dose prescribed to the isocentre was 3971.8 cGy (264.8
cGy / fraction).
9 |
Figure 12: Plan for the first phantom, showing isocentre and 3 dose points (DP1, DP2 and DP3)
(see appendix for large pictures).
Beam InformationBeam ANT RAO LATNom. Acc. Pot.(MV or MeV) 6 6 6Field size X (cm) 10 10 10Field size Y (cm) 10 10 10SSD (cm) 90 86 90Gantry (degrees) 0 315 270
Table 4: Beam information for the first phantom.
10 |
Figure 13: Plan for the second phantom, showing isocentre and 4 dose points (DP1, DP2, DP3 and
DP4) (see appendix for large pictures).
Beam InformationBeam ANT LAO LPONom. Acc. Pot.(MV or MeV) 6 6 6Field size X (cm) 10 10 10Field size Y (cm) 10 10 10SSD (cm) 90 88.5 91.3Gantry (degrees) 0 60 120
Table 5: Beam information for the second phantom.
11 |
Measurements on the Linac
For the experiment with the phantoms, a Farmer dosimeter and an Ionisation Chamber with
a volume of 0.6cc, both from N E Technology were used. A 6MV X-Ray beam with a SSD
of 100cm and a depth of 5cm was used, along with a field size of 10 x 10cm and a Set Dose
(SD) of 400MU. For the first phantom, on the first day, the experiments were conducted at
room temperature of 20.5 oC, and at a pressure of 765.5 mmHg. For the second phantom,
on the second day, the experiments were conducted with the temperature at 20.2 oC and
pressure 759.2 mmHg. A temperature correction factor of 0.9945 was calculated using
equation 2.3, and a Depth Dose Correction to dmax of 0.863 was used for the calculations,
which is a constant for a 6MV linac in experiment. Further, the value of Wρ (density
correction) was taken as 1.000, which is a correction for Perspex to water. The calibration
factor for the ion chambers were 0.794 and 1.034, as two different chambers were used for
the two sets of measurement. The ion recombination factor, Pion, was 1.0042 in both cases.
The following equations were used to calculate the dose delivered:
Dose = Reading x ND x Pion x Ø (P, T) x Wρ / %DD (2.1)
For Phantom: D (cGy) = Reading x ND x P ion x Ø (P, T) x daily calibration correction factor (2.2)
Wρ is the density correction factor;
%DD is the percentage Depth Dose.
ND Calibration factor ion chamber and electrometer.
Pion ion recombination.
12 |
First phantom- daily calibration correction factor = (400MU / 401.9cGy)
Second phantom- daily calibration correction factor = (400MU / 399.1cGy)
Readings were obtained from the Dosemeter and converted to dose (401.9-399.1) using Equation 2.1.
Ø (P, T) = (273 + T / 293) * (760 / P), (2.3)
Where Ø (P, T) is the temperature and pressure correction factor, given by equation 2.3
In the users’ beam, the correction factor for air temperature and air pressure Ø (P, T) is
given as:
Ø (P, T) = ;
and is applied to convert the measured signal to the reference conditions used for the
chamber calibration at the standards laboratory. Note that P and T (in oC) are chamber air
pressure and temperature, respectively, at the time of measurement, while Po and To (in oC)
are the normal conditions used in the standards laboratory.
The temperature of the air in a chamber cavity should be taken as that of the phantom and
this is not necessarily the same as the temperature of the surrounding air. For measurements
in a water phantom the chamber waterproof sleeve should be vented to the atmosphere in
order to obtain a rapid equilibrium between the ambient air and the air in the chamber
cavity.
The ionisation chamber measurements were taken on linear accelerator A (Lin A). The
Linac was used to deliver the 6MV X-ray beam to each phantom separately. During this
process, the ionisation chamber was inserted within the phantom at each dose point, (Iso,
DP1, DP2, DP3 and DP4). Moreover, the radiation beam were delivered as per the plans in
13 |
273 .2+T273 .2+T0
⋅P0
P
figure 12 and 13. The readings were taken with the ionisation chamber are shown in table 8
and 9. These measurements were used with equation 2.2 to calculate the dose in cGy. The
percentage difference between the measured dose and the dose calculated using the PB and
CC algorithms was calculated.
Table 6 shows the two models for Dosemeter and Chambers used to obtain the dose, while
Table 7 summaries the parameters used in the experimental measurements.
Manufacturer Description Part No. Serial No. Local Description
Dosemeter:NE Technology Farmer 2570/1 B 944 & 1297 Field 2 & Field
3Chambers:
NE technology 0.6 cc thimble & graphite
2571 & 2571 A 1884 & 2921 Mk3 & Mk4
Table 6: Dosemeter and ionisation chambers used for the experimental measurements.
Sample depth dose chart for a 6 MV X-ray beam for a treatment distance of 100 cm SSDFirst day Second day
Field Size 10 x 10 cm 10 x 10Source to surface Distance (SSD) 100 cm 100 cm
Depth 5 cm 5 cmSet Dose (SD) 400 mu 400 mu
ND (calibration factor ion chamber & electrometer) 0.794 1.034Pion (ion recombination) 1.0042 1.0042
Temperature 20.5 Centigrade 20.2 Centigrade
Pressure 765.50 mmHg 759.20 mmHgØ (P,T) (Temperature & Pressure correction) 0.9945 1.0017
Wρ (density Correction) 1.000 1.000Depth Dose Correction to dmax 0.863 0.863
Table 7: Parameters used in experimental measurements.
14 |
4. Results and Discussion
4.1 Results
As describe in the Materials and Methods a Set Dose (SD) of 400MU was used. Readings
were obtained from the Dosimeter and converted to dose after accounting for daily
calibration correction factor for first and the second phantoms. Using Equation 2.1 and
Equation 2.2.the calculated values of 400MU=401.9cGy and 400MU=399.1cGy were
obtained respectively.
Table No 8 and Table No 9 summarize the Beam Information for Beams 1, 2 and 3 for
Phantom 1 and Phantom 2 respectively.
Table No 8:
Table No 1 demonstrates percentage difference between Pencil Beam (PB) with Experiment
Measured and Collapsed Cone (CC) with Experiment Measured for first phantom. The results are
as follows:
a. For Isocenter:
i. Experiment Measure for Isocenter ANT Beam one is 106.93 while for CC it is
107.9 and for PB it is 106.9. The % difference between the experiment measure
and that of CC is -0.02 % and between the experiment measure and that of PB is
0.9%.
ii. Experiment Measure for Isocenter RAO Beam two is 60.9 while for CC it is 61
and for PB it is 61.1. The % difference between the experiment measure and that
of CC is 0.1 % and between the experiment measure and that of PB is 0.3%.
15 |
iii. Experiment Measure for Isocenter LAT Beam three is 104.7 while for CC it is
105.7 and for PB it is 106.9. The % difference between the experiment measure
and that of CC is 0.9% and between the experiment measure and that of PB is
2.1%.
b. For DP 1:
i. Experiment Measure for DP1 ANT Beam one is 112.05 while for CC it is
112.7 and for PB it is 112.4. The % difference between the experiment
measure and that of CC is 0.5 % and between the experiment measure and
that of PB is 0.3%.
ii. Experiment Measure for DP1 RAO Beam two is 64.07 while for CC it is
64.7 and for PB it is 65.3. The % difference between the experiment
measure and that of CC is 0.9 % and between the experiment measure and
that of PB is 1.9%.
iii. Experiment Measure for DP1 LAT Beam three is 95.88 while for CC it is
97.3 and for PB it is 98.6. The % difference between the experiment
measure and that of CC is 1.4 % and between the experiment measure and
that of PB is 2.8%.
c. For DP2:
i. Experiment Measure for DP2 ANT Beam one is 98.7 while for CC it is 78.3
and for PB it is 80. The % difference between the experiment measure and
16 |
that of CC is 0.5 % and between the experiment measure and that of PB is
2.7%.
ii. Experiment Measure for DP2 RAO Beam two is 90.7 while for CC it is 71.1
and for PB it is 73.3. The % difference between the experiment measure and
that of CC is -0.6 % and between the experiment measure and that of PB is
2.4%.
iii. Experiment Measure for DP2 LAT Beam three is 117.74 while for CC it is
119.7 and for PB it is 120.6. The % difference between the experiment
measure and that of CC is 1.6 % and between the experiment measure and
that of PB is 2.4%.
d. For DP3:
i. Experiment Measure for DP3 ANT Beam one is 139.04 while for CC it is
139.6 and for PB it is 140.8. The % difference between the experiment
measure and that of CC is 0.4 % and between the experiment measure and
that of PB is 1.2%.
ii. Experiment Measure for DP3 RAO Beam two is 20.12 while for CC it is 27
and for PB it is 28.4. The % difference between the experiment measure and
that of CC is 34.1 % and between the experiment measure and that of PB is
41.1%.
iii. Experiment Measure for DP3 LAT Beam three is 6.7 while for CC it is 6.1
and for PB it is 6.7. The % difference between the experiment measure and
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that of CC is -8.9 % and between the experiment measure and that of PB is
0%.
Table 8: Dosimeter Readings and percentage difference between PB with measured and CC with measured for first phantom.
ANT Beam one
Beam 1% Different
RAO Beam two
Beam 2% Different
LAT Beam three
Beam 3% Different
Total Dose at Isocentre
MU or min / Fraction IN = 451.9 78.4 IN = 487.8
Iso
PB IN = 106.9 - 0.02% 61.1 0.3% IN = 106.9 2.1% 274.9CC IN = 107.9 0.9% 61 0.1% IN = 105.7 0.9% 274.6
Reading from Dosemeter 135.5 77.2 132.7
experiment Measure 106.93 60.9 104.7 272.53
DP1
PB IN = 112.4 0.3% 65.3 1.9% IN = 98.6 2.8%CC IN = 112.7 0.5% 64.7 0.9% IN = 97.3 1.4%
Reading from Dosemeter 142 81.2 121.5
experiment Measure 112.05 64.07 95.88
DP2
PB IN = 80 2.7% 73.3 2.4% IN = 120.6 2.4%CC IN = 78.3 0.5% 71.1 - 0.6% IN = 119.7 1.6%
Reading from Dosemeter 98.7 90.7 149.2
experiment Measure 77.88 71.57 117.74
DP3
PB IN = 140.8 1.2% 28.4 41.1% IN = 6.7 0%CC IN = 139.6 0.4% 27 34.1% IN = 6.1 - 8.9%
Reading from Dosemeter 176.2 25.5 8.5
experiment Measure 139.04 20.12 6.7Percentage difference = [(PB/Measure) * 100] – 100%Percentage difference = [(CC/Measure) * 100] – 100%
For the First Phantom the isocentre plans includes an ANT beam with a 60º wedge, a right
RAO beam with no wedge and a LAT oblique beam with a 60º wedge. The Isocentre dose
for PB and CC algorithms were provided by the OMP treatment planning system. The first
phantom was outlined and the total dose prescribed to the isocentre for CC was 5492.8 cGy
(274.6 cGy / fraction) and for PB the total dose prescribed was 274.9 cGy / fraction. (Table
No 8). Dosimeter and ionization chamber were used to arrive at the Iso- Reading value of
135.5 cGy. Equation 2.2 was used to obtain the Iso Measure value of 106.93 cGy (Fig No
14 & 15, Table No 8).
18 |
Figure 14: First phantom calculated with Pencil Beam (PB)
Figure 15: First phantom calculated with Collapsed Cone (CC)
19 |
Table No 9:
Table No 9 demonstrates percentage difference between Pencil Beam (PB) with Experiment
Measured and Collapsed Cone (CC) with Experiment Measured for Second phantom. The results
are as follows:
a. For Isocenter:
i. Experiment Measure for Isocenter ANT Beam one is 89.3 while for CC it is 90
and for PB it is 90.5. The % difference between the experiment measure and that
of CC is 0.7 % and between the experiment measure and that of PB is 1.3%.
ii. Experiment Measure for Isocenter LAO Beam two is 117.7 while for CC it is
119.3 and for PB it is 120.6. The % difference between the experiment measure
and that of CC is 1.3 % and between the experiment measure and that of PB is
2.4%.
iii. Experiment Measure for Isocenter LPO Beam three is 50.5 while for CC it is
55.5 and for PB it is 55.6. The % difference between the experiment measure
and that of CC is 9.9% and between the experiment measure and that of PB is
10.09%.
b. For DP1:
i. Experiment Measure for DP1 ANT Beam one is 94.7 while for CC it is 94.4
and for PB it is 95.1. The % difference between the experiment measure and
that of CC is -0.1 % and between the experiment measure and that of PB is
0.6%.
20 |
ii. Experiment Measure for DP1 LAO Beam two is 121.9 while for CC it is
121.6 and for PB it is 121.4. The % difference between the experiment
measure and that of CC is -0.2 % and between the experiment measure and
that of PB is -0.4%.
iii. Experiment Measure for DP1 LPO Beam three is 50.03 while for CC it is
50.9 and for PB it is 50.8. The % difference between the experiment
measure and that of CC is 1.7 % and between the experiment measure and
that of PB is 1.5%.
c. For DP 2:
i. Experiment Measure for DP2 ANT Beam one is 82.3 while for CC it is 82.5
and for PB it is 83.5. The % difference between the experiment measure and
that of CC is 0.2 % and between the experiment measure and that of PB is
1.4%.
ii. Experiment Measure for DP2 LAO Beam two is 125.6 while for CC it is
125.6 and for PB it is 127.5. The % difference between the experiment
measure and that of CC is -0.1 % and between the experiment measure and
that of PB is 1.5%.
iii. Experiment Measure for DP2 LPO Beam three is 52.8 while for CC it is
55.5 and for PB it is 55.6. The % difference between the experiment
measure and that of CC is 5.1 % and between the experiment measure and
that of PB is 5.3%.
21 |
d. For DP3:
i. Experiment Measure for DP3 ANT Beam one is 106.3 while for CC it is
107.4 and for PB it is 110.4. The % difference between the experiment
measure and that of CC is 1.03 % and between the experiment measure and
that of PB is 3.8%.
ii. Experiment Measure for DP3 LAO Beam two is 100.7 while for CC it is
101.9 and for PB it is 102. The % difference between the experiment
measure and that of CC is 1.1 % and between the experiment measure and
that of PB is 1.2%.
iii. Experiment Measure for DP3 LPO Beam three is 34.9 while for CC it is 54
and for PB it is 55. The % difference between the experiment measure and
that of CC is 54.7 % and between the experiment measure and that of PB is
57.5%.
e. For DP4:
i. Experiment Measure for DP4 ANT Beam one is 132.5 while for CC it is
136.7 and for PB it is 139.3. The % difference between the experiment
measure and that of CC is 3.1 % and between the experiment measure and
that of PB is 5.1%.
ii. Experiment Measure for DP4 LAO Beam two is 20.8 while for CC it is 24.4
and for PB it is 26.4. The % difference between the experiment measure and
22 |
that of CC is 17.3 % and between the experiment measure and that of PB is
26.9%.
iii. Experiment Measure for DP4 LPO Beam three is 38.7 while for CC it is 39
and for PB it is 39. The % difference between the experiment measure and
that of CC is 0.7 % and between the experiment measure and that of PB is
0.7%.
For the Second Phantom
The isocentric plan includes an ANT beam with a 60º wedge and a left LAO beam with no
wedge. The third field is a left LPO oblique beam with a 60º wedge. The second phantom
was outlined and the total dose prescribed to the isocentre was 3971.8 cGy (264.8 cGy /
fraction) and for PB the total dose prescribed was 266.7 cGy. Dosimeter and ionization
chamber were used to arrive at the Iso- Reading value of 135.5 cGy. Equation 2.2 was used
to obtain the Iso Measure value of 106.93 cGy.
The Isocentre dose for PB and CC algorithms were provided by the OMP treatment
planning system. The first phantom was outlined and the total dose prescribed to the
isocentre for CC was 5492.8 cGy (274.6 cGy / fraction) and for PB the total dose
prescribed was 274.9 cGy / fraction. (Table No 8). Dosimeter and ionization chamber were
used to arrive at the Iso- Reading value of 85.7 cGy and the ISO Measure value of 89.3 cGy
was obtained using Equation 2.2 (Fig No 16 & 17, Table No 9)
23 |
Figure 16: Second phantom calculated with Pencil Beam (PB)
Figure 17: Second phantom calculated with Collapsed Cone (CC)
24 |
Table 9: Dosimeter Readings and percentage difference between PB with measured and CC with measured for second phantom.
ANT Beam one
Beam 1% Different
LAO Beam two
Beam 2% Different
LPO Beam three
Beam 3% Different
Total Dose at Isocentre
Isocentre% Different
MU or min / Fraction IN = 382.4 144.1 IN = 229.2Iso PB IN = 90.5 1.3% 120.6 2.4% IN = 55.6 10.09% 266.7 3.5%
CC IN = 90 0.7% 119.3 1.3% IN = 55.5 9.9% 264.8 2.8%Reading from Dosemeter
85.7 113.0 48.5
experiment Measure 89.3 117.7 50.5 257.5DP1 PB IN = 95.1 0.6% 121.4 - 0.4% IN = 50.8 1.5%
CC IN = 94.4 - 0.1% 121.6 - 0.2% IN = 50.9 1.7%Reading from Dosemeter
90.7 117.0 48.0
experiment Measure 94.5 121.9 50.03DP2 PB IN = 83.5 1.4% 127.5 1.5% IN = 55.6 5.3%
CC IN = 82.5 0.2% 125.4 - 0.1% IN = 55.5 5.1%Reading from Dosemeter
79.0 120.5 50.7
experiment Measure 82.3 125.6 52.8DP3 PB IN = 110.4 3.8% 102 1.2% IN = 55 57.5%
CC IN = 107.4 1.03% 101.9 1.1% IN = 54 54.7%Reading from Dosemeter
102.0 96.7 33.5
experiment Measure 106.3 100.7 34.9DP4 PB IN = 139.3 5.1% 26.4 26.9% IN = 39 0.7%
CC IN = 136.7 3.1% 24.4 17.3% IN = 39 0.7%Reading from Dosemeter
127.2 20.0 37.2
experiment Measure 132.5 20.8 38.7Percentage different = [(PB/Measure) * 100] – 100%Percentage different = [(CC/Measure) * 100] – 100%
25 |
4.2 Discussion
4.2.1 Comparison between Pencil beam (PB) VS Collapsed Cone (CC):
Table 10 shows a comparison between PB and CC data for the First Phantom. As can be
seen in the above table, most of the beam values calculated by the two algorithms show a
variation of between 0 to 3%, except for the DP3 Doses in the case of the RAO and the
lateral beam 3, which show a variation in the range of 5% and 10%, respectively.
Table 10: Comparison of PB and CC algorithms for the First Phantom
Beam ANT Beam 1 RAO Beam 2 LAT Beam 3 PB CC % PB CC % PB CC %
Iso Dose (cGy/Fraction)
106.9 107.9 -0.9 61.1 61 0.1 106.9 105.7 1.1
DP1 Dose (cGy/Fraction)
112.4 112.7 -0.2 65.3 64.7 0.9 98.6 97.3 1.3
DP2 Dose (cGy/Fraction)
80 78.3 2.1 73.3 71.1 3.1 120.6 119.7 0.7
DP3 Dose (cGy/Fraction)
140.8 139.6 0.8 28.4 27 5.1 6.7 6.1 9.8
Figures 14 and 15 demonstrate position of DP 1, DP 2 and DP 3 related to the beams. It is
evident that DP 3 is closer to ANT Beam 1 but away from the RAO Beam 2 and LAT beam
3. The beams 2 and beam 3 reach DP 3 at a tangent. In case of Iso dose there is little
difference in the algorithm for Beam 2 and Beam 3. For DP 1 there is no significant
difference in the algorithm for any of the beams. In case of DP 2 there is a slight variation
in the algorithm for Beam 1 and Beam 2 with a difference of 2.1 % and 3.1 % respectively.
This variation in algorithm is expected and can be explained from the fact that Beam 1 and
26 |
Beam 2 have to pass through air. DP3 Beam 1 passes through only 3cm of water and no air
giving an accurate algorithm. Whereas in case of DP 3 Beam 2, there is a difference of 5.1
% suggesting that DP3 is situated in the low dose Penumbra. The algorithms are less
accurate in low dose areas with an absolute dose difference of less than 1.5 cGy, (Figure 14
and 15). Beam 3 does not pass through DP3 giving PB and CC algorithms values of 6.7
cGy and 6.1 cGy respectively demonstrating a difference of 9.8 % (Table 10). These
observations are similar to a retrospective treatment planning study conducted by
(ASPRADAKIS et al 2006)3, to evaluate the differences in the dose distributions and
monitor units predicted by CC and PB algorithms. They observed that the calculated dose
in unit density medium was within1% for the CC model and up to 2% for PB. In contrast in
low density medium and under full scatter conditions, CC overestimated the dose by 1%
whereas PB overestimated the dose by 9%. A negative value obtained while calculating the
percent difference is suggestive of a CC dose.
Table 11: A comparison between PB and CC data for the second Phantom
Beam ANT Beam 1 LAO Beam 2 LPO Beam 3 PB CC % PB CC % PB CC %
Iso Dose (cGy/Fraction)
90.5 90 0.5 120.6 119.3 1.08 55.6 55.5 0.1
DP1 Dose (cGy/Fraction)
95.1 94.4 0.7 121.4 121.6 -0.1 50.8 50.9 -0.1
DP2 Dose (cGy/Fraction)
83.5 82.5 1.2 127.5 125.4 1.6 55.6 55.5 0.1
DP3 Dose (cGy/Fraction)
110.4 107.4 2.7 102.0 101.9 0.09 55.0 54.0 1.8
DP4 Dose (cGy/Fraction)
139.3 136.7 1.9 26.4 24.4 8.1 39 39 0.0
27 |
Table no 11 shows the beam values calculated by the two algorithms. For Iso dose the
variation is not significant for all the beams. In case of DP 1 LAO Beam 2 and LPO Beam
3 show a variation of -0.1%. For DPI 2 all the beams have an accurate algorithm with a
percent difference of 1.2 and 1.6 for Beam 1 and Beam 2. In case of DP3, Beam 1 has a
difference of 2.7 and Beam 3 has a difference of 1. 8 %. In case of DP4 LAO Beam 2,
shows a maximum variation of 8%. This could be explained by the fact that point DP4 is
located at the edge of beam 2 in the penumbra region.
4.2.2 Comparison between algorithms and experimental data:
Phantom 1
In order to investigate the comparative accuracy of the Pencil Beam and Collapsed Cone
algorithms, the percentage differences were calculated by dividing the dose for each
algorithm by the dose calculated from equation 2.2. Figure 18(a – d) illustrates the accuracy
of each algorithm for each beam for First Phantom.
Fig: 18 a: Difference between PB and CC algorithm for Beam 1 (ANT) Phantom 1
28 |
Beam 1 (ANT)
Dose Points
Measured PB Series CC Series PB - CC
ISO -0.028 0.907 -0.935DP1 0.312 0.58 -0.268DP2 2.722 0.539 2.183DP3 1.264 0.402 0.862
ISO DP1 DP2 DP3-0.50
0.51
1.52
2.53
-0.0280.312
2.722
1.2640.907
0.580.5390.402
Beam 1 (ANT)
PB SeriesCC Series
Dose Point measured
perc
enta
ge d
iffer
ence
s
a)
The difference between PB and CC for Beam 1 (ANT) is maximum at DP2 where it is
found to be 2.183. For rest of the dosage points it is less than 1.
Fig: 18 b: Difference between PB and CC algorithm for Beam 2 (RAO) Phantom 1
The difference between PB and CC for Beam 2 (RAO) ranges from 0.164 to 6.96. It is
maximum at DP3 where it is found to be 6.96 showing a wide variation in the algorithm by
Pencil Beam (PB).
Fig 18 c: Difference between PB and CC algorithm for Beam 3 (LAT) Phantom 1
29 |
ISO DP1 DP2 DP3-100
1020304050
0.3281.9192.417
41.15
0.1640.983 -0.656
34.19
Beam 2 (RAO)
PB SeriesCC Series
Dose Point measuredperc
enta
ge d
iffer
ence
sb) Beam 2 (RAO)
Dose Points
MeasuredPB
SeriesCC
Series PB - CCISO 0.328 0.164 0.164DP1 1.919 0.983 0.936DP2 2.417 -0.656 3.073DP3 41.15 34.19 6.96
ISO DP1 DP2 DP3
-10-8-6-4-2024 2.1012.8362.429
00.9551.4810.66
-8.955
Beam 3 (LAT)
PB SeriesCC Series
Dose Point measured
perc
enta
ge d
iffer
ence
s
c) Beam 3 (LAT)
Dose Points
Measured
PB Series
CC Series PB - CC
ISO 2.101 0.955 1.146DP1 2.836 1.481 1.355DP2 2.429 0.66 1.769DP3 0 -8.955 8.955
For Beam 3 (LAT), the difference ranges from 1.146 to 8.955. The difference is maximum
for DP 3 (8.955) suggesting a very wide variation in the PB algorithm.
Phantom 2
The same exercise is repeated for Phantom 2, and the graphs are again plotted for Beam 1,
(ANT) Beam 2 (LAO) and Beam 3 (LPO) as shown in Figures 19 a – c.
Fig 19 a: Difference between PB and CC algorithm for Beam 1 (ANT) Phantom 2
The difference between PB and CC for Beam 1 (ANT) ranges from 0.56 to 2.823. It is
maximum at DP3 where it is found to be 2.823 showing a slight variation in the algorithm
by Pencil Beam (PB).
Fig 19 b: Difference between PB and CC algorithm for Beam 2 (LAO) Phantom 2
30 |
Beam 1 (ANT)
Dose Points
Measured
PB Series
CC Series PB-CC
Iso 1.343 0.783 0.56
DP1 0.634 -0.105 0.739
DP2 1.458 0.243 1.215
DP3 3.857 1.034 2.823
Dp4 5.132 3.169 1.963
Iso DP2Dp4-1
1
3
5
1.3430.634
1.458
3.8575.132
0.783-0.1050.243
1.034
3.169
Beam 1 (ANT)
PB SeriesCC Series
Dose Point measuredperc
enta
ge d
iffer
ence
s
a)
Iso DP1 DP2 DP3 Dp4-5
0
5
10
15
20
25
30
2.463-0.41 1.5121.29
26.92
1.359 -0.246 -0.159 1.191
17.3
Beam 2 (LAO)
PB SeriesCC Series
Dose Point measured
perc
enta
ge d
iffer
ence
s
b)
Beam 2 (LAO)
Dose Points Measured
PB Series
CC Series PB-CC
Iso 2.463 1.359 1.104
DP1 -0.41 -0.246 -0.164DP2 1.512 -0.159 1.671DP3 1.29 1.191 0.099Dp4 26.92 17.3 9.62
Difference for Beam 2 (LAO) is maximum in DP4 with a variation for PB to the tune of
9.62.
Fig 19 c: Difference between PB and CC algorithm for Beam 3 (LPO) Phantom 2
The difference between PB and CC for Beam 3 (LPO) ranges from -0.199 to 2.87, it is
maximum at DP3 where it is found to be 2.87 showing a slight variation in the algorithm by
Pencil Beam (PB).
Using both the phantoms the difference in the algorithm for PB and CC is fairly
large in DP3 (Phantom 1) and DP 4 (Phantom 4) for Pencil Beam. These findings are
consistent with the conclusions of ASPRADAKIS et al (2006)3, who reported that PB
overestimated the dose by 9%. In this experiment it is apparent that PB tends to
overestimate the algorithm and therefore Collapsing Cone is a much preferred algorithm.
Nisbet et al (2004)27, had similar conclusion while comparing the accuracy of Pencil beam
with that of Collapsing Cone. They recommend usage of Collapsing Cone algorithm while
clinical treatment planning situations where lung is present.
31 |
Beam 3 (LPO)
Dose Points Measured
PB Series
CC Series PB-CC
Iso 10.09 9.9 0.19
DP1 1.539 1.738 -0.199
DP2 5.303 5.113 0.19
DP3 57.59 54.72 2.87
Dp4 0.775 0.775 0
Iso DP2Dp4
020406080
10.091.5395.303
57.59
0.7759.9 1.7385.113
54.72
0.775
Beam 3 (LPO)
PB SeriesCC Series
Dose Point measured
Perc
enta
ge d
if-fe
renc
esc)
Conclusion
1. This study was conducted to compare and contrast two algorithms Pencil Beam (PB)
and Collapsing Cone (CC).
2. Collapsing Cone is found to be more accurate when measured on two phantoms
suggesting that Collapsing cone is a much accurate algorithm for clinical treatment
planning scenario.
3. The experiment clearly shows that Pencil beam tends to overestimates the dose by 9.62
%.
4. Based on this study it is recommended that Collapsing Cone is used as the treatment
algorithm. Conclusions drawn in this study are consisted with findings of other studies.
32 |
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