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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 Information Beam 1ANT 2RPO 3RPO 4ARO Nom. Acc. Pot.(MV or MeV) 6 6 6 6 FX (cm) 8.2 8.2 8.6 8.6 FY (cm) 9.4 10.6 8.2 8.6 SSD (cm) 87.2 86.8 85.6 84.7 Gantry (degrees) 0 223 267 320 Wedge Angle(degrees) 60/60 60/33 60/25 Dose Information: Absolute dose 5500 cGy (275 cGy / fraction) Number of Fraction 20 20 20 20 IN = 424.77 IN = 185.78 IN = 92.85 MU or min / Fraction 16.39 OUT = 0 OUT = 85.22 OUT = 69.34 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 3LPO Beam 4ANT 5MINI-ANT 6LAO THORAX Nom. Acc. Pot. 6 6 6 6 (MV or MeV) FX (cm) 10.6 10.1 9.7 14.2 FY (cm) 9.7 14.3 11 10.1 SSD (cm) 86.3 87.2 87.2 82.4 Gantry (degrees) 120 0 0 60 Wedge 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 IN = 2291.07 IN = 1277.72 IN = 1974.08 IN = 476.08 MU or min / Fraction OUT = 1711.09 OUT = 793.65 OUT = 0 OUT = 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 to Heart Heart to Lung to X,Y axis Lung (cm) heart (cm) (cm) Lung (cm) Skin (cm) Average for 2.8 4.2 4.2 3.3 3.1 17.4 patient(Lateral) Average for patient 2.3 1.9 4.4 1.3 2.6 11.1 (Ant-post) Average for 4 8 4 8 4 28 Phantom(X) Average for 4 4 6 2 4 20 Phantom(Y) MAX in X axis for patients (Lateral) 19 MIN in X axis for patients (Lateral) 15 MAX in Y axis for patients (Ant-post) 13.7 MIN in Y axis for patients (Ant-post) 9.6 RANGE in X axis for patients (Lateral) 4 RANGE in Y axis for patients (Ant-post) 4.1 Table 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 Information Beam ANT RAO LAT Nom. Acc. Pot.(MV or MeV) 6 6 6 Field size X (cm) 10 10 10 Field size Y (cm) 10 10 10 SSD (cm) 90 86 90 Gantry (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 Information Beam ANT LAO LPO Nom. Acc. Pot.(MV or MeV) 6 6 6 Field size X (cm) 10 10 10 Field size Y (cm) 10 10 10 SSD (cm) 90 88.5 91.3 Gantry (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 Pion 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: 273 .2 T P0 Ø (P, T) = ; 273 .2 T0 P 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 | 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 3 Chambers: NE technology 0.6 cc thimble 2571 & 2571 A 1884 & 2921 Mk3 & Mk4 & graphite 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 SSD First day Second day Field Size 10 x 10 cm 10 x 10 Source to surface Distance (SSD) 100 cm 100 cm Depth 5 cm 5 cm Set Dose (SD) 400 mu 400 mu ND (calibration factor ion chamber & electrometer) 0.794 1.034 Pion (ion recombination) 1.0042 1.0042 Temperature 20.5 Centigrade 20.2 Centigrade Pressure 765.50 mmHg 759.20 mmHg Ø (P,T) (Temperature & Pressure correction) 0.9945 1.0017 Wρ (density Correction) 1.000 1.000 Depth 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 17 | 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 Beam 1 RAO Beam Beam 2 LAT Beam Beam 3 Total Dose at one % Different two % Different three % Different Isocentre MU or min / Fraction IN = 451.9 78.4 IN = 487.8 PB IN = 106.9 - 0.02% 61.1 0.3% IN = 106.9 2.1% 274.9 CC IN = 107.9 0.9% 61 0.1% IN = 105.7 0.9% 274.6 so Reading from 135.5 77.2 132.7 Dosemeter experiment Measure 106.93 60.9 104.7 272.53 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% P1 Reading from 142 81.2 121.5 Dosemeter experiment Measure 112.05 64.07 95.88 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% P2 Reading from 98.7 90.7 149.2 Dosemeter experiment Measure 77.88 71.57 117.74 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% P3 Reading from 176.2 25.5 8.5 Dosemeter experiment Measure 139.04 20.12 6.7 Percentage 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 Beam 1 LAO Beam Beam 2 LPO Beam Beam 3 Total Dose at Isocentre one % Different two % Different three % Different Isocentre % Different MU or min / Fraction IN = 382.4 144.1 IN = 229.2 Iso 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 85.7 113.0 48.5 Dosemeter experiment Measure 89.3 117.7 50.5 257.5 DP1 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 90.7 117.0 48.0 Dosemeter experiment Measure 94.5 121.9 50.03 DP2 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 79.0 120.5 50.7 Dosemeter experiment Measure 82.3 125.6 52.8 DP3 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 102.0 96.7 33.5 Dosemeter experiment Measure 106.3 100.7 34.9 DP4 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 127.2 20.0 37.2 Dosemeter experiment Measure 132.5 20.8 38.7 Percentage 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 106.9 107.9 -0.9 61.1 61 0.1 106.9 105.7 1.1 (cGy/Fraction) DP1 Dose 112.4 112.7 -0.2 65.3 64.7 0.9 98.6 97.3 1.3 (cGy/Fraction) DP2 Dose 80 78.3 2.1 73.3 71.1 3.1 120.6 119.7 0.7 (cGy/Fraction) DP3 Dose 140.8 139.6 0.8 28.4 27 5.1 6.7 6.1 9.8 (cGy/Fraction) 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 90.5 90 0.5 120.6 119.3 1.08 55.6 55.5 0.1 (cGy/Fraction) DP1 Dose 95.1 94.4 0.7 121.4 121.6 -0.1 50.8 50.9 -0.1 (cGy/Fraction) DP2 Dose 83.5 82.5 1.2 127.5 125.4 1.6 55.6 55.5 0.1 (cGy/Fraction) DP3 Dose 110.4 107.4 2.7 102.0 101.9 0.09 55.0 54.0 1.8 (cGy/Fraction) DP4 Dose 139.3 136.7 1.9 26.4 24.4 8.1 39 39 0.0 (cGy/Fraction) 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. a Beam 1 (ANT) 2.722 Beam 1 (ANT) 3 percentage differences Dose 2 Points 1.264 0.907 Measured PB Series CC Series PB - CC 1 0.580.5390.402 PB Series ISO -0.028 0.907 -0.935 -0.028 CC Series 0 0.312 DP1 0.312 0.58 -0.268 ISO DP1 DP2 DP3 DP2 2.722 0.539 2.183 -1 DP3 1.264 0.402 0.862 Dose Point measured Fig: 18 a: Difference between PB and CC algorithm for Beam 1 (ANT) Phantom 1 28 | 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. b Beam 2 (RAO) Beam 2 (RAO) 50 41.15 percentage differences 40 Dose 30 34.19 Points PB CC 20 PB Series Measured Series Series PB - CC 1.919 10 0.328 2.417 CC Series ISO 0.328 0.164 0.164 0 0.164 0.983 -0.656 DP1 1.919 0.983 0.936 -10 ISO DP1 DP2 DP3 DP2 2.417 -0.656 3.073 Dose Point measured DP3 41.15 34.19 6.96 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). c Beam 3 (LAT) Beam 3 (LAT) 5 2.1012.8362.429 percentage differences 1.4810.66 0 Dose 0.955 Points PB CC 0 ISO DP1 DP2 DP3 Measured Series Series PB - CC PB Series ISO 2.101 0.955 1.146 -5 CC Series DP1 2.836 1.481 1.355 -8.955 DP2 2.429 0.66 1.769 -10 Dose Point measured DP3 0 -8.955 8.955 Fig 18 c: Difference between PB and CC algorithm for Beam 3 (LAT) Phantom 1 29 | 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. Beam 1 (ANT) a Beam 1 (ANT) Dose 6 Points PB CC percentage differences 5.132 Measured Series Series PB-CC 4 3.857 3.169 Iso 1.343 0.783 0.56 2 PB Series 0.634 -0.105 1.343 1.458 DP1 0.739 0.634 1.034 0.783 0.243 CC Series 1.458 0.243 0 -0.105 DP2 1.215 Dp4 Iso DP1 DP2 DP3 DP3 3.857 1.034 2.823 -2 Dose Point measured Dp4 5.132 3.169 1.963 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). Beam 2 (LAO) 30 b Beam 2 (LAO) Dose percentage differences 26.92 Points PB CC 20 Measured Series Series PB-CC 17.3 Iso 2.463 1.359 1.104 10 PB Series 2.463 1.29 DP1 -0.41 -0.246 -0.164 -0.41 1.512 CC Series 0 1.191 DP2 1.512 -0.159 1.671 1.359 -0.246 -0.159 Iso DP1 DP2 DP3 Dp4 DP3 1.29 1.191 0.099 -10 26.92 17.3 Dose Point measured Dp4 9.62 Fig 19 b: Difference between PB and CC algorithm for Beam 2 (LAO) Phantom 2 30 | Difference for Beam 2 (LAO) is maximum in DP4 with a variation for PB to the tune of 9.62. c Beam 3 (LPO) Beam 3 (LPO) 80 Dose Points PB CC differences Percentage 60 57.59 54.72 Measured Series Series PB-CC 40 Iso 10.09 9.9 0.19 20 10.095.303 1.539 0.775 PB Series 0 9.9 1.738 0.775 DP1 1.539 1.738 -0.199 5.113 CC Series DP2 5.303 5.113 0.19 DP3 57.59 54.72 2.87 Dose Point measured Dp4 0.775 0.775 0 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 | 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 | References [1] Ang, K. & Garden, A. S. 2006. Radiotherapy for Head and Neck Cancers: Indications and Techniques. United States: Lippincott Williams & Wilkins [2] Arnfield, MR, CH Siantar, J. Sirbers, et al. 2000. The Impact of Electron Transport on the Accuracy of Computed Dose. Med Phys 27(6): 1266-74 [3] Aspradakis, M., McCallum, H. M. and Wilson, N. 2006. Dosimetric and Treatment Planning Considerations for Radiotheraphy of the Chest Wall. The British Journal of Radiology, 79 (2006), 828-836 [4] Chetty, IJ, B. Curran, JE Cygler, et al. 2007. Report of the AAPM Task Group No. 105: Issues Associated with Clinical Implementation of Monte Carlo-based Progams (105) [5] Chin, L. & Regine, W. 2008. 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Comparison of Pencil Beam and Collapsing Cone

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