Abstract
Background:
Given the importance of scattered and low doses in secondary cancer caused by radiation treatment, the point dose of critical organs, which were not subjected to radiation treatment in breast cancer radiotherapy, was measured.
Objective:
The purpose of this study is to evaluate the peripheral dose in two techniques of breast cancer radiotherapy with two energies.
Material and Methods:
Eight different plans in two techniques (conventional and conformal) and two photon energies (6 and 15 MeV) were applied to Rando Alderson Phantom’s DICOM images. Nine organs were contoured in the treatment planning system and specified on the phantom. To measure the photon dose, forty-eight thermoluminescence dosimeters (MTS700) were positioned in special places on the above nine organs and plans were applied to Rando phantom with Elekta presice linac. To obtain approximately the same dose distribution in the clinical organ volume, a wedge was used on planes with an energy of 6 MeV photon.
Results:
Point doses in critical organs with 8 different plans demonstrated that scattering in low-energy photon is greater than high-energy photon. In contrast, neutron contamination in high-energy photon is not negligible. Using the wedge and shield impose greater scattering and neutron contamination on patients with low-and high-energy photon, respectively.
Conclusion:
Deciding on techniques and energies required for preparing an acceptable treatment plan in terms of scattering and neutron contamination is a key issue that may affect the probability of secondary cancer in a patient.
Keywords: Photon Dose Measurement , Radiation Therapy , Breast Cancer , Thermoluminescence Dosimeter , Rando Phantom
Introduction
Breast cancer is one of the most common cancers among women in the world [1-3]. Early diagnosis and radiotherapy treatment can decrease the mortality rate [4-6] and increase the survival rate of these patients [6,7]. Despite the advantages of radiotherapy for cancer treatment as one of the third treatment modalities, radiotherapy is a “two-edged swords” that may cause cancer [8-10]. Therefore, dose distribution in critical organs, which are outside the treatment fields, is a crucial matter. Absorbed dose in these organs are due to the photon scattering. The main sources of photon scattering are head leakage, patient (phantom) and secondary collimator scattering [11]. Given the importance of absorbed dose caused by photon and electron scatter and secondary cancer caused by radiation, authors tried to measure out-field critical organ dose in some treatment plans with different techniques [12-16]. Banaee et al. in 2014 [12] measured the peripheral dose for bladder, rectum and prostate cancer in the phantom and 27 patients. They conducted their measurement using two photon energies with Thermoluminescence dosimeters (TLD). Authors assessed the necessity of a testicular shield for high and low energies. Cyriac et al. in 2015 [13] evaluated the peripheral dose for prostate cancer by comparing the treatment planning system (TPS), ionizing chamber and TLDs measurement of the two techniques (3D-Conformal radiation therapy and Intensity modulated radiation therapy). Based on their evaluation, the inaccuracy of TPS calculation for outfield organ dose was proved. Thermoluminescence dosimeters (TLD) was approved as an acceptable tool for evaluating and monitoring the absorbed dose in patients’ organs in different radiotherapy treatment with the same articles [13,17-19]. Based on the literature review, there were not any articles which compare the nine organs’ photon absorbed dose in two techniques and two energies for breast cancer radiation therapy. Therefore, the authors in this study decided to evaluate the organs’ absorbed dose in eight different plans.
The goal of this study is to measure outfield dose of critical organs in breast cancer radiotherapy in Rando phantom. Treatment was administered using two different techniques (conventional and conformal) and two photon energies (6 and 15 MeV) and assessments were made by comparing point doses in nine organs measured by the thermoluminescence dosimeter.
Material and Methods
Treatment plans for breast cancer
To evaluate and compare point dose of the outfield organs in breast cancer, eight different plans containing two techniques (conventional and conformal) and energies (6 and 15 MeV) were applied along with ISO gray treatment planning system (version 4.2.3.50L) to Rando phantom’s DICOM (Digital Imaging and Communications in Medicine) images. The Rando Alderson phantom (Radiology Support Devices (RSD), United Kingdom) DICOM images were taken in the radiology department and the radiation treatment was administered in radiotherapy department using Elekta Precise linear accelerator (Stockholm, Sweden) in Emam Reza hospital of Mashhad, Iran. Prior to the plan, nine organs (right and left lenses, thyroid, right and left lung, right and left kidney, spinal cord, heart and liver, bladder, rectum, uterine) were contoured by the radiation oncologist on DICOM images in TPS and then planning was performed. Plans applied to the phantom were as follows: 2 opposed tangential fields; 2 opposed tangential fields together with super clave and postaxial treatment field using two different energies and techniques. The details of each plan are given in Table 1. Attempts were made to maintain other parameters of treatment plans constant to have reasonable dose distribution and acceptable treatment plans. In all plans, the prescribed dose was 50 cGy in 25 fractionations. Figure 1 illustrates the CT (computed tomography) simulation of the Rando phantom and one of its plans (Conventional+supra+postaxilla, 15MV) in ISOgray TPS.
Table 1.
Eight different treatment plans applied to Rando phantom in experimental measurement with TLD. Lengths and angels are in millimeters and degree, respectively.
| Energy | 6 MeV | 15 MeV | |||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Technique | Conventional | Conformal | Conventional | Conformal | |||||||||||||||||||||
| Plan | 2 opposed tangential fields | 2 opposed tangential fields plus super clave and postaxial | 2 opposed tangential fields | 2 opposed tangential fields plus super clave and postaxial | 2 opposed tangential fields | 2 opposed tangential fields plus super clave and postaxial | 2 opposed tangential fields | 2 opposed tangential fields plus super clave and postaxial | |||||||||||||||||
| Name of Field | LtMed | LtLat | LtMed | LtLat | AP | PA | LtMed | LtLat | LtMed | LtLat | AP | PA | LtMed | LtLat | LtMed | LtLat | AP | PA | LtMed | LtLat | LtMed | LtLat | AP | PA | |
| Field Size | X1 | -100 | -100 | -179 | 0 | 0 | -108 | -90 | -96 | -179 | 0 | 0 | -108 | -100 | -100 | -58 | -51 | 0 | -108 | -90 | -96 | -58 | -51 | 0 | -108 |
| X2 | 100 | 100 | 0 | 179 | 99 | 0 | 90 | 92 | 0 | 179 | 99 | 0 | 100 | 100 | 51 | 63 | 99 | 0 | 97 | 92 | 51 | 63 | 99 | 0 | |
| Y1 | -57.5 | 60 | -50.5 | -50.5 | -30.5 | -10.5 | -50.5 | -50.5 | -50.5 | -50.5 | -30.5 | -10.5 | -57.5 | 60 | -179 | -179 | -30.5 | -10.5 | -50.5 | -50.5 | -179 | -179 | -30.5 | -10.5 | |
| Y2 | 57.5 | 60 | 58 | 63 | 103.5 | 120.5 | 53 | 60.5 | 58 | 63 | 103 | 120.5 | 57.5 | 60 | 0 | 0 | 103.5 | 120.5 | 53 | 60.5 | 0 | 0 | 103.5 | 120.5 | |
| Gantry Angle | 298 | 118 | 298 | 118 | 350 | 170 | 298 | 118 | 298 | 118 | 350 | 170 | 298 | 118 | 298 | 118 | 350 | 170 | 298 | 118 | 298 | 118 | 350 | 170 | |
| Collimator Angle | 96 | 264 | 96 | 264 | 90 | 270 | 96 | 264 | 96 | 264 | 90 | 270 | 96 | 264 | 6 | 354 | 90 | 270 | 96 | 264 | 6 | 354 | 90 | 270 | |
| Couch Rotation | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | |
| Wedge | Yes | Yes | Yes | Yes | No | No | Yes | Yes | Yes | Yes | No | No | No | No | No | No | No | No | No | No | No | No | No | No | |
| Block | No | No | No | No | Yes | Yes | No | No | No | No | No | No | No | No | No | No | Yes | Yes | No | No | No | No | No | No | |
| MLC | No | No | No | No | No | No | Yes | Yes | Yes | Yes | Yes | Yes | No | No | No | No | No | No | Yes | Yes | Yes | Yes | Yes | Yes | |
| MU | 146.0 | 143.9 | 141.4 | 155.4 | 163.6 | 104.5 | 151.1 | 148.9 | 142.7 | 156.4 | 154.8 | 96.26 | 106.6 | 106.5 | 108.3 | 118.8 | 132.8 | 97.3 | 110.7 | 110.6 | 111.5 | 110 | 125.0 | 91.44 | |
Figure1.
(a) CT simulation of the Rando phantom. (b) One of plans (Conventional+ supra+ postaxilla, 15MV) in ISOgray TPS.
TLDs are located in special positions which are determined and embedded in Rando phantom. In this study, to measure the peripheral photon dose in organs, MTS-700 TLDs (TLD Poland, Krakow, Poland) purchased from Poland Company were used to measure point dose in organs. Before using the TLDs, they were calibrated with the photon mode of Elekta precise linac (6 MeV energy) based on the calibration protocols of these TLDs obtained from the company and then point doses of outfield organs were calculated. Some of Rando phantom slices and TLDs’ positions in these slices are illustrated in Figure 2.
Figure2.
Rando phantom slices and TLDs position in some of slices.
Calibration of TLD
Calibration was implemented in three steps: In the first and second steps, the reader calibration factor (RCF) was determined and in the third step, element correction coefficient (ECC) was computed. In the first step, at least 10-15% of total TLDs should be selected as representative. Thus, 15 TLDs from 50 TLDs were chosen randomly and irradiated to achieve a uniform dose under the range of dose distribution which is linear (less than one gray (1 Gy) is acceptable). TLDs were read by a Harshaw TLD reader ( Model 3500- Thermo electron, Solon, OH) and the dimensions provided by the TLD reader for each TLD at this stage were in nano-coulomb (nC). The average of these numbers was calculated and ECC of each TLD was determined using the following formula:
Element correction coefficient(ECC) for representative TLDs= Average of nC for representatives TLD (nC)/nC for each TLD
In this stage, all TLDs were introduced to the TLD reader.
In the second step, representative TLDs were irradiated by a homogeneous and specific dose. The average nC read by the reader was calculated and RCF was computed by the following formula:
Reader calibration factor (RCF) for representative TLDs= Average nC for representatives TLD (nC)/certain (specified) dose(Gy)
In this stage, the reader sensitivity and unit conversion factor are determined.
The third step involved exposing all TLDs to a homogeneous and distinctive dose to determine ECC for each TLD. Here, the unite used by the reader is gray since nC was divided by RCF as follow:
nC/RCF=Dose (Gy)
Therefore, ECC will be computed by dividing the distinctive dose by the above numbers for each TLD.
distinctive dose/Dose (Gy)=ECC (for each TLD)
Based on the TLDs reading, the linearity of TLDs was assessed. To estimate and draw the linearity graph of TLDs, 24 TLDs were selected randomly, and divided into eight groups then irradiated in the photon mode of the Elekta Precise linac (6 MeV) in the range of 110 to 250 cGy. The linearity function was estimated and applied to the numbers which read larger than 1 Gy. Table 2 and Figure 3 represent the number and TLD’s linearity graph.
Table 2.
TLDs results in TLDs linearity calibration.
| Group 1 | Group 2 | Group 3 | Group 4 | Group 5 | Group 6 | Group 7 | Group 8 | |
|---|---|---|---|---|---|---|---|---|
| Radiation Dose (cGy) | 110 | 130 | 150 | 170 | 190 | 210 | 230 | 250 |
| Reading Dose (cGy) | 109.8 | 127.77 | 151.30 | 178.64 | 185.80 | 224.07 | 240.90 | 260.00 |
Figure3.
TLD’s linearity graph.
Experimental dosimetry in Rando Phantom
Given that some articles have reported TPS weakness in estimating doses of outfield organs [20-23], TLDs were chosen to measure the point doses in organs outside the treatment fields. Nine organs (right and left lenses, thyroid, right and left lung, right and left kidney, spinal cord, heart and liver, bladder, rectum, uterine) were located in Rando phantom’s slices and 48 TLDs were placed on the designated position inside organs in different slices of Rando phantom. Based on the organ volume, at least two TLDs were placed in each organ. Thereafter, eight plans applied to Rando phantom with ISOgray TPS were undertaken for Rando phantom. Measurements were repeated three times for greater accuracy.
Results
The point dose measurement in Rando phantom for all eight plans was carried out by thermoluminescence chips. The results of measurements are demonstrated in Table 3 based on the organ point dose which were out of treatment fields. The numbers in Table 3 show the mean and standard deviation of three-time measurement and the number is based on 50 gray in 25 fraction (2 Gy per fraction). For all numbers, TLDs’ background doses deducted from the primary TLD’s absorbed dose and for larger than 1 Gy were inserted in the linearity function to compute the dose. All of these were undertaken to obtain accurate doses absorbed by organs. The point doses were assigned to two primary groups based on the Elekta linac energy (photon mode) and then each group was further divided into four subgroups based on the treatment techniques. In this table, the dimensions of absorbed doses are expressed in milli-Gray (mGy). Figure 4 depicts the glow curve of TLD reading, the first and second plots are glow curves for two TLDs in outfield organs and the third plot is the glow curve for a background TLD which is exported from Harshaw TLD reader (Model 3500).
Table 3.
Organ point doses (mGy) measurement in Rando phantom for different techniques of breast cancer radiation treatment (50 Gy in 25 Fraction)
| Energy | 6 MeV | 15 MeV | ||||||
|---|---|---|---|---|---|---|---|---|
| Techniques | Conventional (2 Tang) | Conformal (2 Tang) | Conventional (2 Tang + Axilla) | Conformal (2 Tang + Axilla) | Conventional (2 Tang) | Conformal (2 Tang) | Conventional (2 Tang + Axilla) | Conformal (2 Tang + Axilla) |
| Organs | ||||||||
| Left Lens | 251.50±10.25 | 170.25±13.75 | 704.00±21.37 | 615.99±36.51 | 215.84±9.13 | 267.60±18.44 | 650.67±22.35 | 491.75±22.91 |
| Right Lens | 218.25±27.25 | 110.25±16.25 | 552.60±9.37 | 486.79±50.92 | 239.35±9.14 | 229.96±19.88 | 556.12±30.00 | 402.19±46.57 |
| Thyroid | 453.50±97.00 | 271.25±78.75 | 2991.67±160.97 | 2781.06±184.73 | 437.77±22.62 | 394.22±93.73 | 2860.17±267.73 | 2948.30±311.74 |
| 593.50±48.25 | 299.75±91.75 | 52535.78±6332.16 | 63833.27±1086.54 | 441.74±13.62 | 410.59±36.31 | 47923.23±1416.09 | 52272.32±1482.23 | |
| Left Lung | 328.50±15.50 | 176.75±5.00 | 62673.11±1064.69 | 66628.35±1555.81 | 233.98±5.68 | 235.38±28.92 | 58941.85±1876.58 | 64394.57±2230.9 |
| 687.50±56.50 | 322.75±14.00 | 51586.44±1364.91 | 30712.86±64.92 | 403.23±81.31 | 449.15±48.08 | 9896.87±1736.61 | 27781.38±3693.87 | |
| 438.50±49.00 | 242.25±11.75 | 4720.30±181.05 | 4707.93±173.29 | 371.35±93.09 | 348.21±80.94 | 3772.62±217.00 | 5262.81±688.63 | |
| 1829.00±240.75 | 1615.25±168.5 | 2441.25±37.54 | 2214.87±48.04 | 1033.82±70.20 | 932.24±168.56 | 1607.47±89.57 | 1730.18±144.66 | |
| 1660.00±171.00 | 2132.25±330.75 | 1959.69±59.38 | 1757.43±37.37 | 1192.09±16.66 | 1223.86±311.72 | 1819.20±79.19 | 1992.32±128.79 | |
| Right Lung | 256.75±23.25 | 148.75±0.75 | 1311.08±116.18 | 1131.55±13.07 | 226.37±24.78 | 266.37±29.15 | 898.70±56.41 | 916.85±72.87 |
| 337.25±20.75 | 191.50±8.75 | 959.74±22.83 | 956.26±6.02 | 242.59±9.36 | 230.76±29.97 | 711.02±69.81 | 781.83±77.34 | |
| 484.00±39.00 | 323.50±25.50 | 594.45±9.17 | 756.24±12.10 | 332.02±28.13 | 361.22±17.59 | 520.83±28.24 | 513.00±40.86 | |
| 471.00±40.25 | 450.50±101.25 | 724.88±21.57 | 671.31±38.65 | 373.38±6.28 | 362.28±62.63 | 648.14±46.41 | 669.54±40.46 | |
| 315.75±25.25 | 277.00±33.50 | 550.29±14.18 | 468.40±15.31 | 216.26±6.80 | 265.08±39.82 | 557.85±29.55 | 482.20±27.49 | |
| Spinal Cord | 238.50±54.50 | 149.75±10.50 | 2033.90±59.15 | 1880.62±62.41 | 195.79±19.75 | 282.48±61.08 | 1439.60±127.65 | 1467.34±176.58 |
| 282.25±67.25 | 139.00±21.50 | 24154.57±437.25 | 19356.11±4454.57 | 178.21±14.91 | 208.53±13.83 | 18872.20±2733.47 | 21122.39±4443.8 | |
| 316.25±31.75 | 189.75±1.50 | 3638.05±123.40 | 4539.73±52.45 | 285.89±30.74 | 269.10±45.06 | 2406.37±386.48 | 4214.59±1158.86 | |
| 231.00±.24.50 | 134.50±3.25 | 1283.43±25.94 | 1273.94±53.08 | 168.50±9.71 | 187.72±21.59 | 905.45±77.64 | 1046.23±109.76 | |
| 252.00±12.750 | 205.25±16.25 | 668.83±21.48 | 671.97±67.48 | 188.65±12.54 | 242.67±33.71 | 609.83±38.74 | 542.77±36.57 | |
| 345.00±19.00 | 222.75±28.0 | 1064.50±25.32 | 834.65±48.58 | 227.70±17.72 | 262.89±38.66 | 912.54±50.56 | 871.03±28.16 | |
| Heart | 1132.00±129.75 | 872.50±101.25 | 1634.69±34.56 | 1560.63±5.38 | 737.73±2.57 | 731.65±134.54 | 1239.47±89.90 | 1408.68±121.26 |
| 1603.00±138.50 | 1342.25±116.25 | 2378.97±104.22 | 2209.76±64.33 | 1056.51±11.16 | 1077.06±205.49 | 1649.77±109.12 | 1898.60±188.68 | |
| 733.25±79.75 | 536.00±66.50 | 1244.55±27.20 | 1139.47±27.50 | 487.43±6.79 | 450.09±49.78 | 991.42±33.68 | 1054.56±70.95 | |
| 1250.75±129.75 | 1162.00±186.00 | 1716.82±50.87 | 1510.17±69.68 | 779.55±8.23 | 802.56±121.22 | 1285.82±135.73 | 1381.26±65.26 | |
| 959.50±89.50 | 826.50±108.75 | 1596.55±26.67 | 1428.29±21.47 | 649.66±41.51 | 647.86±68.30 | 1267.66±97.97 | 1349.24±92.60 | |
| 714.50±74.50 | 617.50±92.25 | 1147.71±13.32 | 1025.85±13.41 | 493.14±14.25 | 502.37±43.64 | 935.98±63.96 | 950.43±39.46 | |
| Liver | 411.00±58.25 | 450.25±63.25 | 592.39±23.47 | 531.81±24.34 | 332.94±13.77 | 366.86±22.10 | 592.37±28.79 | 559.17±30.80 |
| 502.00±18.75 | 550.75±66.25 | 673.29±21.71 | 586.22±41.98 | 334.59±22.08 | 345.60±34.33 | 655.06±51.63 | 641.00±30.80 | |
| 842.50±199.00 | 911.00±158.25 | 924.58±9.10 | 811.56±53.76 | 498.76±13.13 | 585.22±78.12 | 782.81±67.46 | 833.20±20.09 | |
| 917.50±57.50 | 1289.25±267.50 | 1208.55±23.70 | 1084.19±14.34 | 617.42±28.55 | 657.92±108.69 | 1017.02±93.44 | 1059.24±51.76 | |
| 315.25±43.25 | 375.75±56.50 | 435.87±12.13 | 379.77±8.23 | 208.14±6.04 | 240.38±34.95 | 408.57±22.84 | 430.22±58.75 | |
| Right Kidney | 197.00±32.75 | 175.50±27.50 | 262.27±5.43 | 259.57±42.32 | 134.68±15.51 | 156.74±12.27 | 258.01±9.28 | 244.95±48.66 |
| 243.25±30.75 | 293.00±21.75 | 381.88±34.22 | 313.07±21.25 | 203.62±13.45 | 212.98±42.91 | 356.33±47.21 | 324.19±46.87 | |
| 231.25±31.25 | 247.50±41.25 | 333.18±53.84 | 270.77±8.21 | 189.14±8.53 | 181.98±28.30 | 290.60±8.20 | 303.17±78.62 | |
| Left Kidney | 290.75±21.25 | 370.00±41.50 | 465.43±9.40 | 378.61±6.97 | 240.35±7.24 | 223.33±39.46 | 422.06±24.54 | 412.78±34.12 |
| 418.50±28.50 | 462.00±52.00 | 523.87±971 | 473.18±37.93 | 246.24±1.77 | 256.60±39.82 | 468.77±21.08 | 415.47±23.88 | |
| 292.50±22.00 | 394.00±55.50 | 415.73±14.12 | 350.58±20.45 | 229.71±12.20 | 240.44±38.81 | 376.74±24.04 | 319.15±8.96 | |
| Rectum | 41.75±3.00 | 97.50±3.50 | 114.50±6.00 | 98.00±0.75 | 86.50±2.00 | 85.75±2.00 | 118.50±2.50 | 93.25±1.25 |
| 37.00±2.50 | 39.50±3.50 | 118.75±1.00 | 98.75±2.50 | 81.25±1.25 | 80.50±2.25 | 116.50±1.00 | 95.00±4.00 | |
| Bladder | 33.28±2.00 | 84.5±2.25 | 109.75±4.25 | 89.75±1.75 | 77.00±0.25 | 75.50±3.75 | 105.25±0.75 | 89.75±2.50 |
| 39.25±3.00 | 91.75±2.75 | 115.00±1.75 | 94.25±1.25 | 81.75±0.25 | 79.50±4.25 | 111.75±1.25 | 90.75±1.50 | |
| 28.25±2.00 | 81.25±1.75 | 101.75±1.75 | 86.50±1.00 | 75.50±1.00 | 75.25±2.25 | 104.50±0.50 | 85.50±1.00 | |
| 27.00±2.75 | 77.75±1.50 | 97.50±1.00 | 85.50±2.50 | 73.00±1.00 | 72.75±2.00 | 103.75±2.00 | 85.25±0.75 | |
| Uterin | 35.25±9.25 | 81.00±0.50 | 99.75±0.75 | 85.75±1.00 | 74.75±0.75 | 73.50±2.75 | 103.25±1.75 | 86.50±0.75 |
| 33.00±2.50 | 84.25±2.25 | 107.00±0.25 | 89.50±0.50 | 78.50±0.50 | 76.25±3.25 | 109.75±2.25 | 88.25±0.75 | |
| 20.50±1.50 | 72.00±1.75 | 92.50±2.25 | 82.25±2.25 | 70.25±0.00 | 69.25±2.25 | 100.75±1.00 | 84.75±1.00 | |
Figure4.
TLDs glow curve a) and b) TLDs which are in Rando Phantom’s organs. c) background TLDs.
Discussion
In this study, considering the importance of secondary cancer estimation in out-of-field organs in patients who have radiation therapy treatment, the outfield point dose of organs was measured for radiation treatment of breast cancer. According to articles evaluating secondary cancer [8,24-28], the first and important parameter for assessing secondary cancer is absorbed dose in out of radiation fields. In general, the results in this study represent that the point doses of organs closer to treatment fields in comparison with organs distant from treatment fields are larger. It is clear that in paired organs, like eyes and kidneys, the left organ absorbs higher dose. This is obviously related to the inverse-square law.
Plans of 6 MeV energy: As expected, the results suggest that point dose of organs in conventional treatment plans is higher than conformal ones and then TLDs in plans with super clave and postaxilla fields besides tangential fields (4 treatment fields) absorbed greater dose compared to plans with two tangential fields (especially in organs close to the treatment fields such as left-lung, spinal cord and heart). The above results could be explained as follows: treatment fields in conventional plans are wider than that of conformal ones; on the other hand, the multi-leaf collimator, which covers critical organs in conformal plans, produces more scatter photons in treatment fields. The results of conventional and conformal techniques for 6 MeV photon energy indicate that scattering caused by the treatment field size in conventional plans is more effective than the scattering induced by the MLC.
Plans of 15 MeV energy: The point dose of organs in conventional and conformal plans (2 tangential fields) with 15 MeV photon energy provided relatively identical outcomes which could be attributed the following factors: 15 MeV photon energy has greater penetration depth and lower scattering than 6 MeV energy [29]. Therefore, conventional and conformal plans with 15 MeV energy have lower scattering in outfield organs confirming the results by comparing the columns related to 6 MeV and 15 MeV energy plans, respectively. On the other hand, the effect of MLC in conformal plans with 15 MeV energy leads to greater scattering. Thus, it is expected that point doses of organs in these plans (conformal and conventional with 2 tangential fields) will be the same, while other plans with 4 treatment fields in this energy follow the same role in plans with 6 MeV photon energy.
The presence of wedge in plans of 6 MeV is another reason for higher organ point dose in these plans compared to 15 MeV energy. Other sources of scattering are the presence of shield in the super clave and postaxilla treatment fields.
As seen in the results, with increasing the photon energy, the medical physicist has the reasonable dose distribution and dose volume histogram (DVH) with omitting the wedge in phantom plans. On the other hand, using the high energy photon is imposed neutron contamination to the plans. Under this condition, if the medical physicists cover target with an acceptable dose distribution by 6 MeV photon energy and wedge, the use of subfield instead of wedge can be a good suggestion. This suggestion can help to reduce the photons scattering in 6 MeV plans without having neutron contamination.
The finding in this study is comparable with the results in Berris et al. (2012) [6] study in some organs. Their measurement was in a single energy (6 MeV energy) and single technique. The authors measured absorbed dose in a Rando phantom’s organs which are treated for breast cancer with tangential fields (two size) and reported the mean dose in organs. Their outcomes approved results of study in common organs and condition.
Conclusion
Selection of energies and techniques for radiotherapy treatment of breast cancer have a big effect on scatter absorbed dose (which due to the wedge, shields and MLC beside the other common scattering sources) in out of field organs. However, we should consider that this selection is dependent on different factors like the volume of breast, the number of lymph nodes which involved to cancer and the depth of chest wall that need to be treated by the medical oncologist. Therefore, for election of treatment, all those factors and source of scatters should be evaluated.
Acknowledgement
The authors would like to thank the radiotherapy department of Imam Reza hospital (Mashhad, Iran) for providing experimental measurement conditions. The results of this study are based on the data derived from a PhD thesis by Marziyeh Behmadi (grant n. 931169), which was funded by Mashhad University of Medical Sciences (MUMS).
Conflict of Interest:None
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