Assessment of radiation dose to the chest wall and lung of the patients with breast cancer under electron beam therapy MB. Tavakoli 1, M. Saeb 2, H. Emami 3 1 MB Tavakoli, Department of Medical Physics and Medical Engineering, School of Medicine, Isfahan University of Medical Sciences, Isfahan, Iran. E-mail:mbtavakoli@mui.ac.ir 2 M. Saeb, Department of Medical Physics and Medical Engineering, School of Medicine, Isfahan University of Medical Sciences, Isfahan, Iran 3 H. Emami, Department of Radiation Oncology, School of Medicine, Isfahan University of Medical Sciences, Isfahan, Iran 0
Abstract Radiotherapy treatment for patients who have undergone breast saving operation is basically tangential radiation using cobalt or other high-energy photons. In case of patients who experience modified radical mastectomy (MRM), electron beam therapy is the method of choice. Rapid fall off of the percentage depth dose (DD%), homogeneous dose distribution and limited dose to the sub tumoral tissues are the outstanding features of electron therapy. In the case of MRM, chest wall and auxiliary regions should be treated with electron beams. The treatment field includes chest wall, auxiliary, sub and supra clavicular lymph nodes as well as internal mammary nodes. The aim of this work was to study the depth dose of 10 and 13MeV electron beams in the chest wall, lymph nodes and lung when the chest wall was radiated anteriorly. In this study two high-energy linear accelerators (Neptone 10 and Saturn 20) were used as the sources of electron beams. Measurements of the DD% were performed in a Perspex phantom using TLD (TLD-100) dosimetry method. The phantom was designed based on the chest wall CT images of mastectomized patients. The measured DD% to the internal mammary nodes, auxiliary nodes and chest wall were 97.5%, 96% and 98%, respectively when the electron energy was 13 MeV and they were 90.5%, 77% and 99.7% when 10 MeV electrons beams were used. The anterior lung paranchimal DD% was 78% with 13 MeV electron while the posterior part of the lung received 47% of the maximum dose. With 10 MeV electron beams, the values for anterior and posterior parts of the lung were 83% and 45%, respectively. Using the 13 MeV electron beams, internal mammary and auxiliary lymph nodes as well as chest wall received adequate doses, while lungs were excessively exposed to radiation. With 10 MeV electron radiation therapy, internal mammary nodes and chest wall were well exposed to the radiation, yet auxiliary nodes did not receive enough radiation, hence auxiliary lymph nodes should additionally receive radiation from posterior field as well. Keywords: Electron Beam Therapy; Percentage Depth Dose, Chest Wall, Radiation Dose 1
1. Introduction Breast cancer is one of the most common cancers in women. The prevalence age of this cancer is usually between 45 to 75. It has also more incident rate in Jewish and black people. It is less common in Asian and African women. Its treatment of choice is usually surgery or combination of surgery and radiotherapy, chemotherapy and hormone therapy. Radiotherapy is a modality used after surgery to remove probable remained unremoved cancerous cells and also lymph nodes around the excision area such as auxiliary, sternal and sub clavicular nodes. The best radiation therapy modality is assumed to be electron irradiation of the chest wall and also the lymph nodes. Compaired with photon therapy the main advantage of electron therapy is the fast fall off of the depth dose of the electron beam, which results in saving normal underlying tissues especially in lungs. To treat the chest wall and the lymph nodes while saving the lung, it is necessary to have a depth dose of 80% to 90% at the border of the chest wall and the lungs. The choice of electron energy depends on the chest wall thickness and also depth of the lymph nodes. The electron energies of 10 to 13MeV is usually used. The chest wall is usually treated using a single field of electron and in order to compensate the dose to the auxiliary anodes, extra posterior cobalt field is used. The purpose of this work was to compare the measured dose to the target volume and normal tissue with the prescribed dose. 2. Materials and Methods Electron sources used in this research were a Saturn 20 and Nepton 10 Linear accelerators. A phantom of chest wall (without breast) was constructed using Perspex blocks (Perspex is usually easily accessible, hard enough to perform the task, can be cut in different necessary thicknesses and also it is nearly equivalent to the soft tissue). The material used for lung phantom was cork. It is also a material equivalent to lung tissue both in density and atomic number, and hence electron density. The material used for bone phantom was Teflon, which has the properties of bone materials. To obtain the necessary sizes for phantom construction, preliminary measurements of 10 patients referred to the radiotherapy department of Said Al-Shohada hospital were performed. Contour sizes of the patients were obtained for a mean patient size with 95% confidence limits. To measure the percentage depth dose, LiF (Tl) TLD dosimeter was used. In the phantom suitable holes were made at critical locations of the chest wall such as sternal nodes, auxiliary nodes, the border of chest wall and lung and also at different depth in the lung for TLD positioning. To measure the dose at different locations, 47 TLD chips were used. The TLDs were first prepared and calibrated according to the manufacture recommendations. The treatment field was drawn on the phantom chest wall as a rectangle with 26x25cm dimensions by radiotherapist. The irradiation technique was SSD and the applied dose was 200cGy. After positioning the TLDs at the predetermind locations of the phantom and positioning the phantom on the treatment coach of the linacs and adjusting the radiation field, a dose of 200 cgy was applied to the phantom. The above procedure was repeated 6 times for both 10 and 13 MeV electron beams of the two linacs. After each irradiation of the phantom, TLDs were removed and the recorded doses were read using the Solaro 2A TLD reader. 2
3. Results 1-The measured percentage depth doses at different locations with 13MeV electron beam are shown in table 1 to 3. The percentage depth dose at different depths in lung is also shown in graph 1 (SSD=100cm, Field size=26x15cm and chest wall thickness was 2.5cm). 2-The measured percentage depth doses at different locations with 10 MeV electron beam are shown in tables 4 to 6. The percentage depth dose at different depths in lung is also shown in graph 2 (SSD=100cm, field size-26x15 and chest wall thickness=2.5cm). 4. Discussion The aim of this work was to compare the prescribed dose with the measured dose in critical points of the chest wall and the surrounding organs. When the 13MeV electron beam from Saturn 20 linear accelerator was used, the results were as follows: Table 1 shows that the dose at the internal mammary lymph nodes were 195cGy while the prescribed dose was 200 cgy. The difference is 2.5%, which is acceptable (the acceptable error is 5%). Table 2 shows that the measured dose in auxiliary lymph nodes was 192 cgy, which is 4% less than the prescribed dose. In this case, the nodes received the acceptable dose. According to table 3, the measured dose at the chest wall was 186cGy compared with the prescribed dose (190 cgy), the difference is 2.1%, which is acceptable. In graph 1, percentage depth doses at different depths using 13MeV electron are shown. It shows that the dose in the lung was relatively high. The dose in the superior level of the lung was 158 cgy, which is 79% of the prescribed dose. This can cause problems for the safety of the lung. It is also be seen that the fall off of the dose to the lung is much more gradual than water or soft tissue, which means entire lung, received a high dose. When 10 MeV electrons from Nepton Linear accelerator was used the results were as follows: Table table 4 shows that, although a dose of 200cGy was prescribed for 3 cm depth but measured dose at this depth was 181cGy, which is 9.5% less than the necessary dose. This is higher than 5% (permitted errors), and hence should be compensated. Table 5 shows that measured the dose at the auxiliary lymph nodes (129cGy) was mush less than the prescribed dose (167cGy), with a difference of about 29%. The necessary treatment dose should be applied to the lymph nodes using an extra boost field. In some centers this is done using a posterior photon field. According to table 6, the difference between measured dose and prescribed dose in the chest wall was 0.2% (209cGy and 208.5cGy), which is in the acceptable error range, and hence the chest wall at the depth of 3 cm receives an acceptable treatment dose. Graph 2 shows the percentage depth dose at different depths of the lung when 10MeV electron was used. The absorbed dose at the border of the lung and chest wall was 162cGy, which was 81% of the maximum dose and decreases to 45cGy at the posterior border of the lung, which is 23% of the tumor dose. Therefore the dose to the lung is sufficiently low to be harmful. From these results it is suggested that when 10MeV electron is used for the treatment of the chest wall, the dose to the sternal nodes and auxiliary lymph nodes should be compensated. 3
5. References 1-Carlos A, Perez: Principles and Practice of Radiation Oncology. 3 rd Edition, Williams and Wilkins 1999 2-Vincent T, Devita ET, et al: Practice of Oncology. 5 th Edition.1633-1726 (2001). 3-Feletecher GH. Introduction. Tapley N, Ed. Clinical Aplications of the Electron Beam. New York, Wiley, 1976 4-Faiz M Kan, The Physics of Radiation Therapy, 2 nd Ed. Electron Beam Therapy, 1994. 5-IAEA. Absorbed Dose determination in External Beam Radiotherapy: An International Code of Practice for Dosimetry based on Standards dose to Water. 83-85, (1999) 6-Choi MC, Purdy BJ, Abrat FG, Glasgow GP. Variation in output factor Caused by Secondary for 7-16 MeV Electron Beams. Med. Phys. 37(6).(1979). 4
Table 1: Measured doses in the phantom at the sternal nodes position after application of 200 cgy radiation dose from electron beam with 13MeV energy. 1 200 2 190 3 195 4 192 5 197 6 195 Mean 195 Relative to prescribed 97.5% SD 3.54% Prescribed dose 200 Deference between prescribed and Measured dose 2.5% Table 2: Measured doses in the phantom at the auxiliary nodes position after application of 200 cgy radiation dose from electron beam with 13MeV energy. 1 185 2 195 3 190 4 193 5 189 6 197 Mean 192 Relative to prescribed 96% SD 4.4% Prescribed dose 200 Deference between prescribed and Measured dose -4% Table 3: Measured doses in the phantom at the chest wall after application of 200 cgy radiation dose from electron beam with 13MeV energy. 1 182 2 180 3 191 4 191 5 187 6 185 Mean 186 Relative to prescribed 98% SD 4.56% Prescribed dose 190 Deference between prescribed and Measured dose -2.1% 5
Table 4: Measured doses in the phantom at the sternal nodes position after application of 200 cgy radiation dose from electron beam with 10MeV energy. 1 177 2 177 3 188 4 187 5 180 6 179 Mean 181 Relative to prescribed 90.5% SD 4.93% Prescribed dose 200 Deference between prescribed and Measured dose -9.5% Table 5: Measured doses in the phantom at the auxiliary nodes position after application of 200 cgy radiation dose from electron beam with 10MeV energy. 1 120 2 130 3 135 4 129 5 127 6 131 Mean 129 Relative to prescribed 77% SD 5% Prescribed dose 167 Deference between prescribed and Measured dose -29% Table 6: Measured doses in the phantom at the chest wall after application of 200 cgy radiation dose from electron beam with 10MeV energy. 1 205 2 212 3 206 4 210 5 208 6 210 Mean 208.5 Relative to prescribed 99.7% SD 2.66% Prescribed dose 209 Deference between prescribed and Measured dose -0.2% 6
100 90 80 70 %DD 60 50 40 30 20 10 0 0 0.7 1.4 2.1 2.8 3.5 4.2 4.9 5.6 6.3 7 7.7 8.4 9.1 9.8 10.511.211.912.613.3 14 14.7 Depth from skin (cm) Figure 1: Percentage depth dose in the lungs of the phantom at different depths when irradiation dose at the skin was 200cGy using 13MeV electron beams. 120 100 80 60 40 20 0 سوپ حطس زا هير قمع) cm ( ش يريگ هزادنا زد دصرد )cgy( %DD 0 0.7 1.4 2.1 2.8 3.5 4.2 4.9 5.6 6.3 7 7.7 8.4 9.1 9.8 10.5 11.2 11.9 12.6 13.3 14 14.7 Depth from skin (cm) Figure 2: Percentage depth dose in the lungs of the phantom at different depths when irradiation dose at the skin was 200cGy using 10MeV electron beam. 7