[Abstract] Objective evaluation of breast modified radical postoperative CT simulation target positioning the electron beam irradiation of the chest wall and heart and lung irradiated volume and dose distribution. Methods 20 patients with chest wall irradiation indications of breast cancer patients with modified radical mastectomy, row CT simulation, three-dimensional treatment planning system CT image reconstruction of digitized outline the chest wall CTV and the heart, lungs, and other organs at risk, and the calculation of the chest wall and its heart lung dose according to volume and exposed. Chest wall prescription dose for 5 000cGy. Results of the left breast target Dmax (5 536 ± 301) cGy the Dmean to cGy (823 ± 129), D90 (4 543 ± 290) cGy the ipsilateral lung Dmean (1 724 ± 624) cGy , V20 (36 ± 13)%. The heart Dmean (1 008 ± 457) cGy, V30 (13 ± 9)%. The right side of the breast cancer target Dmax (5 554 ± 253) cGy the Dmean to cGy (783 ± 89), D90 (496 ± 101) cGy ipsilateral lung Dmean of (1 416 ± 567) cGy V20 (30 ± 12)%. Conclusion CT simulation to develop chest wall radiotherapy planning of electron beam irradiation, can be more accurate understanding of the target and normal tissue dose distribution, and thus better able to optimize radiation treatment planning.

Key words breast cancer; radical postoperative radiotherapy; electronic line; chest wall

Dosimetry of Electron  beam Radiotherapy with Use of Computed Tomography (CT)  Based on Chest Wall for Postmastectomy Patients

    ZENG Zi  jun, LI Wei  xiong

    Department of Radiotherapy, Cancer Center, Guangdong Provincial People’s Hospital, Guangzhou 510080, ChinaAbstract: Objective To evaluate the dosimetry of electron beam with the use of computed tomography (CT)  based radiotherapy on chest wall for postmastectomy patients. Methods CT simulation was conducted for 20 breast cancer patients having received electron beam chest wall irradiation after postmastectomy, and digitizing reconstitution of the image pictures from spiral CT equipment and outlining the clinical target volume (CTV) and organs at risks (OARs) such as heart and lung, calculating their volume and exposure dose, were done by the 3D treatment planning system (TPS). The prescribed dose for chest wall was 5 000 cGy. Results The maximum dose (Dmax), mean dose (D mean) and dose of 90% CTV (D90) for the left breast cancer were (5 542 ± 279) cGy, (4 809 ± 116) cGy and (4 526 ± 239) cGy respectively. The Dmean and percentage of volume receiving more than 20Gy (V20) and 10Gy (V10) of left lung were (1 616 ± 608) cGy, (33 ± 13)% and (44 ± 16)% respectively. The mean dose (Dmean) and volume receiving more than 30Gy (V30) of heart were (1 008 ± 457) cGy and (13 ± 9)%. The Dmax, Dmean and D90 for the right breast cancer were (5 554 ± 253) cGy, (4 783 ± 89) cGy and (4 496 ± 101) cGy respectively. the Dmean, V20 of right lung were (1 416 ± 567) cGy, (30 ± 12)% respectively. Conclusion By the use of computed tomography (CT)  based to perform the plan of electron  beam radiotherapy on chest wall, we can understand precisely the dosimetry of the target volume and normal structures, and perfect the radiation plan better.

    Key words: Breast cancer; Postmastectomy radiotherapy; Electron beam; Chest wall

 Postoperative radiation therapy is the main treatment in the comprehensive treatment of breast cancer, postoperative radiation therapy can reduce the number of high-risk breast cancer improved local recurrence rate in patients undergoing radical operation and survival rates. Modified radical mastectomy is still the main method of surgical treatment of breast cancer, according to incomplete statistics, accounting for about 95% of all surgical, modified radical reasonable application of postoperative radiotherapy is particularly important in our country. The chest wall is a modified radical mastectomy radiotherapy must be irradiated site, currently many units commonly used electron irradiation radiotherapy technology, our department has been using 6MeV electronic line half plus compensation film the vertical chest wall radiotherapy. With the advent of CT simulation, this study is to understand the target area of ​​the chest wall electron beam irradiation, cardiopulmonary according to volume and dose distribution in order to evaluate the reasonableness of this technology through three-dimensional treatment planning system. The results are reported below.

    1 Materials and Methods

    1.1 The clinical data of August 2006 to May 2007 were treated 20 cases of breast cancer after modified radical mastectomy patients, 13 cases left, the right side of the seven cases, the median age of 49 years (33 to 65 years old). Irradiated chest wall indications: primary tumor T3 above and (or) metastatic axillary lymph nodes ≥ 4, pathologic results are invasive ductal carcinoma. All patients completed after 6 to 8 cycles of adjuvant chemotherapy.

    1.2 positioning method the patient supine on the A mammary bracket, according to the patient’s body to determine the headrest position, the breast frame surface inclination, the fixed position of the ipsilateral upper arm and buttocks crosspiece position to guarantee the level of patients with chest wall, and upper arm full elevation. At the same time in the patient’s body to determine the laser positioning point. Application GE spiral CT the calm breathing state scan, slice thickness 2.5mm, scan range including bilateral whole lung and contralateral breast, and the treatment planning system (CMS) spread through hospital LAN.

    1.3 target delineation target CTV as the chest wall, not including the internal mammary area, range: upper bound for the level of the lower edge of the clavicular head, the lower bound for the contralateral breast under ~~ 2cm within the sector for thoracic vascular outside of the outside back wide muscle leading edge, the former profession as subcutaneous 0.5cm, after the sector as the pleura and lung junction. Vital organs: the ipsilateral lung, heart (left breast).

    1.4 treatment plan

    (1) Select the left and right side of the breast cancer patients each one according to previous radiotherapy method, namely 6MeV electronic line half plus compensation film, rack angle of 0 ° design of radiation treatment planning, and then, under the same conditions, increase energy design radiotherapy school program, until this study dose requirements: ie 90% isodose including the 90% of CTV.

    (2) the remaining patients were to design a suitable electron beam irradiation treatment plan, also meet the 90% isodose curve, including 90% of the CTV requirements, electronic wire and organizations choose different energy compensation film, in order to optimize the treatment plan. Chest wall the CTV irradiation prescription dose of 50Gy.

    1.5 OUTCOME MEASURES through three-dimensional treatment planning system, CTV DVH To evaluate the treatment plan, Dmax Dmean, D90 and V105%, V110%, the average of the ipsilateral lung dose, V20, and the heart of the average dose, V30.

    1.6 Statistical Methods SPSS11. 0 software processing.

    2 Results

    2.1 vertical wild energy electron line compared with oblique field dose

    Randomly selected one to the left side of the breast cancer patients develop radiotherapy by conventional 6MeV vertical irradiation plan. Dose distribution and DVH figure shows dose distribution is poor, only 90% isodose including chest wall, a lower dose of the outside of the chest wall, axillary line dose. So try to increase energy, the until the 15MeV to meet dosimetry requirements, the target the Dmax, Dmean were 5 331cGy, 4 762cGy, the ipsilateral lung Dmean 988cGy. The average dose of heart 1 393cGy. The specific dose, as shown in Figure 1. The electronic line rotation angle of about 33 ° 12MeV 90% isodose curve can basically cover the target area, target target Dmax, Dmean were 282cGy In 4 744cGy, the ipsilateral lung Dmean 405cGy. Heart Dmean 940cGy specific dose distribution, as shown in Figure 2. DVH comparison of the two plans, as shown in Figure 3. Also select one to the right side of the breast cancer patients, the same design the electronic line perpendicular wild with oblique Wild radiotherapy planning, two schemes target Dmax, Dmean for 6 067cGy, the 4 710cGy and 456cGy, 4 598cGy, ipsilateral lung Dmean respectively 054cGy 374cGy and 2, the DVH comparison, as shown in Figure 4. Be seen from the above, whether left or right breast electronic line oblique the wild chest wall irradiation, the target dose distribution than the vertical field irradiation, the maximum dose reduction targets, reduce doses to the lungs, heart.

    2.2 oblique field irradiation target and normal tissue dose distribution

    All plan to meet the dose requirements rack angle rotation angle of approximately (± 30 ° to 35 °), and select a different energy electron line 9MeV 12 cases, 12MeV 8 cases, and half plus organizations compensation film thickness 0.5cm 11 cases, the thickness of 1cm 9 cases. The left breast target Dmax (5 536 ± 301) cGy the Dmean to cGy (823 ± 129), D90 (4 543 ± 290) cGy ipsilateral lung Dmean of (1 724 ± 624) cGy V20 (36 ± 13)%. The heart Dmean (1 008 ± 457) cGy, V30 (13 ± 9)%. The right side of the breast cancer target Dmax (5 554 ± 253) cGy the Dmean to cGy (783 ± 89), D90 (496 ± 101) cGy ipsilateral lung Dmean of (1 416 ± 567) cGy V20 (30 ± 12)%.

    3 Discussion

    Chest wall breast cancer after radiotherapy Standing parts.

    No uniform chest wall irradiation technology, tangential field irradiation with 60Co and high-energy X-ray According to domestic questionnaires, most (45.2%), simple electron beam irradiation (28.3%) also occupy a certain proportion of [1]. Gez et al [2] using electron beam irradiation of chest wall, with tangential field X-ray contrast, the 14-year follow-up found that the local control rate and overall survival between the two are the same.

    Spierer et al [3] reported that the electronic line can cover the target area, reduce the volume of normal tissue by high-dose irradiation, acute and late radiation injury can accept higher local control rate and survival rate. Therefore, we hope that the use of CT simulation of three-dimensional treatment planning system analysis of the chest wall electron irradiation dose distribution. The results of this study found that the vertical irradiation of 6MeV electronic line, lower chest wall outside of dose, can only increase ray energy 15MeV to meet dosimetry requirements, D90 reaches the midline, ipsilateral lung and heart irradiated volume and dose larger. In order to better cover the entire target area, the rack angle must be rotated a certain angle (± 30 ° to 35 °) to make the target to achieve a better dose distribution and reduce the irradiated volume. From the dose distribution and DVH Figure visible, whether left or right breast electronic line oblique the wild chest wall irradiation, the target dose distribution than vertical field irradiation, the target maximum dose reduction, lung, heart irradiated dose reduction. At the same time, we found that the chest wall thickness of individual differences, the range of 0.8 ~ 3cm, the median thickness of 1.4cm, depending on thickness must select the appropriate energy electron line, generally 9 ~ 12MeV, to meet the requirements of dosimetry. So previous technology there is a certain lack of 6MeV electronic line perpendicular to the chest wall irradiation for all patients.

    Irradiated chest wall electronic line with other reported the dosimetry contrast, Spierer [3] reported ipsilateral lung V20 and V30 median heart were 38.4%, 6.8%, this study were 34.9%, 10.38%. The former uses a hybrid electronic wire and at the same time plus compensation films of different thickness, the heart reduced radiation dose. Pierce et al [4] reported the average of the ipsilateral lung V20 and heart V30 (28.75 ± 9.94)% and (5.15 ± 4.7)% of this study were (33 ± 13)% (13 ± 9)% . The former want to irradiation of the internal mammary area, chest wall medial part includes internal mammary radiation field, leading to reduce the volume of lung and heart contained in the chest wall wild. Therefore, the electronic line technology used in this study also need to constantly improve, to further reduce the radiation damage of the lung and heart. The chest wall irradiation Another method is the use of the tangential field of the X-ray. According to Wang et al [5] underwent CT simulation chest wall tangential field dose distribution results reported Dmax, Dmean, V105% and V110%, respectively (5,564 ± 268) cGy (831 ± 352) cGy (18.7 ± 15.4)% and (5.3 ± 7.5)%. The ipsilateral lung Dmean, V20 and heart Dmean of of the left breast, respectively (839 ± 202) cGy, (17 ± 7)% and (338 ± 165) cGy. Right side the breast ipsilateral lung Dmean, V20 (858 ± 191) cGy (16 ± 5)%, this study Dmax Dmean V105% and V110% are smaller than the former, may claim 95% with the former, etc. The dose curves including 95% of the CTV relevant are larger than the former, but the average of the ipsilateral lung dose, V20 and the average dose of the heart. The same Pierce et compare these two radiotherapy plans, results showed that the average dose of electron irradiation than the X-ray tangential field irradiation, but the ipsilateral lung V20 and heart V30 tangential field irradiation significantly higher than the X-ray. The chest wall electron beam irradiation, and the tangential field compared to the X-ray irradiation, lung, heart dose exposure may increase.

    In summary, breast modified radical postoperative chest wall electronic line radiotherapy, preferably through CT simulation, select the appropriate energy electron line in accordance with the different thickness of the chest wall, and select the film of a certain thickness of the tissue compensation in order to meet the requirements of dosimetry.