Since the 21st century, the incidence and mortality rates of female breast cancer have shown an overall increasing trend. In 2020, the rate of female breast cancer reached the top of the global cancer incidence spectrum and global female cancer death spectrum.1 In China, a total of 420,000 new breast cancer cases have been reported in 2020.2 Breast cancer is treated by a combination of different treatment strategies, including surgery, radiation therapy, chemotherapy, and endocrine therapy. The 10-year local recurrence rate in axillary node-positive patients is 46%, which can be decreased to 13% with postoperative radiotherapy.3 The intensity-modulated radiation therapy (IMRT) and cone-beam computed tomography (CBCT) have shown significant improvements with the advancements in radiotherapy technology. IMRT can achieve arbitrary dose distribution for breast cancer and improve dose uniformity of the irradiated area.4,5 Using imaging, the CBCT technology can guide radiation to accurately irradiate within the target area of the breast tissues.6
In breast cancer radiotherapy, the lungs and heart are the main organs at risk (OARs). Reducing the dose to these organs can reduce the radiation-induced long-term cardiovascular and lung damage.7,8 Accurate setup is one of the methods to reduce the exposure dose to OARs under the premise of advanced radiotherapy technology and Radiation Therapy Oncology Group (RTOG) guidelines. The accuracy of setup can be improved by selecting the professional level of therapists along with the optimization of position selection and position fixation mode during radiotherapy. The role of the therapists becomes increasingly important in this dynamic environment.9 Zhou
This study retrospectively analyzed the setup error of each target volume in breast cancer radiotherapy fixed by Styrofoam glue using CBCT and compared it with the fixation device with arm support (Abbreviation: breast bracket). The setup repeatability and PTV margin of the two fixation devices were also compared. The current study also provided a reference basis for the fixation device with good repeatability for breast cancer radiotherapy target areas, including the supraclavicular lymphatic drainage area.
This was a single-institution retrospective study, which was approved by the institutional ethics committee (KY0202002-F-1). The study included the patients, receiving IMRT radiotherapy after breast cancer surgery between March 2021 and August 2022. The inclusion criteria included (1) patients with breast cancer confirmed by pathology, (2) patients with postoperative radiotherapy target, including supraclavicular lymphatic drainage area, (3) patients with good abduction and upper limb lifting function of the affected side, and (4) patients with KPS (Karnofsky) score greater than 80. The exclusion criteria were as follows: (1) the patients receiving chest wall radiotherapy only, (2) the patients with difficulty in upper arm support and unable to meet fixation, and (3) the patients who were unwilling or unable to complete the whole study process.
According to the fixation mode (due to the introduction of Styrofoam postural fixation of breast cancer in routine practice in 2022, the previous fixation was terminated), the selected patients were divided into two groups, including the breast bracket and Styrofoam glue groups. The production process of Styrofoam is shown in Figure 1. In both groups, the mandible of the participants was raised as much as possible, their heads were inclined to the healthy side by 15 degrees, and their arms were raised naturally. The 16-slice large-aperture spiral Computer Tomography (CT) was used to simulate localization in the two groups. The scanning range was 5 cm from submandibular to subdiaphragmatic with a slice thickness and interval of 5 mm. The scanned images were sent to the doctor’s workstation system, and the radiotherapist delineated the target area in the CT positioning image combined with other image data based on the RTOG standard, limited the surrounding organs at risk, and formulated a target dose. The CTV was enlarged to 5 mm to form a PTV. The sketched images were sent to the radiotherapy treatment planning system (TPS), and the radiotherapy plan was prepared by the radiotherapy physicist. The radiotherapy plan was transferred to the accelerator after verification.
Making process of Styrofoam.
The CBCT images were obtained using the on-board image guidance system (OBI) of the Clinac IX linear accelerator purchased from Varian Medical Systems, California, US. The regions of interest were placed in the chest wall and s supraclavicular target area, respectively, as shown in Figure 2. The CBCT image was compared with the positioning CT planning image. Then, the setup errors of the X-axis (left and right), Y-axis (head and foot), Z-axis (front and rear), and the foot of table (RTN) were calculated. Moreover, the error data was recorded after manual adjustment according to the target area position.
ROI Matching.
CROI = chest wall target; SROI = supraclavicular target
According to International Commission on Radiation Units and Measurements (ICRU) reports 50 and 62, there should be a certain distance outside the CTV to reduce the setup error and effects of patient and tissue motion on the target volume. In our institution, Clinicians will follow unified standard for delineation and will delineate an ITV to address the effects of organ motion. For the setup error, a uniform 5 mm external margin is adopted. In this study, we mainly study the CTV (Including ITV)-PTV margin caused by setup error. The marginal calculation formula of Van Herk
The setup time for each fraction of the two fixation techniques was recorded and counted. The time required for the setup was defined as the time when the patient sat on the treatment couch until the therapist walked out of the treatment room after the setup. Each patient was investigated weekly. Subjective comfort A questionnaire survey was conducted for the first treatment to understand patients’ subjective comfort satisfaction with the device. The satisfaction survey comprised eight items, each with a 5-point Likert scale. The dimensions of evaluation included head, neck, and back comfort, mask fit, tightness, temperature, color, anxiety about the fixture, general discomfort, and recommendation of the fixture.
The data were analyzed using SPSS25.0 statistical software. The data counts were expressed as frequencies (n) and percentages (%). The measurement data were expressed as mean ± SD. All the setup errors were taken as absolute values, and the data results were expressed as mean ± standard deviation (x ± s). Between the two groups, the differences in data counts were compared using the χ2 test, while all the other comparisons were performed using t-tests. The correlation of errors in each direction was analyzed using the Pearson correlation analysis. A P-value of < 0.05 was considered statistically significant.
Among 78 patients with breast cancer who received IMRT, 65 patients met the inclusion criteria. Among these 65 patients, 36 patients received Styrofoam fixation, and 29 patients received bracket fixation. A total of 281 CBCT verifications, including 147 Styrofoam and 134 breast bracket verifications, were performed. The clinical data of the two groups were analyzed, and the results are listed in Table 1. No significant differences between the indices were observed (P > 0.05).
Comparison of clinical data between the two groups
48.34 ± 9.58 | 48.90 ± 10.92 | 0.837 | |
0.565 | |||
|
20 (55.6) | 11 (37.9) | |
|
16 (44.4) | 18 (62.1) | |
0.053 | |||
|
7 (16.7) | 9 (31) | |
|
29 (83.3) | 20 (69) | |
0.146 | |||
|
7 (19.4) | 4 (13.8) | |
|
26 (72.2) | 20 (68.9) | |
|
3 (8.4) | 5 (17.3) |
BCS = Breast Conserving Surgery; RM = Radical Mastectomy
The setup error of the Styrofoam glue in the chest wall target area in the left-right direction was less than the breast bracket (1.59 ± 1.47 mm
Comparison of setup errors of the two fixation methods in the chest wall target area and supraclavicular target area (mm,`x ± s)
1.59± 1.47 | 2.05 ± 1.64 | 2.516 | 0.012 | |
|
1.99 ± 1.46 | 2.10 ± 1.59 | 0.611 | 0.541 |
|
1.78 ± 1.47 | 2.00 ± 1.58 | 1.235 | 0.218 |
1.23 ± 0.88 | 1.32 ± 1.16 | 0.620 | 0.536 | |
|
1.23 ± 1.21 | 1.16 ± 1.17 | −0.445 | 0.657 |
|
1.36 ± 1.27 | 1.75 ± 1.55 | 2.003 | 0.046 |
0.48 ± 0.46 | 0.53 ± 0.43 | 1.033 | 0.302 | |
0.47 ± 0.47 | 0.66 ± 0.59 | 2.760 | 0.006 |
CROI = Chest Wall Target; SROI = supraclavicular target
In the breast bracket group, the PTV margins of the chest wall in the X, Y, and Z directions were 6.10 mm, 6.34 mm, and 6.10 mm, respectively, while those of the supraclavicular target area were 3.99 mm, 3.72 mm, and 5.45 mm, respectively. In the Styrofoam group, the PTV margins of the chest wall in the X, Y, and Z directions were 5.01 mm, 5.99 mm, and 5.47 mm, respectively, while those of the supraclavicular target area were 3.69 mm, 3.86 mm, and 4.28 mm, respectively (Table 3). In the chest wall target area, the margin of the Styrofoam group was 17.87% smaller than that of the bracket group in the X direction. In the supraclavicular target area, the margin of the Styrofoam group was 21.47% less than that of the bracket group in the Z direction.
Target volume expansion boundary in the three-dimensional direction in 65 patients in the breast bracket and Styrofoam groups (mm)
CROI | 1.59 | 1.99 | 1.78 | 2.05 | 2.10 | 2.00 | |
SROI | 1.23 | 1.23 | 1.36 | 1.32 | 1.16 | 1.75 | |
CROI | 1.47 | 1.46 | 1.46 | 1.39 | 1.57 | 1.57 | |
SROI | 0.88 | 1.12 | 1.26 | 0.98 | 1.17 | 1.54 | |
CROI | 5.01 | 5.99 | 5.47 | 6.10 | 6.34 | 6.10 | |
SROI | 3.69 | 3.86 | 4.28 | 3.99 | 3.72 | 5.45 |
Note: ∑ was the standard deviation of the systematic error of each patient, and the systematic error of each patient in each direction was the average value of the errors in each direction among all fractions; σ was the mean square value of the random error of each patient, and the random error of each patient in each direction was the standard deviation of the error in each direction among all fractions; and MPTV was the size of the outspread boundary from the clinical target volume to the planned target volume.
The displacement distributions for < 3 mm, 3~5 mm, and > 5mm in the Styrofoam group in the X direction were 75.4%, 23.1%, and 1.5%, respectively, and those in the Y direction were 66.8%, 32.1%, and 1.1%, respectively. In the Z direction, the displacement distributions for the respective frequencies were 69.2%, 29.2%, and 1.6%. In the breast bracket group, the displacement distributions for <3 mm, 3~5 mm, and > 5mm in the X direction were 66.9%, 31.6%, and 1.5%, respectively, while those in the Y direction were 66.9%, 30.8%, and 2.3%, respectively. Moreover, in the Z direction, the displacement distributions for the respective were, 66.9%, 31.6%, and 1.5%, respectively. The probability of < 3 mm of Styrofoam in the left and right direction was significantly greater than that of the bracket group, as shown in Figure 3.
Bar chart of setup error of two fixation methods in chest wall target area.
The displacement distributions for < 3 mm, 3~5 mm, and > 5mm in the Styrofoam group in the X-direction were 89.7%, 9.1%, and 1.2%, respectively, and those in the Y-direction were 84.5%, 15.5%, and 0%, respectively. In the Z-direction, the displacement distributions of respective were 82.8%, 17.2%, and 0%. The displacement distributions for < 3 mm, 3~5 mm, and > 5mm in the breast bracket group in the X-direction were 87.3%, 10.8%, and 1.9%, respectively. In the Y-direction, the displacement distributions were 87.7%, 12.3%, and 0%, while in the Z-direction, they were 76.5%, 20.6%, and 2.9%, respectively, as shown in Figure 4.
Box plot of setup error in supraclavicular region of two fixation methods.
The subjective comfort satisfaction scores of patients in the Styrofoam and bracket groups were 27.50 ± 1.24 and 25.44 ± 1.23 points, respectively, showing a statistically significant difference (P < 0.001). The setup times of the Styrofoam and breast bracket groups were 3.4 ± 1.1 min and 5.5 ± 3.1 min, respectively (P = 0.007).
In the bracket group, the Pearson correlation analysis showed a moderate correlation between the Y-axis and Z-axis direction in the chest wall setup error (r = −0.205), while the supraclavicular target area X-axis setup error showed a weak correlation with the Y and Z-axis directions (r = 0.190 and 0.185). The Z-axis setup error in the Styrofoam group supraclavicular target area was moderately correlated with the X-axis direction and RTN (r = −0.211 and 0.235), as shown in Figure 5.
Scatter plot of setup error between two groups in different directions. The dark color in the upper figure shows the bracket group, and the light color in the lower figure shows the Styrofoam group. The r-values with P-values < 0.05 are indicated in the figure.
C = chest wall; S = supraclavicular
The current study compared the Styrofoam fixation and breast bracket fixation in patients with breast cancer, who underwent postoperative radiotherapy. The results showed that Styrofoam fixation could significantly reduce inter-fractional displacement in the X direction of the chest wall and displacement in the Z and RTN directions of the supraclavicular region. The chest PTV margin of the foam group was 17.87% (5.01 mm
The accuracy of radiotherapy directly affects the success or failure of radiotherapy.17,18 The setup error was relatively large due to the special physiological structure of breast cancer. Errors in breast cancer radiotherapy are related to factors, such as fixation devices, the patient’s position, experience of the radiotherapy therapist, and the patient’s body mass index.19 When the fixture is more comfortable, it reduces the setup error more effectively. Currently, the commonly used molds for breast cancer positioning in various radiotherapy centers include vacuum bags, thermoplastic body films, breast brackets, and Styrofoam. A vacuum bag poses a risk of air leakage and compression deformation during treatment. The thermoplastic phantom could significantly reduce inter-fraction error in IMRT for breast cancer as compared to a vacuum bag. However, it might increase the irradiated skin dose at the irradiated site, thereby exacerbating radiation skin reactions.20,21 Therefore, care should be taken while using thermoplastic masks for fixation. In the breast bracket (a conventional mechanical fixing device), the fixation of the neck and shoulder was uncertain, and it was easier to form a forced body position. Moreover, the repeatability of the clavicle area could not be guaranteed, and the degree of individualization of mold was not as high as that of Styrofoam.
Our results showed that the Styrofoam group had a smaller setup error than the bracket group in the X direction of the chest wall. Zhou C
This study found that Styrofoam fixation in the supraclavicular region had a significantly smaller setup error in the Z direction as compared to that in the bracket group. Zhang Y
The PTV margins of the IMRT target volume in most studies were 5 mm. The current study showed that the calculated PTV margins of the target volume fixed by Styrofoam in the X, Y, and Z directions of the chest wall were 5.01 mm, 5.99 mm, and 5.47 mm, respectively, while those of the supraclavicular target area were 3.69 mm, 3.86 mm, and 4.28 mm, respectively. Moreover, in the breast bracket group, the chest wall boundaries of the calculated PTV margins were 6.10 mm, 6.34 mm, and 6.10 mm in the three directions, while the supraclavicular margins were 3.99 mm, 3.72 mm, and 5.45 mm, respectively. It could be seen that for both fixation devices, a PTV margin of 5 mm was not sufficient on the chest wall, while it was sufficient for the supraclavicular target area. Yao W
The current study showed that the setup errors of the two groups of fixation devices in the Y direction of the chest target area contained more positive values; this indicated that the position of the two groups of patients moved to the foot side. In addition, the couch angle in both groups had extreme values, which might be due to a slightly greater number of patients enrolled after radical mastectomy on the right side. During the late-course treatment, the patient’s arm could not be naturally lifted to the original positioning position due to irradiation and surgery, and the body was affected by traction to shift to the affected side, resulting in coronal rotation. Therefore, the radiotherapist must educate patients with breast cancer to do functional exercises of the affected upper limb after radiotherapy. The data in the supraclavicular target area showed more negative values in the Z direction in both groups, suggesting that both groups collapsed in the neck region. A similar phenomenon was also reported by Svestad JG
The current study analyzed the setup errors in all directions using the Pearson correlations analysis, which has been rarely studied in previous studies. The correlation analysis was used to analyze whether an increase in error in one direction would change the error in the other direction. The results showed that the setup error of bracket fixation in the Y direction in the chest wall region was negatively correlated with that in the Z direction, while the setup error in the X direction in the supraclavicular target area was weakly correlated with that in the Y and Z directions; this was also consistent with clinical practice. When the X direction error became larger, the side deformed the neck in the headrest, resulting in changing the neck error in the Y and Z directions. In the Styrofoam group, the setup error in the Z direction of the supraclavicular target area was negatively correlated with that in the X direction, while it was positively correlated with the RTN. This might be due to the relatively stronger head fixation of Styrofoam (Figure 1 D), and the error in the Z direction, which caused the coronal rotation of the neck. This also showed that the bracket fixation was not as good as the Styrofoam fixation for the patient’s head.
The current study has several limitations. First, only two groups of fixation methods were discussed, and there were many factors, which affected the setup error. Secondly, van Herk’s boundary calculation was performed only for the errors of three horizontal displacements. At larger target volumes, even small rotational errors can lead to dose uncertainty.27 In clinical practice, it is difficult to correct rotational errors and local setup errors using CBCT image guidance. However, a combination of a six-dimensional treatment couch28 and an optical surface detection system (OSMS)29 can be used to solve these problems in a qualified unit. Finally, the effects of breathing on positioning were not explored in this study and are needed to be studied further in the future. The use of artificial intelligence (AI) in radiotherapy is increasing; therefore, it is expected to apply AI techniques for error correction. Mathematical models or computerized deep learning might help in reducing the setup errors of breast cancer in the future. It is also possible to quantify the physical indicators, including weight and body mass index, which will be the subject of future studies. Future studies with larger sample sizes, multifactorial setup errors, and better fixation methods are needed.
In summary, the current study retrospectively analyzed the use of Styrofoam fixation in radiotherapy for patients with lymph node metastasis after breast cancer surgery. The study suggested that the use of Styrofoam could further improve the setup accuracy and setup efficiency of the chest wall and supraclavicular target area, improve patients’ comfort and satisfaction, and decrease the PTV margin distance. This study might provide a reference for the clinical use of Styrofoam glue to fix the postoperative radiotherapy of patients with breast cancer.