Influence of minimum segment width on intensity-modulated radiotherapy plan for left-sided breast cancer after breast conserving surgery

Abstract

To research the influence of the minimum segment width (MSW) on intensity-modulated radiotherapy (IMRT) plan for left breast cancer after breast conserving surgery and provide a reference for plan optimization. Ten patients with left-sided early-stage breast cancer after breast-conserving surgery (BCS) were selected for postoperative radiotherapy. The Monaco 5.11 treatment planning system (TPS) was used, and the optimization parameters of the plan were fixed, while MSW were set to 0.5, 1, 1.5, and 2 cm, and four 5 field-IMRT plans were designed for every patient. The plan quality and execution efficiency of each plan were compared, including planning target volume (PTV) dose, organs at risk (OAR) dose, segments number (SN), monitor units (MU), delivery time (DT), and dose verification results. With the increase of the MSW, the dose indexes of the OAR were similar (P > .05). The D_2%, D_98%, and D_50% of the PTV gradually deviated from the prescription dose, and the uniformity gradually deteriorated (P < .05). The gamma passing rates (GPR) of dose verification and the dose deviation of the PTV were similar (P > .05). The SN, MU, and DT gradually decreased (P < .05), but the amplitude change caused by each increase of 0.5 cm gradually decreased, the decrease of 0.5 to 1 cm was the largest, reaching 16.41%, 21.25%, and 0.35 minutes, respectively, while the 3 values of 1.5 and 2 cm were close. When using Monaco 5.11 TPS to design IMRT plan for left breast cancer after BCS, 1.5 cm MSW could ensure better dose coverage and uniformity of the PTV, as little as possible exposure dose to the OAR, and high dose verification accuracy. The SN, MU, and DT were significantly reduced, and the treatment efficiency was improved.

1. Introduction

Intensity modulated radiotherapy (IMRT) was a highly conformal and complex irradiation technique with good dosimetric advantages. It could improve the dose and conformity of the planning target volume (PTV) by using multiple segments to step by step irradiation, and reduce the dose of the organs at risk (OAR) around the PTV. It had been widely used in radiotherapy of breast cancer after breast conserving surgery (BCS).^[1–4] In Monaco IMRT and volumetric modulated arc therapy (VMAT), the sequence is based on an alternative sliding window pattern that moves all the leaves from the start to the end position in a continuous manner. The minimum segment width (MSW) is a parameter of the Monaco treatment planning system(TPS) used to control the minimum width of the segments, such as the minimum separation of a leaf pair. Therefore, The MSW parameter plays an important role in limiting the formation of narrow segments, thus avoiding the effects of incorrect MLC calibration and leaf positioning accuracy. The MSW directly affects segments shape and segments number (SN), as well as treatment efficiency and accuracy.^[5,6]While the PTV of breast cancer was shown long and narrow in the beam’s eye view of the tangential field, so the segment shape of the IMRT plan was usually long and narrow.^[6] Studies have reported that by modifying parameters such as minimum segment area, minimum segment monitor units, minimum segments number, and MSW. It was possible to reduce monitor units (MU), SN and shorten delivery time (DT), without significantly impacting the dose of the PTV and OAR.^[7–11]This study intends to design IMRT plans of different MSW using Elekta Monaco TPS for left-side breast cancer after BCS, and to study its effect on the dose distribution and execution efficiency of the plan, so as to provide reference for plan design.

2. Materials and methods

2.1. Patient selection

Ten patients with left-sided early-stage breast cancer after BCS were selected for postoperative radiotherapy at our hospital in 2021 from January to December. All patients were pathologically confirmed. Clinical staging included 6 cases of pT1N0M0 stage and 4 cases of pT2N0M0 stage. Invasive ductal carcinoma was observed in 7 cases, while ductal carcinoma in situ was observed in 3 cases, all of which were solitary. The age range of the patients was 30 to 57 years, with a median age of 44 years.

2.2. Position fixation and CT simulation

The patients were positioned supine with hands crossed behind the head and immobilized using a vacuum bag. A Philips Brilliance large-aperture CT scanner was used to perform a CT simulation with the patient in a calm breathing state. The scanning range was from the inferior border of the mandible to 5 cm below the breast fold, including the entire lungs. The slice thickness and interval were both set to 5 mm, with a resolution of 512 × 512. After the scan, the CT images were transferred to the Monaco TPS.

2.3. Target contours

The same senior radiation oncologist delineated clinical target volume (CTV), PTV, heart, left lung, and right lung on the CT images. The CTV included the entire breast tissue, interpectoral lymph nodes, and the lymphatic drainage area of the sub-breast wall. The CTV was expanded 5 mm to create the PTV, the anterior boundary limited to 3 mm below the skin with considering dose buildup effect. The maximum volume, minimum volume, and average volume of the PTV in the 10 patients were 962 cm³, 450 cm³, and 663 cm³, respectively.

2.4. Plan designing

A dynamic 5-field IMRT plan was designed for each patient using the Monaco 5.11 TPS. The treatment machine was the Elekta Synergy linear accelerator, with 40 pairs of leaves, the leaf width at the central projection was 1 cm. The photon beam energy used was 6 MV, and the maximum dose rate was 600 MU/minutes. The prescribed dose of all patients was 50 Gy/25 fractions, 95% of the PTV was required to reach the prescribed dose, and the maximum dose of the PTV was <115%. By adjusting the gantry angles, the optimal incidence angle of 2 pairs of tangent fields, 3 inner tangent fields and 2 outer tangent fields were selected to minimize the overlap of the PTV in the BEV. In order to make the PTV had better conformality, the outer tangent field was not opposite to the inner tangent field as far as possible, and the angle between the inner and outer fields was 3° to 5°. When optimizing the setting, the maximum SN in each field was 50. In order to avoid the PTV underdosing caused by respiratory movement and positioning error during breast cancer irradiation, the autoflash function was used to adjust the leaf positions, ensuring that the anterior edge of the beam extends 1.5 cm beyond the surface of the chest wall skin.^[6] The same objective function, constraints, and Monte Carlo algorithm were used for the optimization calculations, with a dose calculation grid size was 3 mm. Only MSW was modified to 0.5, 1.0, 1.5, and 2.0 cm, 4 plans were obtained for each patient.

2.5. Plan evaluation

After completing the plans, the 4 groups of plans were normalized. The normalization was done by ensuring that the prescription dose covered 95% of the PTV. According to the ICRU Report 83, the approximate maximum dose D_2%, approximate minimum dose D_98%, and median dose D_50% were used to evaluate the dose distribution within the target volume. The conformity index (CI) and homogeneity index (HI) were introduced to assess the dose distribution of the plans. HI = (D_2% − D_98%)/D_50%, CI = (TV_RI × TV_RI)/(TV × V_RI), where D_x% represent the dose received by x% of the target volume, TV_RI was the volume covered by the prescription dose line, TV was the target volume, and V_RI was the volume covered by the prescription dose line. A smaller HI value indicates better uniformity in the dose distribution of the PTV, while the CI value ranges from 0 to 1, with values closer to 1 indicating better conformity of the PTV. The left lung was evaluated using V₅, V₂₀, V₃₀, and D_mean, while the right lung was evaluated using D_mean. The heart was evaluated using V₁₀, V₂₀, V₃₀, V₄₀, and D_mean.

2.6. Dosimetric verification

Plan dosimetric verification was performed using the semiconductor detector array ArcCHECK and its accompanying 3DVH software (Sun Nuclear Corp., USA). The 3DVH software utilized the dose perturbation algorithm. Record and compared SN, MU, DT, GPR under ArcCHECK and 3DVH, as well as the dose deviation within the PTV across the 4 groups of plans.

2.7. Statistical analysis

SPSS20.0 software was used for statistical analysis of the obtained data. The data were presented as mean ± standard deviation (x̄ ± s).The data of 4 MSW groups were compared by one-way ANOVA analysis. If the variance was homogeneous, the LSD method was selected. Otherwise, the Games-Howell method was selected, and P < .05 was considered statistically significant.

3. Results

3.1. PTV doses

The mean values of D_2%, D_98%, D_50%, CI and HI of PTV in IMRT plans of breast cancer with different MSW were similar. Except for CI (P = .699), the difference between groups was statistically significant (P < .05). D_2%, D_98% and D_50% of 0.5 cm were the closest to the prescription dose. With the increase of the MSW, it gradually deviated from the prescription dose. The HI value of 0.5 cm was the closest to 0, which had the best dose uniformity, which were shown in Table 1.

Table 1.

Comparison of doses to the PTV of the IMRT plans with 4 different MSW.

PTV	MSW (cm)				F	P value
	0.5	1	1.5	2
D2% (Gy)	53.42 ± 0.46	53.65 ± 0.46	53.95 ± 0.37	54.39 ± 0.5	8.657	0
D98% (Gy)	48.84 ± 0.42	48.77 ± 0.32	48.59 ± 0.27	48.4 ± 0.32	3.381	.029
D50% (Gy)	51.97 ± 0.51	52.15 ± 0.52	52.45 ± 0.36	52.89 ± 0.48	7.267	.001
CI	0.687 ± 0.043	0.694 ± 0.046	0.685 ± 0.05	0.669 ± 0.054	0.478	.699
HI	0.088 ± 0.015	0.094 ± 0.013	0.101 ± 0.01	0.113 ± 0.013	7.284	.001

CI = conformity index, HI = homogeneity index, IMRT = intensity-modulated radiotherapy, MSW = minimum segment width, PTV = planning target volume.

3.2. OAR doses

The values of OAR were close with different MSW, and there was no significant difference between the groups (P > .05), as shown in Table 2. It shows that the change of the MSW has no effect on the dose protection of the OAR.

Table 2.

Comparison of doses to the OARs of the IMRT plans with 4 different MSW.

OAR	MSW (cm)				F	P
	0.5	1	1.5	2
Llung V5 (%)	40.5 ± 5.11	41.01 ± 5.35	41.12 ± 5.13	41.3 ± 5.46	0.42	.988
Llung V20 (%)	25.11 ± 3.39	25.42 ± 3.68	25.54 ± 3.42	25.76 ± 3.77	0.057	.982
Llung V30 (%)	20.69 ± 3.19	20.97 ± 3.43	21.08 ± 2.96	21.29 ± 3.31	0.06	.98
Llung Dmean (Gy)	12.9 ± 1.73	13.02 ± 1.78	13.09 ± 1.63	13.21 ± 1.82	0.55	.983
Rlung Dmean (Gy)	0.48 ± 0.09	0.48 ± 0.09	0.47 ± 0.1	0.48 ± 0.09	0.008	.999
Heart V10 (%)	14.05 ± 1.22	14.3 ± 0.89	14.29 ± 1.13	14.56 ± 1.04	0.379	.769
Heart V20 (%)	9.89 ± 1.15	10.07 ± 0.83	10.03 ± 1.15	10.24 ± 0.97	1.98	.897
Heart V30 (%)	7.38 ± 1.07	7.41 ± 0.86	7.37 ± 1.19	7.39 ± 0.92	0.003	1
Heart V40 (%)	5.21 ± 1.01	5.04 ± 0.87	5.02 ± 1.06	5.06 ± 0.9	0.078	.972
Heart Dmean (Gy)	6.35 ± 0.58	6.38 ± 0.43	6.37 ± 0.61	6.45 ± 0.47	0.064	.979

IMRT = intensity-modulated radiotherapy, MSW = minimum segment width, OAR = organs at risk.

3.3. SN and MU

With the increase of the MSW, the SN and MU gradually decreased. For every 0.5 cm increase in MSW from 0.5 to 2 cm, the SN decreased by 16.41%, 25.96%, and 28.83% respectively, the MU decreased by 21.25%, 27.45%, and 27.33% respectively, and the difference between the groups was statistically significant (P < .05), as shown in Table 3.

Table 3.

SN and MU for the IMRT plans with 4 different MSW

	MSW (cm)				F	P value
	0.5	1	1.5	2
SN	233.4 ± 13.48	195.1 ± 19.6	172.8 ± 13.31	166.1 ± 16.3	37.737	0
MU	523.2 ± 67.12	412 ± 26.03	379.6 ± 24.58	380.2 ± 27.77	28.205	0

IMRT = intensity-modulated radiotherapy, MSW = minimum segment width, MU = monitor units, SN = segments number.

3.4. Dosimetric verification and delivery time

The GPR of ArcCHECK and 3DVH under 3%/3 mm and 2%/2 mm standards were similar, and the percentage of dose deviation of D_2%, D_98%, and D_50% of the PTV under 3DVH analysis were also similar between groups, and the differences were not statistically significant (P > .05). The DT was reduced by 0.35, 0.44, and 0.42 minutes respectively, and the difference between the groups was statistically significant (P < .05), which were shown in Tables 4 and 5 for details.

Table 4.

GPR and DT for the IMRT plans with 4 different MSW

	MSW (cm)				F	P value
	0.5	1	1.5	2
ArcCHECK-3%/3 mm	98.66 ± 1.46	98.93 ± 1.17	99.01 ± 0.88	98.87 ± 1.01	0.169	.916
ArcCHECK-2%/2 mm	92.55 ± 4.11	93.69 ± 2.51	94.11 ± 3.19	93.47 ± 3.45	0.384	.765
3DVH-3%/3 mm	96.77 ± 1.81	97.67 ± 1.39	97.76 ± 1.25	97.13 ± 2.33	0.715	.55
3DVH-2%/2 mm	86.3 ± 3.39	87.23 ± 3.15	87.37 ± 3.53	86.56 ± 4.65	0.192	.901
DT (min)	2.49 ± 0.19	2.14 ± 0.26	2.05 ± 0.16	2.07 ± 0.27	8.253	0

DT = delivery time, GPR = gamma passing rates, IMRT = intensity-modulated radiotherapy, MSW = minimum segment width.

Table 5.

The deviation of dose volume to the PTV of the IMRT plans with 4 different MSW in 3DVH.

PTV	MSW (cm)				F	P value
	0.5	1	1.5	2
ΔD2% (%)	0.83 ± 0.85	0.82 ± 0.65	0.63 ± 0.51	0.66 ± 0.55	0.261	.853
ΔD98% (%)	1.34 ± 0.83	1.29 ± 0.6	1.17 ± 0.41	0.94 ± 0.52	0.844	.479
ΔD50% (%)	1.02 ± 0.62	1.15 ± 0.71	1.23 ± 0.58	1.22 ± 0.58	0.257	.856

IMRT = intensity-modulated radiotherapy, MSW = minimum segment width, PTV = planning target volume.

4. Discussion

Radiation pneumonitis was an important complication of radiotherapy for breast cancer. The incidence of radiation pneumonitis was significantly correlated with the irradiated volume and average dose of lung tissue. For left-side breast cancer, it was particularly important to reduce the irradiation dose to the heart, and necessary to reduce the risk of ischemic heart disease. Compared with 3-dimensional conformal radiotherapy, IMRT after BCS for breast cancer could significantly improve the coverage, uniformity and conformity of the PTV, and reduce the radiation dose of the ipsilateral lung, heart and surrounding normal tissues.

In the process of designing and optimizing the IMRT plan, there were many factors that affect the dose distribution, including the direction of fields, the number of fields, the minimum segment area, and the MSW. It was necessary to know the influence trend of these parameters on different disease plans under different TPS, and find out the most suitable setting range to achieve the optimization of diverse requirements. Sun et al^[11] studied the influence of the minimum segment area on the dose distribution of automatic IMRT planning for postoperative cervical cancer using the Pinnacle 9.10 planning system. They found that segment area between 14 and 50 cm² resulted in appropriate dose distributions, with no significant difference observed in the dose distribution to the PTV and OAR. Other studies had report the impact of the MSW on treatment plan. Smaller MSW could improve PTV coverage and OAR protection, but would prolong DT and reduce treatment efficiency.^[12–15] Treatment plans with smaller MSW required higher treatment accuracy but were more susceptible to positioning and MLC errors.^[5] Wang et al^[16] used the Monaco TPS to study the effect of the MSW on the quality, execution accuracy and efficiency of the VMAT plan for cervical cancer. The MSW of 1.0 and 1.5 cm could reduce the SN and MU without significantly affecting the plan quality, improve the execution efficiency, and improve the GPR of the dose verification. Hong et al^[17] studied the quality and execution efficiency of the MSW for the 5-field plans of esophageal cancer in the Monaco TPS. 0.5 and 1 cm had better dose distribution, but the execution accuracy and efficiency of the 0.5 cm plan were worse than others. 1 cm was the best choice for IMRT plan under ensuring plan quality, execution accuracy and treatment efficiency.

The PTV of breast cancer was long and curved in the transverse section, taking into account the dose coverage and conformality of the PTV, while taking into account minimize radiation to the lungs and heart, the IMRT plan usually used the tangent field. The maximum overlap of the inner and outer target areas in the field direction was the smallest, and the shape of the segment was usually long, narrow and irregular after reverse optimization. The setting of the MSW would directly affect the complexity of the plan. The results of this study showed that different MSW in left breast IMRT plans had similar parameters for OAR dose distribution. With increasing MSW, parameters such as D_2%, D_98%, and D_50% of the PTV gradually deviated from the prescribed dose, and target homogeneity Deteriorated.^[18] In the dose parameters of the PTV, all 4 plans were able to meet clinical treatment requirements, but the 0.5 cm plan was slightly better than other 3 plans. For the dose verification, the GPR of different MSW were high and similar under 3%/3 mm and 2%/2 mm criteria. There were no statistically significant differences in dose deviations of the PTV, meeting the requirements of plan execution. However, as the MSW gradually increased, the SN, MU and DT all decreased. However, the magnitude of change gradually decreased with each 0.5 cm increase in MSW. Specifically, when increasing from 0.5 to 1 cm, the reductions were the greatest, reaching 16.41%, 21.25%, and 0.35 minutes respectively. When increasing MSW to 1.5 cm, the reductions compared to 1 cm were 9.55%, 6.2% and 0.09 minutes. Further increasing MSW >1.5 cm resulted in similar values for 3 parameters between 1.5 and 2 cm. The decrease of the SN, MU and DT, could reduce patient displacement during treatment, enhance the biological effect of treatment, and improve treatment efficiency. The reduction of the MU could also reduce the high-voltage time and wear of the accelerator, improving the utilization efficiency of the treatment machine.

5. Conclusion

In summary, when using the Monaco TPS to optimize the IMRT plan for left breast cancer after BCS, the choice of 1.5 cm MSW could achieve a good balance between plan quality and execution efficiency. On the premise of ensuring better PTV dose coverage, low OAR dose and high GPR. The SN, MU. and DT were significantly reduced, and the treatment efficiency was improved.

Author contributions

Conceptualization: Ning Wang.

Data curation: Ning Wang, Haitao Sun.

Formal analysis: Haitao Sun.

Investigation: Lijuan Chen, Haitao Sun.

Software: Lijuan Chen.

Supervision: Guosen Huang.

Writing – original draft: Ning Wang.