Set-up error and CTV margin in one-week ultra-hypofractionated whole breast irradiation after breast-conserving surgery based on daily CBCT

doi:10.19401/j.cnki.1007-3639.2025.08.003

Abstract

Abstract:

Background and purpose: Single-week ultra-hypofractionated whole breast irradiation (WBI) after breast-conserving surgery could shorten the treatment duration while ensuring efficacy and safety, making it a viable option for WBI. However, ultra-hypofractionated WBI requires daily image-guided radiotherapy (IGRT), and its impact on setup errors remains unclear. This study aimed to identify factors associated with set-up errors in ultra-hypofractionated WBI guided with daily cone-beam computed tomography (CBCT) and calculate margin expanded from clinical target volume (CTV) to planning target volume (PTV). Methods: This study included patients enrolled in a prospective trial that explored the safety of single-week ultra-hypofractionated WBI (NCT04926766) in Shanghai Ruijin Hospital, which was approved by Shanghai Ruijin Hospital Ethics Committee (No. 2020-352). All patients received CBCT1 after positioning. After correcting errors, patients received CBCT2. CBCT3 was conducted after radiotherapy was completed. The translational errors between CBCT1, CBCT2, and plan CT were initial and residual inter-fractional errors. The translational error between CBCT2 and CBCT3 was an intra-fractional error. The PTV margin was calculated according to the van Herk formula. Results: A total of 34 patients were enrolled in this study, and 510 CBCT images were collected. Daily CBCT significantly reduced set-up error in anterior-posterior (AP), superior-inferior (SI) and right-left (RL) directions (initial inter-fractional error vs residual inter-fractional error: AP, 2.8 mm vs 0.4 mm; SI, 1.6 mm vs 0.5 mm; RL, 1.8 mm vs 0.3 mm, all P<0.001). Higher CTV volume (>402.5 cm³ vs ≤402.5 cm³) was associated with larger residual inter-fractional error (0.5 mm vs 0.3 mm, P=0.023) and intra-fractional error (0.5 mm vs 0.2 mm, P=0.001) in AP direction. Higher CTV volume was also associated with larger residual inter-fractional error in the SI direction (0.6 mm vs 0.5 mm, P=0.037). Higher BMI (>23.2 kg/m² vs ≤23.2 kg/m²) and larger weight (>60.0 kg vs ≤60.0 kg) were associated with larger intra-fractional error in AP direction: 0.7 mm vs 0.2 mm (P <0.001) and 0.5 mm vs 0.2 mm (P=0.033), respectively. Under guidance with daily CBCT, the recommended margins were 2.3 mm in AP direction, 2.8 mm in SI direction, and 2.0 mm in RL direction. However, in patients with CTV volume >402.5 cm³ and BMI>23.2 kg/m², a larger margin was recommended in SI direction: 3.1 mm and 3.4 mm, respectively. Conclusion: The 3 mm margin was feasible under guidance with daily CBCT. The CTV to PTV margin should be larger in patients with higher BMI or CTV volume.

Key words: Breast cancer, Ultra-hypofractionated radiotherapy, Whole breast irradiation, Set-up error, Margin

CLC Number:

R737.9

WANG Mengxiao, FAN Wendong, CAO Jingjing, CHEN Jiayi, CAI Gang, CAO Lu. Set-up error and CTV margin in one-week ultra-hypofractionated whole breast irradiation after breast-conserving surgery based on daily CBCT[J]. China Oncology, 2025, 35(8): 752-760.

Figures/Tables 8

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Characteristics	Patients (n=34)
Age median (range)/year	53.5 (40.0, 74.0)
CTV volume median (range)/cm³	402.5 (143.2, 958.6)
Height median (range)/cm	161.5 (150.0, 168.0)
Weight median (range)/kg	60.0 (45.0, 78.0)
BMI median (range)/ (kg·m^-2)	23.2 (18.0, 28.0)
Laterality n(%)
Left	22 (64.7)
Right	12 (35.3)
Quadrant n(%)
Upper inner	6 (17.6)
Lower inner	4 (11.8)
Upper outer	20 (58.8)
Lower outer	4 (11.8)
Axillary surgery n(%)
Sentinel lymph node biopsy	32 (94.1)
None	2 (5.9)
Pathological tumor stage n(%)
pTis	6 (17.6)
pT1	24 (70.6)
pT2	4 (11.8)
Pathological nodal stage n(%)
pNx	2 (5.9)
pN0	31 (91.2)
pN1mi	1 (2.9)
Subtype n(%)
HR⁺/HER2⁺	2 (5.9)
HR⁺/HER^2-	27 (79.4)
HR^-/HER2⁺	1 (2.9)
HR^-/HER2^-	4 (11.8)
Adjuvant therapy n(%)
None	19 (55.8)
Chemotherapy	12 (35.3)
Chemotherapy+anti-HER2 therapy	3 (8.8)

	AP/mm			SI/mm			RL/mm
	Median (range)	Z value^a/ H value^b	P value	Median (range）	Z value/ H value	P value	Median (range)	Z value/ H value	P value
Age/year		-0.818	0.414		-0.335	0.738		-0.003	0.997
≤53.5	0.5 (0.0, 2.1)			0.5 (0.0, 2.1)			0.3 (0.0, 2.6)
>53.5	0.4 (0.0, 2.4)			0.5 (0.2, 2.0)			0.3 (0.0, 1.9)
CTV/cm³		-2.265	0.023		-2.083	0.037		-1.587	0.112
≤402.5	0.3 (0.0, 2.4)			0.5 (0.0, 2.0)			0.4 (0.0, 2.6)
>402.5	0.5 (0.0, 2.1)			0.6 (0.0, 2.1)			0.2 (0.0, 1.9)
Height/cm		-2.206	0.027		-0.770	0.441		-0.480	0.631
≤161.5	0.4 (0.0, 2.1)			0.5 (0.0, 2.1)			0.2 (0.0, 2.6)
>161.5	0.4 (0.0, 2.4)			0.5 (0.0, 2.0)			0.3 (0.0, 1.9)
Weight/kg		-0.839	0.401		-0.804	0.421		-0.710	0.478
≤60.0	0.4 (0.0, 2.4)			0.5 (0.0, 2.1)			0.4 (0.0, 2.6)
>60.0	0.5 (0.0, 2.1)			0.5 (0.0, 1.9)			0.2 (0.0, 1.9)
BMI/(kg·m^-2)		-0.252	0.801		-1.701	0.089		-0.848	0.396
≤23.2	0.4 (0.0, 2.4)			0.5 (0.0, 2.0)			0.4 (0.0, 2.6)
>23.2	0.4 (0.0, 2.1)			0.5 (0.0, 2.1)			0.3 (0.0, 1.9)
Laterality		-0.987	0.324		-1.299	0.194		-1.520	0.129
Left	0.4 (0.0, 2.1)			0.5 (0.0, 1.9)			0.3 (0.0, 2.6)
Right	0.6 (0.0, 2.4)			0.5 (0.1, 2.1)			0.4 (0.0, 1.9)
Quadrant		1.862	0.601		7.222	0.065		2.754	0.431
Upper inner	0.5 (0.0, 2.4)			0.5 (0.0, 2.0)			0.4 (0.0, 2.6)
Lower inner	0.5 (0.0, 1.3)			0.8 (0.1, 1.8)			0.2 (0.0, 0.9)
Upper outer	0.4 (0.0, 2.1)			0.5 (0.0, 2.1)			0.4 (0.0, 1.9)
Lower outer	0.4 (0.1, 2.1)			0.8 (0.0, 1.9)			0.3 (0.0, 1.0)
Adjuvant therapy		-1.495	0.135		-1.414	0.157		-0.370	0.711
None	0.4 (0.0, 2.1)			0.5 (0.0, 2.1)			0.3 (0.0, 2.6)
Yes	0.5 (0.0, 2.4)			0.6 (0.0, 2.0)			0.3 (0.0, 1.4)

Item	AP/mm			SI/mm			RL/mm
Item	Median (range)	Z value^a/ H value^b	P value	Median (range)	Z value/ H value	P value	Median (range)	Z value/ H value	P value
Age/year		-1.109	0.267		-0.713	0.476		-0.743	0.457
≤53.5	0.2 (0.0, 3.9)			0.8 (0.0, 4.4)			0.5 (0.0, 3.1)
>53.5	0.4 (0.0, 2.6)			0.6 (0.0, 3.0)			0.2 (0.0, 2.8)
CTV/cm³		-3.308	0.001		-1.286	0.198		-1.156	0.248
≤402.5	0.2 (0.0, 2.4)			0.5 (0.0, 4.4)			0.2 (0.0, 3.1)
>402.5	0.5 (0.0, 3.9)			0.9 (0.0, 3.5)			0.4 (0.0, 2.5)
Height/cm		-1.329	0.184		-0.347	0.729		-0.555	0.579
≤161.5	0.2 (0.0, 3.9)			0.6 (0.0, 4.4)			0.3 (0.0, 3.1)
>161.5	0.4 (0.0, 2.6)			0.7 (0.0, 3.5)			0.3 (0.0, 2.2)
Weight/kg		-2.129	0.033		-0.461	0.645		-0.597	0.550
≤60.0	0.2 (0.0, 3.9)			0.7 (0.0, 4.4)			0.3 (0.0, 3.1)
>60.0	0.5 (0.0, 2.6)			0.6 (0.0, 3.5)			0.5 (0.0, 1.9)
BMI/(kg·m^-2)		-3.506	<0.001		-1.624	0.104		-1.501	0.133
≤23.2	0.2 (0.0, 2.3)			0.5 (0.0, 3.5)			0.3 (0.0, 3.1)
>23.2	0.7 (0.0, 3.9)			0.8 (0.0, 4.4)			0.5 (0.0, 2.5)
Laterality		-0.350	0.726		-1.571	0.116		-1.692	0.091
Left	0.3 (0.0, 2.5)			0.6 (0.0, 3.0)			0.6 (0.0, 3.1)
Right	0.3 (0.0, 3.9)			0.9 (0.0, 4.4)			0.2 (0.0, 2.8)
Quadrant		1.336	0.721		10.375	0.016		7.106	0.069
Upper inner	0.2 (0.0, 1.1)			0.3 (0.0, 2.4)			0.2 (0.0, 2.2)
Lower inner	0.2 (0.0, 2.4)			1.3 (0.0, 4.4)			0.3 (0.0, 1.5)
Upper outer	0.3 (0.0, 3.9)			0.5 (0.0, 3.0)			0.5 (0.0, 3.1)
Lower outer	0.3 (0.0, 2.2)			0.9 (0.0, 3.0)			0.9 (0.0, 1.9)
Adjuvant therapy		-0.297	0.766		-1.841	0.066		-0.224	0.823
None	0.2 (0.0, 3.9)			0.8 (0.0, 4.4)			0.3 (0.0, 3.1)
Yes	0.4 (0.0, 2.3)			0.3 (0.0, 3.5)			0.3 (0.0, 2.2)

Item	Margin without daily CBCT/mm			Margin with daily CBCT/mm
Item	AP	SI	RL	AP	SI	RL
Overall cohort (n=34)	8.9	7.2	7.1	2.3	2.8	2.0
Categorized by BMI
≤23.2kg/m² (n=17)	9.0	6.3	5.6	1.8	2.3	2.0
>23.2kg/m² (n=17)	8.6	7.9	8.4	2.6	3.4	2.0
Categorized by height
≤161.5 cm (n=17)	8.6	4.6	6.8	2.2	2.7	2.3
>161.5 cm (n=17)	9.2	8.6	7.5	2.3	2.9	1.8
Categorized by weight
≤60.0 kg (n=19)	8.7	6.2	6.0	2.2	2.7	2.2
>60.0 kg (n=15)	9.2	8.2	8.2	2.4	2.9	1.7
Categorized by CTV
≤402.5 cm³ (n=17)	6.8	6.4	5.9	1.6	2.6	2.1
>402.5 cm³ (n=17)	10.1	8.0	8.2	2.8	3.1	2.0
Categorized by quadrant
Upper inner quadrant (n=6)	6.5	5.5	5.1	1.8	2.2	2.0
Lower inner quadrant (n=4)	10.7	5.8	4.5	1.8	2.5	1.8
Upper outer quadrant (n=20)	9.1	7.7	7.5	2.2	2.6	2.1
Lower outer quadrant (n=4)	7.3	7.8	7.5	2.9	4.2	2.1