Patients
Sixty-four patients with left-sided early-stage breast cancer who were enrolled and received DIBH radiotherapy in our institution were eligible for this study. Table 1 shows the patients’ characteristics. All patients received breast-conserving surgery and consecutive DIBH-RT with 42.56 Gy in 16 fractions. The average age of the patients was 52 (47–59) years, and their average height, weight, and body mass index (BMI) were 156.2 (154–160) cm, 53 (47–59.7) kg, and 21.9 (19.9–23.8) kg/m2, respectively. The quadrant distribution of primary lesions included an upper-outer area of 34 patients, upper-inner area of 13 patients, lower-outer area of 14 patients, lower-inner area of 4 patients, and central area or nipple of 2 patients. Three patients had lesions on the lower-outer and upper-outer boundaries, which were counted in both areas.
Table 1. Patient characteristics
Characteristics
|
Median
|
Range (IQR)
|
Age
|
52
|
32–74 (47–59)
|
Height (cm)
|
156.2
|
145–165.3 (154–160)
|
Weight (kg)
|
53
|
39–79.6 (47–59.7)
|
BMI (kg/m2)
|
21.9
|
16.7–30.1 (19.9–23.8)
|
Quadrants
|
|
Upper-outer
|
34
|
Upper-inner
|
13
|
Lower-outer
|
14
|
Lower-inner
|
4
|
Central portion, nipple
|
2
|
Three patients with lower-outer and lower-inner lesions were counted in both quadrants. BMI, body mass index; IQR, interquartile range
Setup reproductivity assessment
A total of 30 patients were evaluated. Figures 3a-d show the histograms of inter- and intra-fractional displacement in X and Y directions. The inter-fractional displacement of DX_inter, DY_inter, and D2D_inter were -0.07 ± 1.69, 1.02 ± 2.09, and 2.32 ± 1.30, respectively. The intra-fractional displacement of DX_intra, DY_intra, and D2D_intra were -0.14 ± 0.57, -0.06 ± 0.36, and 0.55 ± 0.43, respectively. The D2D_ALL was 2.46 ± 1.24, and the calculated D2D_ALL + 2SD was 4.94 mm. The results showed that our setup margin (5 mm) satisfied the required margin.
Treatment time
The entry-to-exit time of 64 DIBH-RT patients and 44 right breast cancer patients treated in FB (Right-FB) were recorded. Figure 4 shows the summary of the entry-to-exit time for each fraction. The median (IQR) time in DIBH and Right-FB were as follows: all fractions, 9 (8-11) minutes in DIBH and 7 (6-9) minutes in Right-FB (P = 3.95e-23); first fractions, 22 (19-27) minutes in DIBH and 16 (13-25) minutes in Right-FB (P = 0.0083); and 2-16 fractions, 9 (8-11) minutes in DIBH and 7 (6-8) minutes in Right-FB (P = 1.39e-50).
The mean MU per fraction was 307 (SD 11.3, IQR 301-312), and the beam-on time for a dose rate of 600 MU/minutes was approximately 30 seconds.
Plan comparison
We compared the volumes and doses of all contours in FB and DIBH plans. The median (IQR) volumes (mL) of all contours were as follows: CTV, 230 (149-283) in FB and 232 (160-287) in DIBH (P = 0.43); HEART, 545 (484-599) in FB and 547 (479-594) in DIBH (P = 0.31); LAD, 1.98 (1.64-2.47) in FB and 2.05 (1.71-2.45) in DIBH (P = 0.24); LUNG, 2325 (2022-2577) in FB and 3670 (3245-3960) in DIBH (P = 2.77e-29). There was a significant volume increase in DIBH for LUNG, but the other contours were not significantly different between FB and DIBH.
The median (IQR) doses of all contours (Gy) were as follows: CTV_D95, 41.0 (40.6–41.4) in FB and 41.9 (40.7–41.5) in DIBH (P = 0.33); CTV_D50, 42.9 (42.6–43.1) in FB and 42.9 (42.6–43.1) in DIBH (P = 0.30); HEART mean dose, 1.67 (1.10–2.45) in FB and 0.71 (0.59–0.81) in DIBH (P = 9.72e-15); LAD mean dose, 13.0 (5.1–17.7) in FB and 2.31 (1.79–3.22) in DIBH (P = 6.65e-15); LUNG mean dose, 2.42 (2.04–3.00) in FB and 2.03 (1.72–2.42) in DIBH (P = 6.80e-8). The LUNG_V20 was 4.64 (3.72–5.83) (%) in FB and 3.71 (2.94–4.48) (%) in DIBH (P = 1.05e-9). There was a significant dose reduction in DIBH for HEART, LAD, and LUNG mean doses, while CTV_D95 and CTV_D50 doses between FB and DIBH settings were not statistically different. The dose differences between FB and DIBH settings are summarized in Table 2.
Table 2. Summary of the comparison between the FB and DIBH plans
Contour volume (mL)
|
FB plan
Median value (IQR)
|
DIBH plan
Median value (IQR)
|
P value
|
CTV
|
230 (149–283)
|
232 (160–287)
|
.432
|
HEART
|
545 (484–599)
|
547 (479–594)
|
.311
|
LAD
|
1.98 (1.64–2.47)
|
2.05 (1.71–2.45)
|
.243
|
LUNG
|
2325 (2022–2577)
|
3670 (3245–3960)
|
2.77e^-29*
|
Contour dose
|
|
|
CTV_D95 (Gy)
|
41.0 (40.6–41.4)
|
41.9 (40.7–41.5)
|
.333
|
CTV_D50 (Gy)
|
42.9 (42.6–43.1)
|
42.9 (42.6–43.1)
|
.302
|
HEART mean dose (Gy)
|
1.67 (1.10–2.45)
|
0.71 (0.59–0.81)
|
9.72e^-15*
|
LAD mean dose (Gy)
|
13.0 (5.1–17.7)
|
2.31 (1.79–3.22)
|
6.65e^-15*
|
LUNG mean dose (Gy)
|
2.42 (2.04–3.00)
|
2.03 (1.72–2.42)
|
6.80e^-8*
|
LUNG_V20 (%)
|
4.64 (3.72–5.83)
|
3.71 (2.94–4.48)
|
1.05e^-9*
|
CTV, clinical target volume; CTV_D50, the dose of 50% of CTV volume received; CTV_D95, the dose of 95% of CTV volume received; DIBH, deep inspiration breath-holding; FB, free-breathing; IQR, interquartile range; LAD, left anterior descending artery; LUNG_V20, the percent volume of LUNG receiving 20 Gy or more. * indicates a significant difference (p < 0.05, 2-sided paired t-test)