ESTRO 2024 - Abstract Book

S512

Clinical - Breast

ESTRO 2024

In the DBCG IMN2 study, locoregional radiotherapy was 3D-based and included the regional nodes but only IMNI in right-sided patients, whereas IMNI was omitted in left-sided patients. Systemic therapy consisted of contemporary modern agents. The DBCG guidelines recommended that the clinical target volume (CTV) of the IMN was covered with 90% of the prescribed dose (V90%). From the DBCG RT Nation study, treatment plans for patients treated in 2008-14 were collected 3 . The CTVn_IMN was only delineated in right-sided patients (IMN_old). Left and right-sided CTVn_IMN structures were auto segmented with in-house developed deep learning (DL) models trained on IMN delineations following the ESTRO guidelines (IMN_new). The IMN_new was manually inspected if volume and/or length deviated more than 5% of the average and corrected if needed. Additionally, due to the cranial discordance, the IMN_new was separated into IMN_new_cranial and IMN_new_caudal. The cut-off was defined as 3.7 cm caudal to the most cranial IMN_new slice. The heart was auto-segmented by another in-house developed DL model, while original lung delineations were used. From the original treatment plan, the V90% of IMN structures was derived. OAR dose metrics included mean heart dose (MHD) and mean lung dose (MLD). Differences between groups were tested with the Wilcoxon rank-sum or signed-rank test.

Results:

Of 4,544 patients, treatment plans for 2,893 (63.7%) patients were available. In the quality assurance process for IMN_new segmentations, 520 were detected for inspection, resulting in 127 manual corrections. In right-sided patients, the median V90% IMN_old (91.5%) was higher than IMN_new (74.0%), p<0.0001. Dose coverage in IMN_new_caudal was comparable to IMN_old, figure 1. Comparing IMN_new_caudal by laterality, the median V90% was 94.0% in right-sided patients and 20.1% in left-sided patients, table 1 and figure 2. MHDs were low, though higher in left-sided patients (2.3 Gy) than in right-sided patients (1.2 Gy). Lung doses were higher in right-sided patients than in left-sided patients, table 1.

Left n=1,517 (52,4%)

Right n=1376 (47,6 %)

p-value

CTV/OAR

V90%

CTV,

median

(IQR)

IMN_old* IMN_new

(%) (%) (%) (%)

N/A 26.0 34.1 20.1

91.5 74.0 39.6 94.0

(55.8-99.3) (48.2-85.8) (16.6-66.7) (58.0-100)

(10.8-50.9) (12.7-56.1) (0.7-55.5)

<0.0001 <0.0001 <0.0001

IMN_new_cranial IMN_new_caudal

OAR,

median

(IQR)

MHD

(Gy)

2.3

(1.6-3.7)

1.2

(0.8-1.5)

<0.0001 <0.0001

MLD (Gy)

12.8 (10.5-14.8)

15.9 (14.4-16.7)

Table 1. CTV, clinical target volume; OAR, organ at risk; IMN, internal mammary node; MHD, mean heart dose; MLD, mean lung dose; V90% CTV, CTV covered by 90% of the prescribed dose. Thus, e.g. for IMN_new_caudal, the table shows that 50% of right-sided patients had a 90% dose coverage of 94% or more of the IMN volume. Wilcoxon rank-sum test was used for p-values. *IMN_old was available for 1130 right-sided patients as one center did not delineate a designated IMN structure, but delineated the entire nodal volume as one volume.