ESTRO 38 Abstract book

S711 ESTRO 38

level III (10.0%) than in level II (9.8%) or in the internal mammary region (9.5%) compard to the healty Ln. Figure 1 depicts a comparison of healthy Ln (a) vs pathological Ln (b) in the analyzed 34 patients.

reported no additional benefit of the MR scans at all due to patient motion during scanning.

Conclusion MR-guidance facilitated delineation of IM vessels due to the improved visualization of the IM vessels when visualization on pCT was poor. Using a T1w scan only already provided adequate visualization. Our recommendation is to use non-contrast MR imaging for IMN target volume delineation when MR scanning facilities are readily available. EP-1299 Lymphatics in breast cancer: healthy nodes versus metastastic nodes L. Ernst 1 , K.J. Borm 1 , S.E. Combs 1 , M. Oechsner 1 , M. Düsberg 1 , M.N. Duma 1 1 Klinikum rechts der Isar- TU München, Department of Radiation Oncology, München, Germany Purpose or Objective Recently, a comprehensive study on PET positive lymph node (Ln) metastases in breast cancer patients has been published (Borm KJ et al. , IJROBP 2018). Nonetheless it is unclear if healthy Ln follow the same distribution pattern. The aim of this study was to detect all visible non pathological Ln in CT data sets in the same patient collective and compare them to the Ln metastases . Material and Methods The recently published dataset included 235 patients. Herein we report the (non pathological) contralateral lymph node distribution on the first patients from the same collective. For 34 patients we contoured all visible axillary/paraclavicular and internal mammary Ln contralateral to the primary tumor site. Patients with contralateral (or bilateral) lymph node metastases were excluded from this study. A total of 400 Ln were contoured. The size and localization of the Ln was recorded. The Ln were transferred to a standard patient using rigid and non-rigid registrations to create a “healthy Ln atlas”. Out of 400 Ln, 380 could be successfully transferred to the standard patient. This atlas was compared to the pattern of Ln metastases and to the ESTRO clinical target volume (CTV) contouring guidelines. Results The average diameter of the healthy Ln was 0.85 ± 0.37 cm and the mean volume was 0.20 cm 3 ± 0.30 cm 3 . Most healthy Ln were found in level I (n = 277; 69.25%), level IV (n = 63; 15.75%) and level II (n = 39; 9.75%), followed by the internal mammary region (n = 17; 4.25%) and level III (n = 4; 1.00%). Similar to this, pathological Ln were also predominantly found in level I (54.5%) and level IV (13.8%). Nonetheless, metastases were more often localized in

Of 380 healthy Ln 124 (32.6%) were completely within , 128 in partly within (33.7%) and 128 outside (33.7%) the ESTRO CTV , respectively. The largest differences with regard to the CTV coverage (compared to the previous analysis of pathological lymph nodes) were found in the supraclavicular and internal mammary region (see table 1).

Ln Metastas es

Health y Ln

partl y insid e 48.7 % 59.6 % 62.1 % 62.5 % 67.3 % 53.8 %

partl y insid e

insid e

outsid e

outside

inside

16.5%

34.8 % 26.3 % 22.4 %

25.3% 38.3 % 10.8% 43.2 %

36.4%

Level I

14.0%

46.0%

Level II

15.5%

0.0% 0.0% 100.0 %

Level III

31.3%

supraclavicu lar region

6.3% 80.0% 7.3% 12.7%

internal mammary region

29.1%

3.6% 70.6% 29.4 %

0.0%

21.2%

25.0 %

33.7% 33.7 %

32.6%

total

Conclusion Similar to Ln metastases, heatly lymph node accumulate in certain “hot spots” within the lymphatic drainage system. Even though the these hot spots seem to be located in similar regions, there are relevant differences (ESTRO CTV coverage) between the lymph node pattern in healthy and pathological tissue that need to be further investigated.