The clinical target volume delineation of para aortic lymph node region for adjuvant radiotherapy based on the regularity of lymph node metastasis in biliary tract cancer

Abstract Background: To determine the high-risk lymphatic drainage area and the precise delineation of para aortic clinical target volume(CTV) for biliary tract cancer(BTC) according to the regularity of lymph node metastasis(LNM). Methods: The imaging records of patients with BTC from 2015 to 2020 were analyzed retrospectively. The metastasis risk for different lymph node regions was assessed. The impact of different recurrence sites on survival was evaluated by Kaplan–Meier method. The diagnosed para aortic lymph nodes(PALNs) were manually mapped to standard axial CT images. Referring to the distance from the volume center of lymph nodes to the edge of aorta, the asymmetric CTV expansion from para aortic were defined. Results: A total of 251 LNMs were identified in study cohort. These lymph nodes mainly concentrated in group 16a2 (13%) and 16b1 (17%). In validation cohort, a total of 93 LNMs were diagnosed and the highest involvement region was 16b1(33%), followed by 16a2. The upper boundary of 16a2 and the lower boundary of 16b1 were defined as the superior border and the inferior border of para aortic CTV，respectively. 18 mm in the front, 12 mm on the left and 24 mm on the right for para aortic CTV were considered as the optimal expansion range. The CTV expansion included 96% (73/76) of PALNs in 16a2 and 16b1 in study cohort and 96% (47/49) of PALNs in validation cohort. Besides, the prognosis in patients with PALN recurrence was significantly better than that in patients with distant organ recurrence(Hazard Ratio(HR)=0.219; 95% confidence interval（CI）0.067-0.711; P=0.006) and was not significantly worse than that of patients without PALN (HR=3.271; 95%CI 0.704-15.185; P=0.111). Conclusions: The lymph metastasis risk of PALN 16a2 and 16b1 was the highest and Based on the distribution of PALNs, a new para aortic CTV was defined to construct a more accurate target volume.