324 Background: Historically, radiation therapy for testicular seminoma has targeted the retroperitoneal (RPLN) and pelvic lymph nodes via standard anterior-posterior/posterior-anterior fields based upon skeletal anatomy. This study was undertaken to identify the location of involved lymph nodes and their relationship to both bony and vascular anatomy in stage II patients. Methods: Scans of 69 patients with stage II seminoma diagnosed between 1996 and 2011 were reviewed. 55% of patients had left-sided, 44% right-sided, and 1% bilateral primaries. The position of each node was recorded and then transferred to a standardized CT template scan based upon its relation to skeletal and vascular anatomy. Para-aortic fields were overlaid on the template, extending from T10/T11 to L5/S1 and bounded laterally by the contralateral transverse processes and ipsilateral renal hilum. Extended fields included ipsilateral iliac lymph node chains. The location of involved nodes within these conventional fields was assessed. Results: 113 nodes were identified as radiographically positive. 88% of involved nodes were located in the RPLN region with the remaining 12% in the pelvis. 98% of positive RPLN nodes were within a 2.2cm lateral and 1.8cm anterior expansion of the aorta between L1/L2 and L5/S1. No positive nodes were identified within the renal hilar region or superior to L1 for patients with left-sided seminomas. For right-sided seminomas, no positive nodes were superior to L2, though there was one renal hilar node. <2% of all nodes fell outside the extended treatment field. Conclusions: Nodal metastases from a contemporary cohort of stage II seminoma patients generally localized to a smaller area than is targeted using conventional fields. Notably, no positive nodes were identified superior to L1, on the contralateral side of vertebral bodies, or in the left renal hilum. Modified treatment fields based on vascular rather than bony anatomy may allow for a significant decrease in normal tissue irradiation and its associated toxicities. Validation of these results may inform guidelines for a redefined clinical target volume and allow for more targeted radiation delivery in the treatment of seminoma.