Background: In testicular germ cell tumor (GCT) staging, current guidelines lack consensus in the measurement of retroperitoneal lymph node metastasis, including the recommended plane and dimension. Therefore, our objective was to investigate current guideline recommendations for staging of retroperitoneal lymph nodes and their influence on clinical stage (cS), relapse rate and therapy. Methods: In this explorative study we retrospectively analyzed 154 cSI GCT patients who underwent orchiectomy without any adjuvant therapy and had a follow-up of ≥ 24 months. Here, cSI was defined as retroperitoneal lymph nodes <10mm in axial short-axis diameter (SAD). In initial staging imaging the largest retroperitoneal lymph node was measured considering the different dimensions (SAD and long-axis diameter (LAD)) in the three different radiological planes: axial, sagittal and coronal. Results: In the studied cohort overall survival (OS) was 100% and relapse free survival was 82% with a mean follow-up of 95 months. By using axial SAD (RECIST 1.1) all patients were classified as cSI. Based on axial LAD (German S3 guideline) or LAD in any plane (EAU, ESMO, onkopedia and AJCC), significantly more patients would be classified as cSIIA (0% vs. 38% vs. 52%) or even cSIIB (0% vs. 1% vs. 25%; p<0.001). Overtreatment would occur in 0%, 31%, and 61% (p<0.001) for axial SAD, axial LAD and LAD in any plane, while undertreatment would affect 18%, 10%, and 2% (p<0.001), respectively. A stage-appropriate therapy would hypothetically require the following number of chemotherapy cycles (BEP or Carboplatin): optionally 34 for axial SAD vs. up to 361 for LAD in any plane. Conclusions: We found a huge variety of clinical stages according to different lymph node staging recommendations in current guidelines. With a 100% OS rate in the entire cohort and considering the balance between the risks of overtreatment and undertreatment, with stage-adapted treatment at relapse, we strongly recommend adopting axial SAD in accordance with RECIST 1.1. Prospective validation is warranted to standardize guideline recommendations.