2.1. Study population

The current SEER database samples 34.6% of the US population and approximates it in demographic composition and cancer incidence [10]. Within the SEER database 2004–2017, we identified all patients ≥18 and ≤75 yr of age with histologically confirmed, metastatic (clinical or pathological) renal cell carcinoma (International Classification of Disease for Oncology site code C64.9). Histological subtypes according to the 2016 World Health Organization and Heidelberg classification included clear cell renal cell carcinoma ([ccRCC] code 8310), non–clear cell renal cell carcinoma ([non-ccRCC] papillary and chromophobe, code 8260 and 8317), and mRCC histology with unknown further histological classification ([unknownRCC] code 8312) [10], [11], [12], [13]. Of these patients, CN-treated patients with renal vein tumor thrombus (pT3a-TT; CS-extension code: 601), infradiaphragmatic IVC tumor thrombus (pT3b; CS-extension code: 610), supradiaphragmatic IVC tumor thrombus/IVC wall infiltration (CS-extension code: 620), or IVC tumor thrombus not specified further (pT3NOS; CS-extension code: 625), adapted from the seventh American Joint Committee on Cancer staging system, represented the current study population [10], [14]. Patients with pT3NOS (n = 101) were excluded from further analyses. Moreover, in sensitivity analyses, we relied on all pT3a mRCC patients (pT3a; CS-extension codes: 400, 450, 460, 600, 601, and 605), instead of only mRCC patients with renal vein tumor thrombus (pT3a-TT) [10], [14]. Patients with unknown lymph node dissection status (defined as missing information regarding lymph node yield [unknown]; n = 10) or bilateral renal cell carcinoma n = 11) were excluded. Overall mortality (OM) was defined as death, irrespective of the underlying cause. Follow-up was defined as time from diagnosis to the end of study period, loss to follow-up, or death.