Metastatic Tumour Burden as Prognostic Factor in Nivolumab-Treated Metastatic Renal Cell Carcinoma: An Exploratory Study

Introduction: Prognostic assessment remains fundamental in metastatic renal cell carcinoma (mRCC). Established prognostic models, such as the International Metastatic Renal Cell Carcinoma Database Consortium (IMDC) and Meet-URO classifications, are widely used but do not directly quantify the metastatic tumour burden. We evaluated the prognostic value of metastatic tumour burden in nivolumab-treated mRCC and developed an exploratory composite prognostic score integrating the tumour burden with systemic inflammation and performance status. Patients and Methods: We retrospectively analysed consecutive patients with mRCC initiating nivolumab as a second- or third-line therapy at a single UK centre between March 2017 and November 2024. Their baseline clinical, laboratory and radiological data were collected. The metastatic tumour burden was defined as involvement of ≥2 metastatic organ sites, excluding lymph nodal, soft tissue, renal, pancreatic and thyroid metastases, while including bone metastases. The Bath score combined the metastatic tumour burden, systemic immune-inflammation index (SII) and Karnofsky performance status (KPS). Overall survival (OS) was analysed using Kaplan–Meier methods and a Cox regression, and the prognostic performance was compared using Harrell’s concordance index (C-index). Results: Fifty-one patients were included. After a median follow-up of 46.1 months, the median OS was 30.4 months. In the univariate analyses, a KPS < 80%, high SII, elevated platelet count and ≥2 metastatic organ sites were significantly associated with inferior OS. A KPS < 80% was the only variable retaining statistical significance in the multivariate analysis. The Bath score was significantly associated with OS (log-rank p < 0.001) and showed a numerically higher C-index than the IMDC and Meet-URO in this cohort (0.779 [optimism corrected, 0.760] vs. 0.641 and 0.706, respectively), with separation of risk groups. Conclusions: In this real-world cohort of nivolumab-treated mRCC, the metastatic tumour burden, systemic inflammation, and performance status were associated with survival. The Bath score should be regarded as exploratory and hypothesis-generating and requires validation in larger contemporary cohorts.