PSMA PET/CT imaging biomarkers are increasingly investigated as predictors of response and survival outcomes following [¹⁷⁷Lu]Lu-PSMA radioligand therapy in prostate cancer. This review summarizes the current evidence and clinical relevance of these biomarkers. As [¹⁷⁷Lu]Lu-PSMA radioligand therapy is increasingly being introduced into earlier treatment lines of prostate cancer management, the identification of reliable imaging biomarkers for response prediction and outcome assessment is becoming increasingly important. Quantitative PSMA PET/CT parameters such as whole-body (WB) mean standardized uptake value (SUV_mean), WB PSMA-positive tumor volume, and tumor load, as well as qualitative features including disease distribution and phenotype (e.g., visceral metastases and FDG-avid, PSMA-negative discordant lesions), have been associated with treatment response, progression-free survival, and overall survival. However, the clinical utility of these imaging biomarkers is limited by heterogeneous study designs, small cohorts, and a lack of standardization in image acquisition, quantification, and cutoff definitions. This review critically appraises available data on baseline PSMA PET/CT-based prognostic and potentially predictive biomarkers for RLT and identifies the methodological barriers that must be addressed before these biomarkers can be implemented in clinical practice.

Introduction: Septic arthritis of the knee caused by the combination of Rothia mucilaginosa and Erysipelothrix rhusiopathiae is extremely rare. E. rhusiopathiae is a rare zoonotic pathogen that primarily affects individuals with occupational exposure to animals, while R. mucilaginosa can cause severe infections, particularly in immunocompromised patients. Case Presentation: A 59-year-old male underwent right knee arthroscopy in 2019 due to meniscal degeneration. Two weeks later, activity-related pain appeared. Magnetic resonance imaging showed proliferative synovitis, and joint aspiration revealed the presence of E. rhusiopathiae, which was treated with ciprofloxacin. As inflammation persisted, arthroscopic synovectomy was performed. Cultures revealed R. mucilaginosa and E. rhusiopathiae, prompting treatment with vancomycin and clindamycin. Despite repeated synovectomies, symptoms remained. After knee trauma in 2023, infection recurred. A two-stage total knee arthroplasty was performed in early 2024. At this time, another pathogen was isolated. At 12-month follow-up, the patient’s function and alignment had improved significantly. Conclusions: The described case highlights the importance of anamnesis, early diagnostics, and knowledge about the possible resistances of rare pathogens to ensure appropriate treatment of the illness.