Background: Sarcopenia and cancer-associated cachexia are common in oncology, characterized by progressive muscle wasting, metabolic dysregulation, and impaired treatment tolerance. Radiological modalities such as computed tomography (CT), dual-energy X-ray absorptiometry (DEXA), ultrasound (US), and magnetic resonance imaging (MRI) are increasingly utilized to quantify skeletal muscle mass and guide nutritional interventions. Objectives: This systematic review aimed to synthesize evidence on radiological assessment of muscle mass and nutrition-related interventions in cancer and chronic disease patients, evaluating prognostic value, clinical outcomes, and therapeutic implications. Methods: Following PRISMA guidelines, a comprehensive search of PubMed, Embase, Scopus, and Web of Science identified studies published between 2009 and 2022. Eligible studies included randomized controlled trials, observational designs, and systematic reviews assessing radiological techniques for muscle mass evaluation or nutritional interventions in oncology and chronic disease populations. Primary outcomes included prevalence and detection of low skeletal muscle mass (LSMM), while secondary outcomes assessed survival, postoperative complications, treatment-related toxicity, quality of life, and functional performance. Results: Eight studies met inclusion criteria, encompassing populations with gastric, esophageal, colorectal, cervical, pancreatic, and advanced solid tumors, as well as end-stage renal disease and chronic kidney disease. Radiological assessments, particularly CT, consistently demonstrated superior accuracy compared to anthropometry or bioelectrical impedance analysis for detecting LSMM. Nutritional interventions—including whey protein, omega-3 fatty acids, artificial intelligence–assisted support, and smartphone app–based programs—improved nutritional markers, reduced treatment-related toxicity, preserved muscle quality, and enhanced quality of life. However, one randomized trial in elderly men found no added benefit of peri-exercise protein supplementation when baseline dietary protein intake was adequate. Conclusions: Radiological evaluation of muscle mass provides reliable and reproducible assessment of sarcopenia and cachexia, with prognostic and therapeutic relevance in cancer care. CT remains the gold standard, though ultrasound shows promise as a bedside alternative. Nutritional interventions tailored by imaging findings may mitigate toxicity and improve patient outcomes. Standardization of diagnostic thresholds and integration of artificial intelligence are critical for future clinical translation.