Complex sentence: Cancer-related fatigue is a chronic fatigue (persistent fatigue not relieved by rest), the exact process of fatigue is unknown and therefore could be related to Chronic Fatigue Syndrome, however Chronic Fatigue Syndrome is not directly associated with cancer.
Split into simpler sentences: Cancer-related fatigue manifests as a chronic fatigue (persistent fatigue not relieved by rest). The exact process of fatigue remains unknown-it may relate to Chronic Fatigue Syndrome; however Chronic Fatigue Syndrome is not directly associated with cancer.