• Standardized measurement of how a malignancy impacts a patient's daily living abilities.
• Determination of eligibility for systemic anticancer therapies (chemotherapy, immunotherapy).
• Primary inclusion/exclusion criterion for oncologic clinical trials.
• Independent prognostic indicator for patients with advanced malignancy.

PS should be assessed at every clinical encounter, as changes often precede radiographic evidence of disease progression or treatment toxicity.

• Grade 0: Fully active.
• Grade 1: Restricted strenuous activity; ambulatory/light work.
• Grade 2: Capable of self-care; up >50% of waking hours; unable to work.
• Grade 3: Limited self-care; confined to bed/chair >50% of waking hours.
• Grade 4: Completely disabled; totally confined.
• Grade 5: Dead.

• Azam et al. (2019) demonstrated high inter-rater reliability across consultants, registrars, and specialist nurses.
• The "Nurse Factor": Oncology nurses often provide more conservative (poorer) PS ratings than physicians because they are typically more aware of a patient’s actual social situation and level of dependence.
• Scenario Complexity: PS 2 and PS 3 represent the highest degree of variability, particularly when comorbidities (e.g., Rheumatoid Arthritis) or cognitive impairments are present.

• ECOG 0–1: Usually fit for standard systemic therapy and clinical trials.
• ECOG 2: High risk for toxicity; consider dose reductions or single-agent therapy.
• ECOG 3–4: In advanced malignancy, risks of systemic treatment often outweigh benefits; Best Supportive Care (BSC) is frequently the most appropriate path.

Key elements of the ECOG scale first appeared in 1960 (Zubrod et al.). It was developed by the Eastern Cooperative Oncology Group (now ECOG-ACRIN) to ensure consistency in measuring disease impact across multiple participating hospitals.