Definition of Oligometastatic Disease
Oligometastatic disease is defined as a limited metastatic state with no more than five metastatic lesions, confirmed by comprehensive imaging including PET-CT and brain MRI, where all lesions are amenable to local ablative treatment (surgery and/or radiotherapy), and without diffuse organ involvement such as malignant pleural, pericardial, peritoneal, or leptomeningeal carcinomatosis. 1
Core Diagnostic Criteria
The 2023 ESMO guidelines establish the following standardized criteria for oligometastatic disease:
- Number of metastases: Does not exceed five lesions 1
- Imaging confirmation: Oligometastatic state must be confirmed by PET-CT and brain MRI to exclude additional occult sites 1
- Treatment feasibility: Local ablative treatment of all lesions by surgery and/or radiotherapy must be deemed technically feasible 1
- Exclusion of diffuse disease: No involvement of diffuse organs such as malignant pleural, pericardial, peritoneal, or leptomeningeal carcinomatosis 1
Additional Defining Features Across Cancer Types
While the five-metastasis threshold is most widely accepted, some guidelines provide additional context:
- Organ involvement: Some definitions limit involvement to up to two metastatic sites or organs 1
- Primary tumor status: A controlled (or optionally resected) primary tumor is preferred but not always mandatory 1, 2
- Size considerations: Limited size of metastatic lesions, though specific thresholds vary by organ and treatment modality 1
Temporal Classification of Oligometastatic Disease
Oligometastatic disease is further classified based on timing of presentation:
- Synchronous oligometastases: Limited metastases present at initial cancer diagnosis (typically within 6 months of primary diagnosis) 1, 3
- Metachronous oligometastases: Metastases identified after treatment of the primary tumor, with metachronous presentation associated with better prognosis 1, 4
- Oligoprogression: Progression of a limited number of metastatic lesions (typically ≤5) while all other sites remain controlled under ongoing systemic therapy 5, 6
- Oligopersistence: An induced state where formerly polymetastatic disease responds to treatment and decreases to fewer than 5 sites of active disease 5, 6
Clinical Context and Treatment Intent
The definition emphasizes treatment intent rather than just anatomic extent:
- Curative intent: The goal is achieving complete remission or "no evidence of disease" (NED) status through aggressive local treatment 1
- Long-term disease control: Treatment aims for prolonged survival and potentially cure, not just palliation 1, 7
- Multidisciplinary assessment: Requires specialist MDT input including medical oncology, radiation oncology, surgical oncology, and radiology 1, 2
Important Caveats and Pitfalls
The upper limit of five metastases is not absolute—some clinical scenarios may consider more lesions if complete eradication remains feasible, though this should be the exception rather than the rule. 1 Most clinical trials have included patients with ≤3 metastases, and in meta-analyses, approximately 90% of patients had a single metastasis, suggesting more conservative patient selection in practice. 1
Comprehensive staging is mandatory—failure to perform PET-CT and brain MRI may result in underestimating disease burden and inappropriately classifying patients as oligometastatic when they have occult widespread disease. 1, 5
Not all five-or-fewer metastases qualify—lesions must be safely treatable without excessive morbidity, and diffuse organ involvement (carcinomatosis) automatically excludes oligometastatic classification regardless of lesion count. 1