cancer of unknown primary
Contents
Systematic approach to the patient presenting with a mass of unknown origin or metastatic disease without an identified primary. CUP is a rigorous diagnosis of exclusion reached after standardised workup — not a label for an incomplete investigation. Favourable subsets exist with curative or site-directed treatment potential.
the master sequence
Image → biopsy → pathology → staging → treatment.
Deviating from this sequence — particularly biopsying before imaging — delays diagnosis and may destroy curative options (especially sarcoma, ovarian). See red-flag presentations of solid tumours for urgency triage and organ-specific red flags.
Oncologic emergencies supersede diagnostic workup. Cord compression, SVC syndrome, hypercalcaemia, raised ICP → treat immediately. See oncologic emergencies.
recognising the clinical footprint
systemic clues
- B symptoms — fever without source, drenching sweats, weight loss >5% in 6–12 months
- risk factors — smoking, asbestos, prior malignancy, immunosuppression
- age >40 + B symptoms = high pre-test probability for malignancy
local mechanics
- pain — bone metastases, organ invasion
- obstruction — dysphagia, jaundice, bowel obstruction
- bleeding — haemoptysis, haematuria, haematochezia
the sarcoma red flag
Any soft tissue mass with these features = sarcoma until proven otherwise — refer to sarcoma centre before excision:
-
5 cm
- enlarging rapidly
- firm or hard
- fixed to adjacent structures
- deep to fascia
lymphadenopathy — site-to-primary associations
| Site | Likely primary |
|---|---|
| left supraclavicular (Virchow) | GI — stomach, pancreas |
| right supraclavicular | thoracic — lung, oesophagus |
| cervical | head & neck |
| axillary | breast, melanoma |
| inguinal | GU, skin, lower extremity |
Malignant features: >2 cm, hard or rubbery, fixed, persistent >4–6 weeks, associated B symptoms.
Supraclavicular adenopathy in any adult demands urgent malignancy workup regardless of other features.
imaging selection
| Modality | Best for |
|---|---|
| ultrasound | thyroid, testicular, superficial soft tissue, real-time biopsy guidance |
| CT chest/abdomen/pelvis (with contrast) | first-line workhorse for suspected solid malignancy — identifies primary, maps burden, guides biopsy |
| MRI | brain, spine, liver characterisation, pelvic organs, soft tissue sarcoma delineation |
| PET-CT | H&N SCC with unknown primary, curable CUP, lymphoma staging — not a first-line study for every mass |
PET-CT and tumour markers — when they help vs harm
Evidence-supported use:
- PET-CT: H&N SCC with unknown primary, curable CUP evaluation, lymphoma staging
- AFP + β-hCG (germ cell), PSA (men with adenocarcinoma metastases), CA-125 (ovarian suspicion)
Do not use indiscriminately — CEA/CA19-9 are elevated in smoking, cirrhosis, and inflammation. Broad panels generate false positives, false reassurance, and delayed workup. See red-flag presentations of solid tumours#tumour markers — use appropriately.
Order PET-CT and tumour markers only when the result will directly change a management decision.
tissue acquisition
golden rules
- architecture over cells — core needle biopsy (image-guided) is gold standard; provides architecture for histology, grading, IHC, and molecular profiling
- image before biopsy — unplanned biopsy (especially sarcoma) contaminates tissue planes, compromises staging, and destroys curative surgical options
- target the periphery — biopsying central necrosis yields non-diagnostic material; target the viable, enhancing edge
Lacks architecture for reliable subclassification. Always obtain core or excisional biopsy.
biopsy target hierarchy (safety > accessibility > yield)
| Rank | Target | Notes |
|---|---|---|
| 1 | peripheral lymph node | safest, highest yield |
| 2 | superficial soft tissue lesion | |
| 3 | liver lesion (image-guided) | |
| 4 | lung lesion (CT-guided) | higher complication risk |
| 5 | bone lesion | technically difficult; decalcification may ruin molecular testing — use only if necessary |
For biopsy contraindications (ovarian, sarcoma) → see red-flag presentations of solid tumours#tissue diagnosis & biopsy strategy.
immunohistochemistry interpretation
IHC narrows the differential but rarely establishes the primary with certainty.
CK7/CK20 framework
| Pattern | Suggests |
|---|---|
| CK7+ / CK20− | lung, breast, gynaecological |
| CK7− / CK20+ | colorectal |
key standalone markers
| Marker | Association |
|---|---|
| TTF-1 | lung adenocarcinoma, thyroid |
| CDX2 | GI origin |
| PAX8 | renal, gynaecological |
| PSA | prostate |
| S100 / SOX10 | melanoma |
reverse-engineering metastatic spread
When metastases are found, the site narrows the differential for the occult primary:
| Metastatic site | Common primaries |
|---|---|
| liver | colorectal, pancreas, breast, gastric |
| bone | prostate, breast, lung, renal, thyroid (see common solid tumours#bone metastases — lytic vs sclerotic) |
| brain | lung, melanoma, breast, renal |
| pleura | lung, breast, mesothelioma |
| peritoneum | ovarian, GI (appendix, colorectal) |
defining CUP
Metastatic epithelial or neuroendocrine malignancy where no primary is identified after standardised workup (history, examination, baseline labs, CT chest/abdomen/pelvis, directed histology with IHC).
the diagnostic trajectory
| Stage | Definition |
|---|---|
| MUO (malignancy of undefined origin) | initial presentation, workup not yet started |
| provisional CUP | workup in progress, executing the diagnostic sequence |
| confirmed CUP | primary remains hidden after complete standardised investigation |
CUP is reached after systematic investigation — it is not a label for an incomplete workup.
favourable vs unfavourable phenotypes
Favourable CUP (curative or site-directed treatment intent):
- women with peritoneal carcinomatosis (presumed ovarian → treat as ovarian)
- squamous cell carcinoma in cervical nodes (presumed H&N → curative chemoradiation)
- young men with germ cell tumour pattern (midline mass, elevated AFP/β-hCG)
- single site of metastatic disease (oligometastatic → consider local treatment)
- neuroendocrine histology
Unfavourable CUP (palliative/empiric intent):
- adenocarcinoma with liver metastases
- multiple visceral sites
- men with bone-only metastases without prostate histology
- poor performance status
next-generation sequencing in CUP
- identifies actionable driver mutations (EGFR, BRAF, MSI-H), TMB for immunotherapy candidacy, and predicts tissue of origin via gene expression profiling (60–85% of cases)
- when to consider (ESMO 2023): young/fit patients with good performance status, when result may alter management (e.g. clinical trial eligibility), after oncology discussion
- NGS does not replace clinical and histopathological reasoning — not a first-line investigation for a deteriorating patient
baseline laboratory phenotyping
Core panel: CBC, CMP, LFTs, coagulation, urinalysis, β-hCG (reproductive-age females). See red-flag presentations of solid tumours#baseline investigations for full details.
pattern recognition
| Pattern | Significance |
|---|---|
| elevated ALP + normal transaminases | bone metastases, not liver disease → consider bone scintigraphy |
| cytopenias + elevated LDH | marrow infiltration or high tumour burden (prognostic in lymphoma, germ cell) |
| elevated creatinine | essential for IV contrast planning |
| hypercalcaemia | screen for treatable oncologic emergency (see oncologic emergencies#hypercalcaemia of malignancy) |
disposition — admit vs expedite outpatient
| Admit | Expedite outpatient |
|---|---|
| active/suspected oncologic emergency | clinically stable, no emergencies |
| uncontrolled pain or dyspnoea | symptoms controlled |
| urgent image-guided biopsy required | reliable follow-up within 1–2 weeks confirmed |
| rapidly declining performance status | rapid diagnostic access (same-week CT/biopsy) guaranteed |
| unsafe for discharge |
Safest disposition depends on acuity and system reliability, not tumour stage alone.
multidisciplinary involvement
- medical oncology — systemic treatment, genomic profiling, CUP pathway coordination
- radiation oncology — cord compression, brain metastases, oligometastatic curative radiotherapy
- interventional radiology — image-guided biopsy of deep lesions (liver, lung, retroperitoneum)
- surgery — sarcoma excision planning, solitary metastasis resection, obstruction relief
- palliative care — early referral improves symptom control, quality of life, and overall survival
common diagnostic pitfalls
- indiscriminate tumour markers — generates false positives, delays focused workup
- biopsying before imaging — contaminates planes, eliminates surgical cure
- FNA alone for lymphoma — lacks architecture for subclassification
- ignoring emergencies — failing to treat hypercalcaemia or cord compression before pursuing diagnosis
- neglecting symptom control — violates patient-centred care and reduces compliance with workup
- no follow-up arranged — discharging without a confirmed next appointment