Cancer of Unknown Primary (CUP)

Overview and definition

• CUP = histologically confirmed metastatic cancer for which the anatomic primary site is not identified after a standardized diagnostic workup.
• Accounts for a small but meaningful share of cancers; incidence has fallen with better imaging, immunohistochemistry (IHC), and molecular profiling.
• Central task: separate the ~15 to 20% of patients with a favorable, treatable subset (managed like a known primary) from the majority with unfavorable CUP (generally poorer prognosis, empiric or molecularly guided therapy).
• Main histologic groups: adenocarcinoma (well/moderately differentiated), poorly differentiated carcinoma/adenocarcinoma, squamous cell carcinoma, and undifferentiated neoplasm (rule out lymphoma, melanoma, sarcoma, germ cell).

Standard diagnostic workup

• History and physical: including breast, pelvic, testicular, prostate/rectal, skin, and lymph node exam; review prior specimens/scars.
• Baseline labs: CBC, chemistries, LFTs; symptom-directed tumor markers.
• Imaging: CT chest/abdomen/pelvis. Mammography (women with adenocarcinoma, esp. axillary nodes). FDG-PET/CT is especially useful in squamous cervical adenopathy and single-site disease (identifies a primary in roughly 30% of squamous unknown-primary cases). Endoscopy directed by symptoms/pathology.
• Biopsy and pathology: adequate core (not just cytology) when possible; IHC is the backbone of tissue-of-origin assignment.
• Serum markers as adjuncts: AFP and beta-hCG (germ cell), PSA (men with adenocarcinoma/bone metastases), CA-125 (peritoneal carcinomatosis in women), CA 19-9/CEA (nonspecific).

Immunohistochemistry (IHC) panel

Favorable, treatable subsets

Squamous cervical-node CUP (head and neck)

• FDG-PET/CT identifies a primary in roughly 30% of squamous unknown-primary cases.
• FNA of the node is the first-choice biopsy; if nondiagnostic, repeat FNA with or without core biopsy.
• HR-HPV testing of the nodal specimen is routine (level II and III nodes); EBV testing (EBER ISH) should be considered, especially if HPV-negative or nasopharyngeal origin is suspected:
  • p16 positive: confirm with HPV-specific testing; HPV-confirmed nodal disease with no identified primary is treated as an oropharyngeal primary (designated T0).
  • EBV positive: treat as a nasopharyngeal primary.
  • Both negative (T0: no primary identified after workup; TX would mean the primary cannot be assessed): if histology remains inconclusive, perform excisional biopsy sited so the incision can be incorporated into a subsequent neck dissection (an inappropriate open biopsy can seed the surgical field).
• If SCC and a primary above the clavicle is suspected (high neck node): examination under anesthesia with directed biopsies of the base of tongue, nasopharynx, and hypopharynx, plus bilateral simple tonsillectomies (rising incidence of HPV-associated tonsillar cancer).
• Consider skin, upper esophagus, and lung as other squamous sources; thyroid, salivary gland, or infraclavicular sites for adenocarcinoma; and lymphoma (may need core biopsy).
• Treatment: mainstay is neck dissection followed by RT (covering likely primary sites), with concurrent cisplatin when extracapsular (nodal) extension is present. Because most cases are now p16-positive oropharyngeal, definitive chemoradiotherapy is an accepted alternative standard.

Role of molecular / gene-expression profiling

• Tissue-of-origin (gene-expression) classifiers can predict a putative primary when IHC is inconclusive, but assigning a site does not consistently translate into a survival benefit over empiric therapy; use to complement, not replace, clinicopathologic judgment.
• Comprehensive genomic profiling (NGS) is standard to find actionable alterations and biomarkers: MSI-H/dMMR or TMB-high (pembrolizumab, tumor-agnostic), NTRK fusions (larotrectinib, entrectinib), RET, BRAF V600E, HER2, and others.
• Molecularly guided therapy: the randomized phase 2 CUPISCO trial (Krämer, Lancet 2024) showed improved progression-free survival (6.1 vs 4.4 mo; HR 0.72) with molecularly guided therapy versus continued platinum-based chemotherapy in patients with unfavorable non-squamous CUP who achieved disease control after 3 cycles of platinum induction, supporting upfront comprehensive genomic profiling.

Empiric therapy for unfavorable CUP

• After excluding favorable subsets and actionable targets, treatment is empiric and largely palliative; prognosis is generally poor.
• Standard empiric chemotherapy: a platinum plus taxane doublet (e.g., carboplatin plus paclitaxel); gemcitabine-based combinations are alternatives.
• Immunotherapy: pembrolizumab for MSI-H/dMMR or TMB-high tumors; checkpoint inhibitors are under study more broadly in CUP.
• Best supportive care is appropriate for poor performance status; align intensity with goals of care.

High-yield CUP pearls

• Always first ask: is this a favorable, treatable subset? (~15 to 20% of CUP).
• Woman + isolated axillary nodes = treat as breast; woman + peritoneal carcinomatosis = treat as ovarian.
• Squamous cervical nodes = treat as head and neck (test HPV/EBV; bilateral tonsillectomy; cisplatin if extracapsular extension).
• Young man + poorly differentiated midline tumor = treat as extragonadal germ cell (check AFP/beta-hCG, i(12p); potentially curable with platinum).
• Neuroendocrine and single-site CUP are also favorable; men with blastic bone metastases and high PSA get androgen deprivation.
• IHC first (CK7/CK20 then site-directed markers; always exclude lymphoma with CD45); reserve gene-expression profiling for inconclusive cases.
• Comprehensive genomic profiling up front: MSI-H/TMB-high (pembrolizumab), NTRK fusions, and other actionable targets; molecularly guided therapy improved PFS in unfavorable CUP (CUPISCO).
• Unfavorable CUP: empiric platinum plus taxane; poor prognosis; integrate supportive care.