Pathology: Tumor Identification

Study Notes

MUO (Metastatic Unknown Origin) is a clinical disorder where a patient presents with histologically confirmed metastatic cancer, but standard diagnostic investigations fail to identify the primary site.
Accounts for 2-9% of all human cancers.
Considered to be the 7th-8th most frequent malignant tumor.

Average age at diagnosis of 60 to 75 years.
Heterogeneous tumors with a wide variety of clinical presentations.
Absence of the primary, early dissemination, aggressiveness, and unpredictability of metastatic pattern.
About 80% of patients with MUO have a poor prognosis and median overall survival (OS) of 3 to 10 months.

Complete history taking and detailed review of past biopsies or malignancies.
Physical examination including breast, genitourinary, pelvic, and rectal examinations.
Routine laboratory tests (e.g., complete blood count, electrolytes, liver function tests, creatinine, calcium).
Occult blood stool testing and tumor markers (e.g., CA15-3, PSA).
Imaging:
- Computerized tomography (CT) is the most frequently used imaging modality.
- Magnetic resonance imaging (MRI).
- Breast Ultrasound and MAMOGRAPHY.
- Positron emission tomography (PET) scan and a computed tomography (CT) (PET/CT) for radical therapy planning.
Histopathology:
- Carcinoma (adenocarcinoma, squamous carcinoma): CK (cytokeratin).
- Lymphoma: LCA (LEUCOCYTE COMMON ANTIGEN).
- Melanoma: S100, HMB45.
- Sarcoma: VIMENTINE, S100.
Molecular analysis: Neoplasm retains gene expression profile based on cellular origin.
Endoscopy: Symptoms-guided investigational approach.

Isolated inguinal adenopathy (SCC): Local excision +/- radiotherapy.
Woman with adenocarcinoma involving axillary lymph node: Managed as breast cancer.
Woman with papillary adenocarcinoma of peritoneal cavity: Managed as ovarian cancer.
Adenocarcinoma with colon profile: Managed as colon cancer.
Men with bone metastasis from adenocarcinoma with elevated PSA: Managed as prostate cancer.

Male gender, older age (≥65 years), multiple comorbidities: Treated with empirical chemotherapy ± radiotherapy.
Metastases involving multiple organs (e.g., liver, lung, bone): Treated with empirical chemotherapy ± radiotherapy.
Peritoneal metastases; multiple cerebral metastases; non-papillary malignant ascites (adenocarcinoma): Treated with empirical chemotherapy ± radiotherapy.

Consider adjuvant radiation therapy after lymph node dissection (if the disease is limited to a single nodal site).
Consider palliative radiotherapy for symptomatic patients with uncontrolled pain, impending pathologic fracture, or impending cord compression.