Cancer of unknown primary origin
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Synonyms and Keywords: Occult cancer; CUP; Metastases of unknown primary origin; Unknown primary origin neoplasm; Cancers of unknown primary site; Neoplasms of unknown primary site; Carcinoma of unknown primary; Cancer of unknown origin; Cancer of unknown primary; Unknown primary tumors; UPT; Metastatic malignant neoplasms
Cancer of unknown primary origin or CUP is the diagnosis of metastatic cancer when the anatomic site of origin remains unidentified. Cancer of unknown primary origin is common, and it accounts for 2–5% of all cancers. Cancer of unknown primary origin may be classified according to pathology findings into 4 groups: adenocarcinomas, poorly differentiated carcinomas, squamous cell carcinomas, and carcinomas with neuroendocrine differentiation. The majority of patients with cancer of unknown primary origin may be initially asymptomatic. Early clinical features include fatigue, weight-loss, and loss of appetite. If left untreated, the majority of patients with cancer of unknown primary origin may progress to develop multiple organ failure, heart failure, and death. The treatment for cancer of unknown primary origin will depend on several factors, such as: metastatic origin, biopsy findings, patients age, and performance status. The 5-year survival of patients with cancer of unknown primary origin is less than 2%.
- Cancer of unknown primary origin was first described in 1980.
- From 1980 to 1990, the definition of unknown primary cancer was based on imaging results.
- Cancer of unknown primary origin may be classified into 4 groups initially according to light microscopy of the biopsy:
- The exact pathogenesis of cancer of unknown primary origin is not fully understood.
- Cancer of unknown primary, like other cancers, arises from one cell that has managed to escape regulation and produces a tumor at a certain site (the site of origin) and consequently metastasizes to other parts of the body.
- It is thought that the site of origin of CUP remains unknown due to one of the following theories:
- Remaining small and undetectable clinically
- Disappearing after metastasizing
- Elimination by body's defense
- It remains unknown whether cancers of unknown primaries are genetically or phenotypically distinct from metastasis with known origins.
- The cause of CUP has not been identified.
- Several studies have evaluated the chromosomal and molecular anomalies found in cancers of unknown primary; however, they could not identify unique causes of metastasis of unknown primaries relative to those with known primary origins.
- Overexpression of several genes has been noted in patients with cancer of unknown primary including Ras, p53, Bcl-2, and Her-2.
Differentiating Cancer of Unknown Primary Origin from Other Diseases
- Cancer of unknown primary is a diagnosis of exclusion; hence, all other differentials in addition to primary tumor location must be ruled out prior to diagnosis.
- Cancer of unknown primary origin must be differentiated from other diseases that cause sudden weight-loss, fatigue, and loss of appetite, such as:
Epidemiology and Demographics
- The prevalence of cancer of unknown primary origin is approximately 10 cases per 100,000 individuals worldwide.
- Cancer of unknown primary origin is common, and it accounts for 3–5% of all malignant epithelial tumors.
- Cancer of unknown primary origin is the fourth most common cause of cancer-related death.
- Patients of all age groups may develop cancer of unknown primary origin.
- Cancer of unknown primary origin is more commonly observed among adults and elderly patients with a mean age of 59 upon presentation.
- Cancer of unknown primary origin affects men and women equally except for squamous cell carcinomas where males are affected twice as frequently as females.
- There is no racial predilection to cancer of unknown primary origin.
- There are no established risk factors for cancer of unknown primary origin.
Natural History, Complications and Prognosis
- Early clinical features include fatigue, weight-loss, and loss of appetite.
- Cancers of unknown primary origin are characterized by their aggressiveness, early dissemination, and unpredictable metastasis.
- Upon presentation, around 60% of patients with cancer of unknown primary have two or more affected sites.
- The most common sites of metastasis in cancer of unknown origin are lymph nodes, lungs, liver, bones, and pleura.
- If left untreated, the majority of patients with cancer of unknown primary origin may progress to develop multiple organ failure, heart failure, and death.
- Common complications of cancer of unknown primary origin, may include: hypercalcemia, adrenal insufficiency, and inappropriate antidiuretic syndrome, hematologic disorders, and malignant effusions.
- Prognosis is generally poor, and the average survival time of patients with cancer of unknown primary origin is approximately 6-12 months after diagnosis.
- Sites of metastasis and prognosis differ with the types of CUPs:
|Type||Common sites of metastasis||Characteristics||5-year survival rates|
|Adenocarcinoma||Liver, lungs, bones, axillary lymph nodes, and peritnoneum||Most common type of CUP||5%|
|Squamous cell carcinoma||Cervical lymph nodes||More common in males||30%|
|Neuroendocrine carcinoma||Bone marrow, bone lesions, lymph nodes, and lungs||Predominating type in children||17%|
|Undifferntiated||Lungs and lymph nodes||Rapid tumor growth||13-16%|
- The diagnosis of cancer of unknown primary origin is made when the following diagnostic criteria are met: tissue biopsy indicating malignancy and all known primary origins of cancer are ruled out.
- The search of the primary tumor's anatomical location has proven to be very challenging and costly; nevertheless, it is pursued with focus on the types of tumors that might benefit from efficient and targeted treatment.
- The initial approach may vary on a case-by-case basis; however, the diagnosis is made after histopathological and clinical testing:
- The histopathological examination of the tissue includes microscopic evaluation followed by immunohistochemical analysis.
- Clinical diagnostic tests must be exhausted in order to aid in identifying the primary origin and this includes:
- Cancer of unknown primary origin may be asymptomatic.
- Symptoms of cancer of unknown primary origin may include the following:
- Patients with cancer of unknown primary origin usually appear cachexic.
- There are no remarkable findings in the physical examination of these patients; the presentation is variable and depends on many factors including the primary tumor's origin and extent of organs involved.
- There are no specific diagnostic laboratory findings associated with cancer on unknown primary origin; however, certain abnormal laboratory findings may help in locating the primary origin of the tumor.
- There are no ECG findings associated with cancer of unknown primary origin.
- A chest x-ray is a prerequisite in the diagnosis of cancer of unknown primary; however, it's value is of less significance when it comes to locating the primary tumor or differentiating it from a metastatic lesion.
- A mammography may be helpful in the identification of the primary origin of the tumor in women presenting with metastatic adenocarcinoma.
Echocardiography or Ultrasound
- There are no specific echocardiography or ultrasound findings associated with cancer of unknown primary origin. However, a testicular or breast ultrasound may be helpful in the localization of the primary origin of the tumor in certain cases.
- CT scan may be helpful in the diagnosis of cancer of unknown primary origin. It is considered one of the most valuable diagnostic tools in the search for the primary origin of the tumor.
- CT scanning has provided a 20% increase in diagnostic accuracy relative to other imaging modalities.
- There are no MRI findings associated with cancer of unknown primary origin. However, a breast MRI may be helpful in the detection of the tumor's primary origin in females with isolated axillary lymphadenopathy and suspected primary breast carcinoma.
Other Imaging Findings
- There are no other imaging findings associated with cancer of unknown primary origin.
Other Diagnostic Studies
- Other diagnostic studies for cancer of unknown primary that may help establish the origin of the tumor include:
- The algorithm below explains the specific situations where certain diagnostic studies are recommended:
|Tumor of unknown primary origin|
|• Full medical history and physical exam|
• Basic blood and biochemical analysis
• CT scan of chest, abdomen, and pelvis
|Squamous cell carcinoma||Neuroendocrine carcinoma||Adenocarcinoma and poorly differntiated carcinoma|
• Plasma chromogranin A
|Additional tests specific to location of metastasis||Additional tests speciifc to gender|
|Cervical lymphadenopathy||Inguinal lymphadenopathy||Bone metastasis||Men||Women|
• CT scan of head and neck
• Diagnostic bilateral tonsillectomy
|• Complete clinical examination of external genital organs|
• Pelvic CT scan or ultrasound
• Colposcopy (if female)
• Complete clinical examination of head and neck
• Bone scintigraphy
• Xrays of painful areas
• Pelvic ultrasound or CT scan
|Additional tests specific to location of metastasis|
|Midline and/or mediastinal lymphadenopathy||Cervical and/or supraclavicular lymphadenopathy||Axillary lymphadenopathy||Liver metastasis||Lung metastasis||Bone metastasis||Single metastasis||Pleural effusion||Periotneal effusion|
|• Testicular ultrasound|
• Chest and abdominal CT scan
|• Testicular ultrasound|
• Head and neck CT scan
• EBV testing (to rule out undifferentiated nasopharyngeal carcinoma)
• Breast ultrasound
• Breast MRI
• αFP assay (if undifferntiated carcinoma)
• Testicular ultrasound
• Chest and abdominal CT scan
|• Bone scintigraphy|
• Xrays of painful areas
|• Full body CT scan|
• Bone scintigraphy
|• Chest CT scan||Women:|
• Abdominal and pelvic CT scan
- There is no treatment for cancer of unknown primary origin; the mainstay of therapy is supportive care.
- The treatment for cancer of unknown primary origin will depend on several factors, such as: metastatic origin, biopsy findings, patients age, and performance status.
- Medical therapy for cancer of unknown primary origin should be adjusted on an individual basis and according to well-defined clinicopathologic subsets.
- The table below summarizes different types of medical therapy strategies for cancer of unknown primary origin.
| Treatment for cancer of unknown primary origin|
Adapted from the European Society of Medical Oncology
Platinum based combination chemotherapy
Peritoneal carcinomatosis in female
Platinum based chemotherapy
Isolated axillary nodal metastases in female
Identical to breast cancer with similar nodal involvement
Irradiation for N1-N2 disease.
Liver, bone, or multiple-site metastases of adenocarcinoma
Low toxicity chemotherapy of palliative orientation or best supportive care are acceptable
- Surgical intervention is not recommended for the management of cancer of unknown primary origin.
- There are no primary preventive measures available for cancer of unknown primary origin.
- There is no evidence that follow-up of asymptomatic patients is needed.
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