Occult primary cancer: A truly hidden metastatic cancer

Occult primary cancer: A truly hidden metastatic cancer

Occult primary cancer is a rare metastatic cancer type whose origin remains unidentified following the extensive clinical examination. This is why it is also known as cancer of unknown primary (CUP) origin.

It accounts for 2.3% to 5% of new malignancies throughout the world and is considered to be the 3rd or 4th common cause of death due to cancers due to raised diagnostic sensitivity. However, a decrease can be seen in the incidence of CUP because of improved diagnosis of primary tumors.

Basic knowledge about these forms of rare cancers can significantly help people to be attentive and diagnose the problem on time. It will also reduce mortality and morbidity associated with these cancers in the future. Therefore, one should be aware of CUP.

Let’s explore the fundamental information about occult primary cancer or CUP to get to know about disease more closely.


  • What is occult primary cancer?
  • Classification of occult primary cancer
  • Types of occult primary cancer
  • Risk factors of occult primary cancer
  • Signs and symptoms of occult primary cancer
  • Treatment of occult primary cancer

What is occult primary cancer?

Occult primary cancer is a cancer that has been spread to various parts of your body but the place from where it originated is not known. It is often found in the lymph nodes, liver, lung, peritoneum (lining of the bowel) or bone.

On the basis of clinical signs, atypical pattern of metastasis and biological characteristics, the occult primary cancer is found to be a different malignancy from the other familiar primary tumors. As per hypotheses, pathogenesis of occult primary cancers are:

  • Early primary cancer that leads to speedy progression of metastasis.
  • Stem cell producing cancer without any primary cancer or premalignant lesion.
  • Chromosomal instability leading to aggressive disease presentation and chemo resistance

Causes of occult primary cancer

Cancers are usually caused by mutations in certain genes of the body leading to abnormal growth of cells. These abnormal cells accumulate to form tumors. Further these tumors/cancer cells break and form metastatic tumors that spread to the brain, bones, nearby lymph nodes and other body parts.

Scientifically speaking, occult primary tumors are caused by:

  • Activation of angiogenesis (formation of new blood vessels which promotes tumor growth)
  • Over expression of oncogenes (cancer causing genes)
  • Activation of intracellular signals such as protein kinase B (AKT)

Types of occult primary cancer

Based on light microscopic evaluations, occult primary cancer can be majorly differentiated into -

Well or moderately differentiated adenocarcinoma

About 60% out of 100% cases of occult primary cancers are adenocarcinoma. This is the cancer of gland cells, which lines the certain organs of the body.

The primary adenocarcinoma includes colon cancer, lung cancer, hepatobiliary cancer, pancreatic cancer, prostate cancer and kidney cancer. The most frequent sites for secondary cancers are the lungs, liver, lymph nodes, liver and bones.

Poorly differentiated carcinoma

Poorly differentiated carcinoma is another most common type of occult primary cancer after adenocarcinoma. This type of carcinoma mainly affects the younger population. 20% of patients account to suffer from poorly differentiated carcinoma; further 10% of patients known to suffer from poorly differentiated adenocarcinoma.

Squamous cell carcinoma (SSC)

Squamous cells are the epithelial cells with thin and flat appearance. This form of carcinoma is most commonly found in the lymph nodes of the neck, which is also known as cervical lymph nodes. Other than this, this can also be present in the groin.

Neuroendocrine tumors

The neuroendocrine tumors of unknown origin account for 3 out of 100 occult primary tumors. In these, small cell carcinoma is the most aggressive type of neuro-endocrine carcinoma, which spreads very fast. Neuroendocrine systems contain the cells which act like nerve cells in one way and like hormones on the other way. This tumor often spreads to the lungs.

Except these, people can also suffer from some other types of cancer of unknown primary, like extragonadal germ cell tumor, sarcoma, lymphoma and melanoma (in lymphatic system).

Risk factors of occult primary cancer

Though there is no information of the exact type of cancer, it is difficult to say what factors increase the risk of occult primary cancer. However, clinical trials present some information regarding the occult primary cancer risk factors.

  • Smoking can be one of the prime risk factors of occult primary cancer. Majority of patients suffering from this cancer have a history of smoking. Further, as per the autopsy reports of many occult primary cancers, these cancers spread to the liver, lungs, kidneys, pancreas, esophagus, larynx and throat and smoking enhances the risk of these cancers.
  • Other than smoking, weight, diet and nutrition also found to have an association with forms of occult primary cancers that started in colon, stomach, rectum and ovaries.
  • Further harmful UV radiations emitted from the sun also known to enhance the risk of these cancers. These rays are known to increase the risk of melanoma (skin cancer), which can further cause of CUP.

However, these factors may or may not enhance the risk of cancer of an unknown primary. Even in people who may have one or more of the risk factors above, it’s not possible to know for sure if these factors contributed to cancer.

Signs and symptoms of occult primary cancer

Depending on the site where cancer spreads, the signs and symptoms of occult primary cancer can be different. There are some general symptoms which can be the indication there is something not right in your body. You should consult the doctor, if you feel:

  • Intense long-lasting pain in any one part of body
  • Thickening or lump in any body part
  • Uncommon discharge or bleeding
  • Hoarseness in the voice of cough that doesn't resolve
  • Night sweats
  • Change in bowel habits, viz. frequent urination, constipation or diarrhea
  • Fever for no reason
  • Unexpected loss of appetite or weight loss

Treatment of occult primary cancer

Occult primary cancers do not respond to systemic treatments and chemotherapy is the alternative. Chemotherapy includes the use of anti-cancer drugs and the type of drug which will be used is dependent on some factors such as results of diagnostic tests, side effects of the drugs, your overall health and medical problems such as heart problems.

Surgery can be used, if the cancer is restricted to one organ, like lymph nodes. Further, the type of surgery depends upon the site of the cancer. It can also be done with the combination of radiation therapy or chemotherapy. Other than these doctors can also use proton therapy or targeted therapies as per the condition and stage of the disease.

Prevention is the first step to keep yourself safe from such unpredicted unknown types of cancers. Follow a healthy lifestyle, restrict yourself from having alcohol, improve your quality of life, smoke and keep aware of the above mentioned signs and symptoms.

All these precautions can definitely help to keep the body fit and diagnose any problem on time, so the treatment can be given at the right time. Be attentive and defeat cancer.

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