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Malignant Neoplasm of Unknown Primary Site B052

Last amended 
28 October 2019
Current RMA Instruments
Reasonable Hypothesis
80 of 2014
Balance of Probabilities
81 of 2014
Changes from previous Instruments

SOP Bulletin 177

ICD Coding
  • ICD-9-CM Codes: 199
  • ICD-10-AM Codes: C80
Brief description

This diagnosis applies when a metastasis from a malignant neoplasm has been found, but the site where the neoplasm first arose cannot been identified.  That a tumour is a metastasis and not a primary site will generally be apparent from a mismatch between the location of the tumour and the tissue type (e.g. carcinoma in a lymph node), or the presence of multiple metastases (often at a site where metastases are common e.g. lung, liver, bone), or on other clinical or pathological grounds.  Histology will generally show adenocarcinoma, squamous carcinoma, neuroendocrine carcinoma, or a poorly differentiated tumour.

Confirming the diagnosis

This diagnosis requires the presence of malignancy, where the primary site cannot be determined after a complete history, physical examination and appropriate investigations have been carried out.  The extent of appropriate investigation will depend on the patient’s clinical condition and be decided by the treating practitioners. Investigations will generally include a complete blood count, urinalysis, basic serum chemistries, computed tomography (CT) or magnetic resonance imaging (MRI) of the chest, abdomen, and pelvis, together with testing for prostate cancer in men and breast cancer in women. 

Histopathologic confirmation of the presence of a malignancy and of the tissue type will be routinely required except in cases where it is not possible or not warranted by the condition of the patient. 

If a particular primary site is considered probable after appropriate evaluation has been carried out then a diagnosis of malignant neoplasm of unknown primary site would not apply.

The relevant medical specialist will vary with the clinical circumstances.

Conditions excluded from SOP

  • Hodgkin’s lymphoma*
  • Malignant melanoma*
  • Metastatic neoplasm where the primary site has been identified - use the relevant SOP
  • Non-Hodgkin lymphoma*
  • Soft tissue sarcoma*

* another SOP applies

Clinical onset

The condition could present in a wide range of ways including with symptoms specific to a metastasis, non-specific symptoms, detection of a lump on examination, or incidental detection on testing performed for another indication.

Clinical worsening

The only SOP worsening factor is for inability to obtain appropriate clinical management.  Generally with this condition effective treatment is difficult, disease progression is likely and the prognosis is poor.