TUMOUR MARKER

Tumour Marker

Tumour Marker

Introduction

The discovery of tumour markers has revolutionized the detection, diagnosis and management of some cancers.

As with any test, there are caveats, and a poor understanding of these may lead to misuse of the test, false reassurance and consequent under diagnosis of cancer, or false positives and over the investigation of patients. A recent hospital audit concluded that 90 per cent of requests for tumour markers were inappropriate, resulting in about pounds 250,000 of wasted healthcare expenditure per year.1 Given the pressure on GPs to refer less, prescribe less and request fewer investigations, the need to use tests appropriately has never been greater (Bosl, 2001, pp: 1508).

Uses of tumour markers

Most tumour markers are proteins, often produced by tumour cells. They may serve to aid diagnosis, stage disease or augment tumour monitoring and response to treatment. Reality, however, rarely reflects the ideal and tumour markers are no exception. In an ideal world, each tumour marker would be 100 per cent specific and sensitive to the tumour in question, with a 100 per cent negative and 100 per cent positive predictive value, and levels that reflect tumour mass and recurrence of disease; the tumour would also be treatable.

In addition, the test should be cost-effective, acceptable for the patient, repeatable and appropriate for screening purposes.

PSA

One of the most commonly requested tumour markers is PSA. As its name suggests, it is organ specific, but it is also commonly raised in benign conditions of the prostate such as BPH, UTI, acute urinary retention, prostatitis and after urethral catheterisation.

The National Academy of Clinical Biochemistry (NACB) recommends that PSA should not be used for screening purposes and its use as a screening tool remains controversial in the medical literature.

PSA testing is, however, helpful in diagnosis if used in the context of clinical suspicion. It is also used in the detection of recurrence, monitoring and predicting prognosis. PSA may be falsely low by up to 50 per cent in patients taking 5-alpha reductase inhibitors such as finasteride. There is also evidence to suggest that concurrent statin use may lower PSA levels and that statins should be stopped in the weeks before testing.

Although higher levels of PSA increase the likelihood of malignancy, about 15 per cent of men with histological evidence of prostate cancer have a PSA <4g/L. This may be partly explained by highly undifferentiated disease. Despite this, 25 per cent of men with a PSA of 4-10g/L and 50 per cent of men with a PSA >10g/L will have malignancy. PSA levels in excess of 100g/L are usually, but not exclusively, suggestive of metastases.

Carcinoembryonic antigen (CEA)

Doctors have been criticised for requesting tumour markers in patients without symptoms suggestive of cancer and for inappropriately using markers to diagnose cancers.

CEA is often misused in this way. It should not be routinely used in screening or diagnosis and its uses mainly lie in establishing prognosis, monitoring disease and assessing response to ...