Dr Kevin McCarroll
Medical Matters
The recent controversy regarding the Cervical Check programme in Ireland has brought into focus the importance of cancer screening programmes. But in particular, it has also emphasised the limitations with interpretation of screening tests and the need for ongoing quality assurance.
The Pap smear test was introduced in the 1940’s and is used in detecting abnormalites in cells from the cervix. As such, it is not a diagnostic test and results indicate the probability of a cervical pre-cancerous or cancerous lesion helping to identify women who require further investigation or follow-up. Overall, it is estimated to have reduced the incidence of cervical cancer by 60-90%.
About 95% of all cervical cancer is caused by the Human Papilloma virus (HPV) of which there are about 40 types affecting the genital tract. HPV 16 and 18 account for about 70% of all cases causing cancer and for which vaccination is available. HPV is the same virus that causes warts and in the cervix it can lead to abnormal changes in the cells (Cervical Intra-epithelial Neoplasia or CIN).
An early form called CIN 1 will often regress, though CIN 3 (which is more advanced) has a high risk of progressing to cancer. In fact, up to 20-30% of those with CIN 3 will develop cervical cancer within 10 years.
The smear test involves sampling the cervix for cells which are then examined under the microscope with the human eye looking for abnormalities, a process which has significant subjectivity. Indeed, the false negative rate from smear testing is in the order of about 10% and noteworthingly there can also be false positive results too.
Transition
However, the slow transition from premaligancy to cancer means that screening every three years should usually identify any changes that can be followed up or treated early. When CIN 2 or 3 is present, visualisation of the cervix with colopsocopy is perfomed, abnormal tissue can be removed and a biopsy is taken.
The screening programme in Ireland has led to about 50,000 high grade and 40,000 low grade abnormalities being detected, and also resulted in about 1,500 cancer diagnoses.
Importantly, it has also led to preventative treatment that has resulted in a 7% drop in the rate of cervical cancers per year since 2010 . Indeed, largely as a result of screening, about 50% of cervical cancer diagnoses now occur at the earliest stage.
Concerns regarding the incorrect reporting of cervical smears has rightly caused a review of the screening programme. Indeed, the HSE is planning to introduce new HPV cervical screening later this year. Testing for the presence of HPV in cervical cells obtained in a similar way to the smear test may provide up to 70% better protection versus the conventional method.
Extension
The HSE also provides screening for breast cancer with mammography every two years for all woman aged between 50 and 64 with an extension of the programme for those up to 69 under way.
While under 1% of all those screened will be found to have cancer, by detecting disease early, screening appears to reduce the risk of breast cancer death by about 25%. In fact, over 11,500 cases of cancer have been detected since the screening programme began in 2000.
It’s important to point out, however, that in about 10% of cases mammography will miss a cancer, such is the limitations of screening.
Indeed, in recent weeks there has been reports of a rise in potential medicolegal cases against BreastCheck who provide screening. However, sometimes failure to identify disease falls within an acceptable false negative rate for mammography.
Screening to detect bowel cancer at an early stage when there is often no symptoms is also provided for by the HSE if you are aged between 60-69.
This involves collecting a small sample of stool which is sent off for analysis to detect for trace amounts of blood not visible to the eye. If positive, further tests including a colonoscopy (camera test of the bowel) may be required.
However, even with a full screening programme, about 75% of bowel cancers will still present with symptoms and the false negative rate is as high as 30%, while false positives occur about 10% of the time. Despite this, it may reduce the overall mortality from colorectal cancer by 15%.
Screening for prostate cancer is also possible with a PSA blood test, though in about 15% of cases there can be a falsely normal result. Furthermore, in may instances establishing the diagnosis may not alter treatment as tumours are usually slow growing.
In summary, screening for some cancers can detect disease early and reduce mortality. However, false negative results highlight the importance of being vigilant at all times for symptoms of cancer which can also develop between screening tests.
Research into developing better methods of screening and ensuring ongoing review of quality assurance of screening programmes is crucial.
Dr Kevin McCarroll is a Consultant Physician in Geriatric Medicine, St James’s Hospital, Dublin.
