Cancer experts are now calling for cytological smears to be offered at regular intervals for at least 25 years after a woman has had severe dysplasia/CIS (carcinoma in situ).
CIS is not cancer but close to it as some cells look cancerous but are superficially in the mucosa (the soft skin-like layer that lines many body cavities such as the nasal and genital passages) and not in any tissue.
Researchers in Sweden studied data from the National Swedish Cancer Register, which included information recorded between 1958 and 2002 on 132,493 women who had a diagnosis of severe dysplasia/CIS.
They found that 881 women had developed cervical cancer and 111 women had vaginal cancer more than one year after the CIS diagnosis.
Women with such a diagnosis are more than twice as likely to develop cancer as the general female population.
They also found that there was an increasing risk of cervical cancer if the woman was older at the time of diagnosis, with a much higher risk for women aged over 50.
The risk also grew as the decades went by as the researchers found that women were twice as likely to develop invasive cervical cancer after diagnosis of CIS if that diagnosis was made in the period 1991-2000 as in the period 1958-1970. This could be due to changes in the forms of treatment in different decades.
The observed number of cases of women who developed vaginal cancer was almost seven times higher than expected.
The authors say: "Although most women with high-grade dysplasia have been protected from invasive cancer it must be considered a failure of the medical service when women participate in screening, their pre-cancerous lesions are found and they subject themselves to treatment of those lesions, presumably participate in follow-up programmes and still develop invasive cancer."
They conclude that follow-up care has, so far, been insufficient and women should be offered cytological smears at regular intervals for at least 25 years after treatment. Long term follow up should not stop for women when they reach the age of 60 if they were older than 35-40 at the time of treatment for CIS.
This view is reiterated in accompanying BMJ editorial, which suggests that women treated for CIN3 should have long term screening, even if beyond the normal age limit of regular screening.