Monday, 19 February 2018

What are the objectives of cancer prevention programs?

Vinay Prasad and Adam Cifu in "Ending Medical Reversal, Improving outcomes, Saving lives" affirmed that in order to interpret the meaning of secondary cancer prevention programs, three objectives must be kept in mind: 1) cancer ought to be discovered ahead of time, 2) specific mortality ought to be reduced and 3) overall mortality should be decreased.

The authors say that what really matters is objective number 3, given that the first two are purely instrumental. After all, if a healthy person accepts a screening, this is supposed to be because he or she wants to live longer. Unfortunately, the data shows that preventive programs (cancer of the colon, prostate, breast, cervix and lung) obtain the following results (with small nuances among them): a) objective 1: achieved, b) objective 2: weak, and c) objective 3: not reached.

The book contains an evaluation of a colorectal cancer prevention program (30 years of follow-up) that registered a specific mortality (objective 2) of 1.28%, while in the control group the figure was 1.92% (a significant difference), but when the general mortality was analyzed (objective 3), the comparison was between 71.11% and 71.09% (not significant). The survival of people is influenced by many factors, most of them not related to the healhtcare, but if we stick to our environment, it could happen that an intensive policy in the detection and early treatment of cancers achieved respectable successes in goal 2, but that, on the other hand, would lead to shortening life as a consequence of the aggressiveness of the therapies used.

If we focus on objective 1, the authors illustrate that things, on the theoretical level, should go as follows:

At the commencement of the preventive program, many more cancers should appear in the initial stage (subclinical) and some more in advanced stages (the hidden ones). If the program achieves its first objective, after a certain time, it should be seen how the increase in incipient cancers would decrease the number of advanced cases, which would be a prelude to the fact that it’s on the way to achieving goal 2.

With this graph in mind, I read an evaluation of the Dutch breast cancer prevention program "Effectiveness of and over diagnosis from Mammography screening in the Netherlands: population based study" (all women aged 50 to 75 years with a mammogram every two years for a period of 24 years, from 1989 to 2012), which shows the graph on the right. As observed, the detection of women with stage 1 cancers has increased significantly and, to a lesser extent, carcinomas in situ, but there is no significant fall in the incidence of stage 2-4 cancers, which indicate that objective 1 is having problems, as the following conclusions of the work show:

a) The Dutch breast cancer prevention program is having a very small (not significant) impact on reducing the number of breast cancers in advanced stages.

b) The induced over diagnosis reaches between one third and one half of the cancers detected, which means that a considerable number of women receive disproportionate treatment of tumours that would not have reached more advanced stages.

c) Specific mortality has had a significant decrease since 1995, despite the fact that researchers have only managed to attribute to the prevention program between 0% and 5% of said fall (according to the adjustment scenario) and up to 28% to improvements in treatments.

d) There is no reference to improvements in objective 3, the true purpose of preventive programs.

If you are tempted to undergo a secondary cancer prevention program (or influence someone to do so), don’t forget to ask about the three objectives, but especially insist on the third.

Cristina Roure has published a post about Prasad and Cifu’s book by: "I do therefore I am or the bias of intervention in medicine"

Jordi Varela

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