RE: Radiation Ups Breast Cancer Risk

[DamschenGA <damschenga@mkf.ornl.gov>]

I'm a little confused by the statistics tossed around here -

Number of women tracked: 				3436
Number of breast cancer cases: 			   12
Number of breast cancer cases before age 25:	     4
% of women w/breast cancer that get it before 25:	  0.2
ratio of irradiated/non-irradiated cases:		   20

Now, a little math:

Expected cancer cases in 3436 women: 12 / 20 = 0.6 cases
Expected cancer cases before age 25:    0.6 cases x 0.002 = 0.0012 cases
before age 25

Am I supposed to believe that if I picked 3436 women at random I should not
find a single breast cancer case in the group? While the observed rate
certainly looks significant based on the information provided, I wonder if
the report would have read very differently if they had found a total of one
case (and it occurred in a woman under 25). "Forty percent increase in
breast cancer rate" and "100% of the breast cancer occurred before age 25,
compared with 0.2% in the normal population" perhaps?

What does the 95% confidence band for the "normal rate" look like for a
sample this size? How many times the "normal" cancer rate is the high end of
the 95% confidence band? What doses were these women exposed to? How many
times background were their doses? For that matter, how many times the legal
limits for occupational workers and the general public were their doses? How
many of these women would have lived to the age where they developed breast
cancer if they hadn't received radiotherapy? More importantly, will any
member of the public reading the article ask any of these questions?

I would really like to know what the report actually says and who did the
calculations for the quoted statistics.

Gary Damschen
damschenga@mkf.ornl.gov

All musings my own, etc.

_____________________________

>RADIATION UPS BREAST CANCER RISK
>
>Associated Press
>
>Atlanta--Girls treated for childhood cancer with chest radiation are 20
>times more likely to develop breast cancer later in life than other women,
>and run an extremely high risk of getting it by their early 20s, a study
>suggests...It traced medical records of 3,436 girls treated for cancer at
>the institution between 1962 and 1995...They found that 12 of these girls
>later developed breast cancer, about 20 times the rate of the general
>female population.  Four of them, or one-third, developed breast cancer
>before age 25... Normally, only 0.2 percent of breast cancers occur before
>age 25...
>