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Lecture 5

# Medical Biophysics 3503G Lecture 5: Medical biophysics lec 3

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Western University

Medical Biophysics

Medical Biophysics 3503G

James Lacefield

Winter

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Aside 1: MD terminology
Sensitivity = true positive fraction
Specificity = true negative fraction (TNF)
TNF = 1.0 – FPF
Overdiagnosis
In context of screening mammography, have exam that produces an overdiagnosis a false-
positive result or a true-positive result?
It’s true +ve in over diagnosed patients you said you found something by imaging test, so
something was really there, so was a true result
Overdiagnosis when you found tumour that would not have progressed to metasiss but was
treated anyway, so get side effects of treatment without benefits
TPF is in the dominator, all the cases that were cancer present by gold standard and in the
numerator is the number diagnosed properly
Fpf total number of patients that were no cancer
ROC procedure
Now must reconsider, all possible plasces can put threshold and compure new tpf and fpf
changes diagnoiss for patients near threshold
You could put threshold at begiing of chart and say everyone has cancer this would mean all
red bars, are above threshold and got those people right, and therefore tpf is 1…all blue bars
are above threshold, saying people who don’t have cnacer have it, so FPF is also 1
Construct empirical ROC curve
plot (FPF, TPF) pairs obtained at each threshold
each point is a row of FPF and TPF from the table
if you’re doing this experimentally, tend to connect each of data pint with straight line, and we
flag unrealisticness by calling it empirical curve
we are determining receiver operating characteristic for imaging system
when we present the result graphically, the graph is called ROC curve
empirical prt is we connect our measured points with straight lines
Chance line
the red-dashed line is called the chance line (TPF = FPF) accuracy expected fi diagnosies
assigned randomly
this is ROC curve you would get if you assigned diagnosis randomly, instead of clinical
observationchance line is like flipping coin to make a diagnosis, flipping coin is gives you one point we flip
a coint what’s the chance we get it right, it’s 1/2 , have 50% to get a heads, and will say that’s
positive so TPF is 0.5
next patient comes into door doesn’t have cancer, flip coin, whats the chaince to get them
wrong, we also have have 50% chance of calling that paitetnt positive, so FPF is 0.5
this is what you expect to get if you assigne diagnosis, not by any clinical observation, but by
making disecsion using something that has nothing to do with health of patient, ex. what I
rolled on the dice
The other commone misperception is the chance line means you have 50% chance of being
correct not true you must work out possibilities, where on chance line set decision threshold,
and how many patients in population do or don’t have disease
Perfect detector
is the orange solid curve
all roc curve

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