What is PSA?

PSA stands for Prostate Specific Antigen. PSA is producted by the prostate gland. PSA level often increases in case of prostate cancer, prostatitis and other prostate diseases.

How is PSA velocity calculated ?

PSA velocity was calculated as change from the first PSA reading, standardized as PSA change per month.
Given:
PSA ₁: the first PSA reading
DT : the doubling time in month
V : the velocity
Then, V = PSA ₁+ 2 ^1/DT

What are the parameters of the formulas ?

Number of days in a year: 365.25
Number of days in a month: 365.25/12

I found sligthly different results elsewhere !

Curve fitting equations are used to fit your data to a theoritical curve. Several methods exist to do this job.
One method, first transforms your data into a line, then applies to this line the least squares fitting method. Another method (the one used here), applies least square fitting method directly to your data. This method was used because it weights the data points equally. You will find some explanations with an example plot here.

Weisstein, Eric W. Least Squares Fitting--Exponential.From MathWorld--A Wolfram Web Resource.
http://mathworld.wolfram.com/LeastSquaresFittingExponential.html.