Background: We determined the best model available for natural follicle decline in healthy women and used this to calculate the radiosensitivity of the human oocyte.
Methods: Ovarian failure was diagnosed in six patients with a median age of 13.2 years (range 12.5-16.0) who were treated with total body irradiation (14.4 Gy) at 11.5 years of age (4.9-15.1). We previously estimated the dose of radiation required to destroy 50% of the oocytes (LD(50)) to be <4 Gy. This estimate is an oversimplification, because decay represents an instantaneous rate of temporal change based upon the remaining population pool, expressed as a differential equation: dy/dx = -y[0.0595 + 3716/(11780 + y)], with initial value y(0) = 701 200.
Results: Solving the differential equation, we have estimated the number of follicles left after irradiation given as sol(51 - s + r), where r equals age at treatment, s equals age at diagnosis of ovarian failure, and 51 years is the average age of menopause. The surviving fraction of oocytes as a percentage is 100 times this value divided by sol(r). The mean surviving fraction for the six cases is 0.66%. We obtain a function, g(z), which decreases in value from 100% at zero dosage to mean value at dosage z = 14.4 Gy. We have g(z) = 10(mx+c), where c = log(10)100 = 2, and m = [log(10)(0.66) - c]/14.4. Solving g(z) = 50 gives an LD(50) of 1.99.
Conclusions: Based on new data and a revised mathematical model of natural oocyte decline, we have determined the surviving fraction of oocytes following irradiation and estimate the LD(50) of the human oocyte to be <2 Gy.