Measurement and calculation of ovary and fetus dose in extended field radiotherapy for 10 MV X rays

For patients undergoing extended and inverted Y field irradiation for the treatment of Hodgkin's disease, it is desirable to know the ovary and fetus dose accurately. Using a phantom loaded with thermoluminescent LiF dosimeters, doses as a function of distance from the edge of the field were measured for typical mantle, periaortic and inverted Y fields. In addition, ovarian dose for situation (1) ovaries located at midline behind uterus, and (2) ovaries displaced bilaterally to the iliac wings were measured and compared. Our results show that in situation (1) the dose is higher by a factor of 3 as opposed to situation (2). In addition, we show that the use of additional blocking to shield the patient from collimator leakage reduces the fetal dose significantly for mantle field irradiation. Our results indicate that Clarkson's algorithm can be applied with reasonable accuracy to estimate doses for points well outside the irradiated volume.