IMRT in a Pregnant Patient: How to Reduce the Fetal Dose?

The purpose of our study was to find a solution for fetal dose reduction during head-and-neck intensity modulated radiation therapy (IMRT) of a pregnant patient. The first step was optimization of the IMRT treatment plan with as few monitor units (MUs) as possible, while maintaining an acceptable dose distribution. The peripheral dose originating from the final IMRT plan was measured at distances reaching from the most proximal to the most distal fetal position, along the accelerator's longitudinal axis, using an anthropomorphic phantom extended with water-equivalent plastic. The measured peripheral dose was divided into leakage, and internal and collimator scatter, to find the degree to which each component influences the peripheral dose to build an appropriate shield. Collimator scatter was the greatest contributor to the peripheral dose throughout the range of the growing fetus. A shield was built and placed beneath the accelerator head, extending caudally from the field edge, to function as an extra collimator jaw. This shield reduced the fetal dose by a factor of 3.5. The peripheral dose components were also measured for simple rectangular fields and also here the collimator scatter was the greatest contributor to the peripheral dose. Therefore, the shielding used for the IMRT treatment of our patient could also be used when shielding in conventional radiotherapy. It is important for a radiation therapy department to be prepared for treatment of a pregnant patient to shield the fetus efficiently.