Evaluation of a Dose to Embryo for Prospective Palliative Spine Treatment of a Pregnant Patient
Abstract
Purpose
The purpose of this study is to evaluate allowed treatment dose for a palliative treatment of spinal lesion. NCRP report 174 recommends the limit of a total dose to embryo of 0.1Gy during a course of radiation treatment. We performed beam phantom scatter measurements of doses away from the treatment area and compare to predictions obtained from Eclipse treatment planning system.
Methods
Phantom measurements were obtained in solid water platform mimicking the geometrical dimensions of the prospective patient (30cmX21cmX60cm). Treatment plan was created in the TPS for a treatment of a single vertebrae using 2 opposing half beams (to reduce the divergence in the direction of the embryo) with the field size of 2.5x8cm. 0.6cc PTW Farmer chamber was placed at 11cm depth at a distance from the field proximal edge at 2.5cm increments (2.5cm – 35cm range). The attenuation coefficient, the ratio of field center dose (prescription dose) to the out of field scatter at each given point was measured for 3 different linear accelerator systems – Varian TrueBeam, Varian Edge HD MLC and Electa Infinity/Agility. The dose at embryo was also calculated in Varian Eclipse 15.6 for the comparison
Results
Eclipse TPS had limited success in reproducing scatter data beyond 2.5cm distance from the fields edge and the calculated scatter defaulted to zero for more than 10cm distances, therefore it’s applications are not practicable for evaluation of doses to an embryo. All measured attenuation curves shown similar trend, with linear trend for attenuation in 2.5cm–17.5cm range and quadratic growth in 20-35cm range with significantly steeper slope. The attenuation at 20cm was 300-500 depending on the delivery system/energy with the highest value achieved for 6xFFF beam.
Conclusion
Based on this phantom review NCRP 174 embryo dose limit is achievable for targets 15+cm away from the embryo.