Measurement of Fetal Dose for Pregnant Patients Undergoing Emergency Treatment of Lung Cancer
Abstract
Purpose
To measure the fetal dose delivered to pregnant lung cancer patients using AP/PA fields with different beam divergences for consideration of phantom scatter and different distances from the Linac head for evaluation of head leakage.
Methods
Three 6 MV AP/PA plans were created with inferior half-beam block (IHB) (smallest divergence, highest head leakage), symmetric beams (symm) (medium divergence, medium head leakage), and superior half-beam block (SHB) (largest divergence, lowest head leakage). These plans delivered 800 cGy in 1 fraction with a field size of 18.6 cm x 12.5 cm and 900 collimator rotation to further reduce head leakage. The peripheral dose (PD) was measured with a RANDO phantom at depths of 5 cm and 10 cm using an ionization chamber 5 cm, 10 cm, 15 cm, 20 cm, and 25 cm away from the field edge.
Results
At a depth of 5 cm, the PD measured at 5 cm, 10 cm, 15 cm, 20 cm, and 25 cm were 16.3 cGy, 9.7 cGy, 2.5 cGy, 2.6 cGy, and 1.3 cGy with IHB plan, 23 cGy, 12.3 cGy, 4.9 cGy, 1.9 cGy, and 1.4 cGy with the symm plan, and 19.5 cGy, 9.9 cGy, 4.4 cGy, 1.6 cGy, and 1.2 cGy with SHB plan. The doses measured at 5 cm and 10 cm depth were comparable, indicating PD is confounded by internal scatter and head leakage.
Conclusion
All plans are safe for treatment of pregnant patients when the fetus is greater than 15 cm away from the field edge, all measured doses were below the 10 cGy limit recommended by TG-36 for fetal dose. When the distance was <15 cm from the field edge, the IHB plan contributed the lowest dose to the fetus. We recommend conducting measurements to determine ideal beam arrangements for pregnant patients.