Room: Exhibit Hall | Forum 3
Purpose: Evaluate secondary neutron dose equivalent on an axial plane representing a fetus from pencil beam scanning (PBS) proton therapy of a pregnant patient.
Methods: A pregnant patient was treated for a brain tumor with two posterior PBS proton therapy treatment fields with initial proton energies of 156.5 and 165.4 MeV. The secondary neutron dose equivalent was measured during treatment using bubble detectors placed at four points on an axial plane around the patient 10 cm inferior to the umbilicus in an effort to best represent the fetal location. The neutron dose equivalent was also measured with bubble detectors located at the patientâ€™s thorax, umbilicus, and feet. The bubbles generated within the bubble detectors were visually counted three times, rotating the detector between each counting. A correction factor was applied to the bubble detector measurements to account for the difference in average quality factor of the neutron calibration field and the neutron field generated during treatment.
Results: The neutron dose equivalent was measured to be 8 ÂµSv/Gy at the thorax and decreased to 0.5 ÂµSv/Gy at the feet. On the plane representative of the fetus, the measured neutron dose equivalent ranged from 0.8 to 1.9 ÂµSv/Gy with an average of 1.4 ÂµSv/Gy. The average dose equivalent per treatment Gray measured on the plane representative of the fetal position resulted in a total neutron dose equivalent of 70 ÂµSv from a 50 Gy(RBE) PBS proton therapy treatment.
Conclusion: The measurements from the bubble detectors are comparable to those reported from Monte Carlo simulations by Geng et al. (2016) which ranged from 1.5 ÂµSv/Gy to 2.5 ÂµSv/Gy depending on the stage of gestation from a PBS proton therapy brain treatment with two posterior fields. The total neutron dose equivalent is below the threshold for mental retardation, small head syndrome, and growth retardation.