Author: Ahmet S. Ayan, Estelle Batin, Austin M. Faught, Peter E. Klages, Eunsin Lee, Collin Nappi, Mina Okello, Meghana Ramani, Zachary X. Richards, Parisa Sadeghi 👨🔬
Affiliation: Department of Radiation Oncology, The Ohio State University Wexner Medical Center, The Ohio State University Wexner Medical Center, Ohio State University, The Ohio State University 🌍
Purpose: Neutron doses are not modelled in proton treatment planning systems, so measurements were taken with two independent dosimeter types to quantify representative neutron fetal doses in pencil beam scanning proton treatment of cranial tumors.
Methods: A RANDO phantom was set up in HFS orientation at a Varian ProBeam360 proton gantry to mimic patient positioning for treatment of a brain tumor. Measurements were taken using two different types of neutron detectors: a WENDI-2 neutron detector (Ludlum), and a personal neutron dosimeter (Bubble Technology Industries: BD-PND). Measurements were taken with each detector placed inferior to slice 21 of the RANDO phantom to acquire independent measurements of representative neutron doses to a fetus.
Multiple variations of the planned parallel-opposed treatment fields delivering 4000 cGy (RBE) in 10 fractions were delivered: coplanar parallel-opposed at the planned gantry angles (with and without 2 and 5 cm range shifters (RS)), planned fields as vertex fields, and parallel-opposed at all cardinal angles.
Results: For parallel-opposed fields at planned gantry angles, the neutron dose was 2.5 and 2.7 uSv/Gy (WENDI-2), and 2.75 uSv/Gy (BD-PND) for two measurements. At non-planned cardinal angles, the measured doses ranged between 2.5-3.1 uSv/Gy (WENDI-2). For RS 2 and 5 cm, neutron doses measured 4.8 and 7.6 uSv/Gy, respectively using WENDI-2, and 4.1 and 6.5 uSv/Gy, respectively using BD-PND. For vertex fields, field 1 delivered 11.4 uSv/Gy (WENDI-2) and 6.25 uSv/Gy (BD-PND), and field 2 delivered 5.1 uSv/Gy (WENDI-2) and 6 uSv/Gy (BD-PND).
Conclusion: The WENDI-2 and BD-PND detectors showed good agreement in measurements for all but one vertex field. The neutron dose was on the order of 2.8 uSv/Gy for coplanar fields and increased by a factor of 2-3 with the addition of range shifters. Vertex-delivered fields increased the neutron dose by as much as a factor of 4 suggesting directionality effects.