Spot Scanning Proton Therapy for Pregnant Patients with Brain and Head and Neck Tumors

Abstract

When radiotherapy is medically necessary, x-ray based treatments (XRT) have traditionally been used to treat pregnant patients. Treatment planning and delivery techniques may be modified to minimize dose to the fetus but results in less optimal plans due to avoiding posterior beams or arcs. Monte Carlo calculations and published case studies suggest spot scanning proton therapy (PRT) reduces the equivalent dose to the fetus by a factor of 10 compared to XRT and does not require modified treatment planning techniques. However, due to concern for dose uncertainties and neutron scatter with PRT, few centers have adopted PRT over XRT for pregnant patients. The purpose of this work is to perform a retrospective study on the pregnant patients previously treated at our institution with XRT to measure the equivalent dose that would be delivered to the fetus with spot scanning PRT compared to XRT. PRT plans were made for seven pregnant patients, 4 brain tumors and 3 head and neck tumors, who had received XRT. Due to the finite range of protons, the fetal exposure is dominated by neutrons and not the primary beam. Thus, no beam arrangement modifications were required to minimize fetal dose for PRT plans. Fetal dose measurements were performed with the patient plans using a Rando phantom and Wendi-2 (Thermo Scientific) meter placed at the phantom's abdomen. The Wendi-2 measures ambient dose equivalent, which accounts for the biological effect of the neutron energies. Measurements were made at various distances from isocenter to the center of the detector. The total dose equivalent from PRT at several out of field distances was compared to that from XRT. Patient specific measurements were used to determine the total fetal dose from each modality, accounting for the changing position of the fetus each week of the mother's treatment. The imaging dose for standard of practice imaging, including verification CT scans and daily alignment imaging, was also evaluated using a similar setup with a Fluke 451 dose meter. The average measured fetal equivalent dose for the brain plans was 0.4 mSv for PRT and 7 mSv for XRT. For the head and neck plans, it was 6 mSv for PRT and 90 mSv for XRT. The dose from PRT was consistently at least a factor of 10 less than the XRT plans. In addition, the PRT plans were preferred by the physicians when considering tumor coverage and other normal tissue sparing. Daily imaging added between 0.05 and 1.5 mSv to the total dose in the PRT treatments. This retrospective study showed that when treating brain or head and neck tumors in pregnant patients, the equivalent dose a fetus would receive with PRT is approximately a factor of 10 less than XRT without making any compromises in treatment planning. These results support changing the standard of practice to utilizing spot scanning PRT as the preferred method for treating pregnant patients with brain or head and neck tumors when available instead of XRT. We have brought this process to clinic at our center.