Comparative analysis of fetal dose sparing between a C-arm linac and an O-ring linac in a SIB-VMAT sarcoma treatment for a pregnant patient: A technical note and case report.

Purpose: To compare the effect of two linacs designs on fetal dose sparing on a pregnant patient, including estimation of the fetal dose, and the effect of a lead apron.

Methods: A patient with a high-grade sarcoma located in the right knee/lower thigh was prescribed 51 Gy (1.7 Gy/Fx) with a simultaneous-integrated-boost (SIB) of 60 Gy to a smaller volume, starting in the 26th gestational week. Volumetric modulated radiation therapy (VMAT) plans with 6MV-FFF were developed using identical dosimetric constraints on a Varian Truebeam Edge with HD-MLC and a Varian Halcyon with double-stacked MLC. Based on patient dimension measurements, an anthropomorphic phantom was constructed using a Rando phantom and saline bags in the patient's Vac-Lok bag. Phantom measurements were performed using OSLDs and TLDs placed at three different planes, corresponding to the pubis, the umbilicus, and the fundus based on patient measurements and projected gestational age, to estimate the fetal dose. Three experimental scenarios were measured, each with CBCT-based image guidance for an accurate, reproducible setup: Edge, Halcyon, and Halcyon with a tri-folded lead apron (0.5 mm × 3 = 1.5 mm Pb) over the phantom abdomen.

Results: Plan quality and total MUs are comparable between the Edge and Halcyon plans. The OSLD-measured whole-course dose to the pubis, umbilicus, and fundus were 18.8, 13.1, and 11.7 cGy, respectively, on Halcyon, on average 27.8% lower than Edge. The repeatability within either dosimeter was good, although TLD showed systematically lower doses. Importantly, both dosimetry systems showed a lower measured fetal dose for the Halcyon plan compared with the Edge plan. Adding a tri-folded lead apron over the abdomen did not meaningfully lower the measured dose.

Conclusion: In this case study, Halcyon demonstrated a better sparing of out-of-field fetal dose compared to TrueBeam Edge. It was shown that lead aprons do not provide additional fetal dose sparing.