Unique Considerations When Delivering Radiation Therapy Treatment to a Pregnant Patient: Inadvisable Delivery of Noncoplanar Beams
Abstract
Purpose
To provide a clinical example of the effect on estimated fetal dose from delivering part of a pregnant cranial radiotherapy patient’s treatment using noncoplanar VMAT arcs, even when those beams are oriented towards the superior direction.
Methods
A cranial radiotherapy treatment was planned for a pregnant patient, with two of four FFF VMAT arcs to be delivered with a 45-degree couch (aka. table/floor) rotation that oriented the beam from the inferior towards the superior direction. These noncoplanar beams brought the linac head closer to the patient’s body, raising concerns about incident head leakage (https://doi.org/10.1007/978-981-10-9023-3_100). Phantom measurements were performed to obtain a fetal dose estimate after the first treatment fraction, leading to immediate replanning of the treatment using two coplanar FFF VMAT arcs only. The new coplanar treatment was used for all subsequent fractions. Based on the patient geometry, a humanoid phantom was combined with water-equivalent plastic blocks and positioned to allow calibrated ionisation chambers to be placed on the surface and at 10 cm depth (eg. https://doi.org/10.1016/j.meddos.2021.03.008), 47 cm from the inferior edge of the treatment target, to obtain estimates of the fetal dose and the expected in vivo surface dose from each treatment plan.
Results
The coplanar method produced a clinically acceptable treatment plan while achieving substantial reductions in measured dose at the approximate fetal location (19.4% at surface and 29.3% at depth), compared to the noncoplanar treatment plan.
Conclusion
There is an obvious logic in avoiding noncoplanar beams oriented towards the inferior direction (ie. towards the fetus) in the rare cases when cranial radiotherapy treatments of pregnant patients are justified. The results of this unusual case study highlight the additional disadvantages of orienting noncoplanar beams towards the superior direction.