  Introduction: Treating pregnant women in the radiotherapy clinic is a rare
occurrence. When it does occur, it is vital that the dose received by the
developing embryo or foetus is understood as fully as possible. This study
presents the first investigation of foetal doses delivered during helical
tomotherapy treatments.
  Materials & Methods: Six treatment plans were delivered to an anthropomorphic
phantom using a tomotherapy machine. These included treatments of the brain,
unilateral and bilateral head-and-neck, chest wall, and upper lung.
Measurements of foetal dose were made with an ionisation chamber positioned at
various locations longitudinally within the phantom to simulate a variety of
patient anatomies.
  Results: All measurements were below the established limit of 100 mGy for a
high risk of damage during the first trimester. The largest dose encountered
was 75 mGy (0.125% of prescription dose). The majority of treatments with
measurement positions less than 30 cm fell into the range of uncertain risk (50
- 100 mGy). All treatments with measurement positions beyond 30 cm fell into
the low risk category (< 50 mGy).
  Conclusions: For the cases in this study, tomotherapy resulted in foetal
doses that are at least on par with, if not significantly lower than, similar
3D conformal or intensity-modulated treatments delivered with other devices.
Recommendations were also provided for estimating foetal doses from tomotherapy
plans.
