Room: Karl Dean Ballroom A1
Purpose: TG-36 recommends the use of shielding bridges to reduce dose to the fetus. However, bridges can be time-consuming and expensive to produce and pose a major safety concern. The purpose of this work was to develop effective treatment planning and shielding methods while minimizing fetal dose and ensuring patient safety.
Methods: A patient with a brain tumor in her first trimester of pregnancy was treated in our department to 5940 cGy in 33 fractions. Two series of experiments were performed using an anthropomorphic and solid water phantom. The first series established optimal parameters related to treatment planning such as linac (Novalis vs TrueBeam), energy (6X vs 6XFFF), gantry angle, and shielding (lead sheets placed on couch). For the second series, 20 plans (3D, conformal arc, IMRT) were created. IMRT plans were evaluated for optimal leaf motion calculation algorithm (step-and-shoot vs sliding window and with/without jaw tracking). Four plans were identified with acceptable dose distribution and minimal expected fetal dose. Phantom dosimetry was measured at the location of maximal exposure during treatment (18 weeks, head-to-umbilicus distance 70 cm, various depths) using ion chambers and verified with OSLDs and film badges. In-vivo dosimetry was performed using film badges.
Results: The first series results showed a reduction in dose when using 6XFFF vs 6X (up to 30%). Posterior oblique beams reduced dose up to 33%. Custom lead shielding reduced dose by 13%. The final dose measurement for the chosen plan (Step-and-shoot, jaw tracking 3-field IMRT) over the entire treatment course was approximately 0.2 cGy. This dose was judged to offer minimal risk.
Conclusion: By optimizing the treatment plan and utilizing simple lead posterior shielding laid on the couch, the use of a cumbersome and dangerous lead bridge can be avoided while substantially reducing fetal dose, increasing patient safety, and maintaining plan quality.