Use of megavoltage computed tomography with image registration for high-dose rate treatment planning of an XXXX using a custom oral mold applicator with embedded lead shielding.

@entity1 @entity5
PURPOSE: A @entity1 with a @entity5 was treated with high-dose rate brachytherapy using an oral applicator with embedded lead shielding making conventional simulation, using either kilovoltage computed tomography or radiographs, impossible because of scatter artifact. METHODS AND MATERIALS: Treatment simulation was accomplished using megavoltage computed tomography (MVCT) simulation on a helical tomotherapy unit. Because of difficulty in visualization of the catheters on the @entity1 MVCT images, Velocity AI image registration software (Velocity Medical Solutions, Atlanta, GA) was used to register an MVCT of the applicator itself with the @entity1 MVCT simulation. The treatment plan was manually optimized to prescribe 4Gy/fraction to the gross @entity5 volume. RESULTS: The @entity1 tolerated the treatment well, with no evidence of disease 6 months after treatment. Thermoluminescent dosimeter measurements showed that the shielding reduced the dose by up to 90%, depending on the location of the thermoluminescent dosimeter. While the @entity1 was treated using dose distributions calculated in a homogeneous medium (Task Group-43), an approximation of the true dose distributions was retrospectively calculated using Acuros (Varian Medical Systems Inc., Palo Alto, CA), which accounts for heterogeneities in the @entity1 . DISCUSSION: Use of the MVCT with image registration allowed treatment planning in the presence of lead shielding. Dose-volume histograms showed that recalculation of the dose using heterogeneity correction did not affect the dose to the gross @entity5 volume, but that the dose to normal structures (maxilla and mandible) was reduced by the lead shielding. CONCLUSION: The use of MVCT and image registration allows for optimized planning in the presence of shielding, which would not be possible with conventional kilovoltage computed tomography.
