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

PURPOSE: A patient with a lateral oral tongue cancer 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 patient MVCT images, Velocity AI image registration software (Velocity Medical Solutions, Atlanta, GA) was used to register an MVCT of the applicator itself with the patient MVCT simulation. The treatment plan was manually optimized to prescribe 4 Gy/fraction to the gross tumor volume. RESULTS: The patient 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 patient 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 patient. 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 tumor 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. (C) 2011 American Brachytherapy Society. Published by Elsevier Inc. All rights reserved.