Room: Karl Dean Ballroom A1
Purpose: Most of the Intrabeam (Carl Zeiss) breasts IORT utilize the TARGIT prescription protocol of 20 Gy at the surface of the applicator in terms of dose to water. The radiation source is historically calibrated using a large ionization chamber. However, the recent V4.0 source calibration utilizing a smaller ionization chamber results in 16 â€“ 28% higher dose rates at the surface of the applicators. The Radiance (GMV), a Monte Carlo based treatment planning system, enables heterogeneous dose calculations utilizing a CT scan of the patient and V4.0 source calibration.
Methods: Patients received a pre-operative CT immediately after needle localization, when a wire marking the center of the tumor was inserted. After the CT scan the patient undergoes the lumpectomy followed by Intrabeam radiation using the TARGIT prescription. Following the radiation a treatment plan is created in Radiance using V4.0 calibration with the actual applicator using the pre-operative CT scan. The applicator is centered at the wire marking the center of the tumor, replacing the tissue removed by the surgeon. For cases in which the patient received more than one Intrabeam treatment to the same breast, composite plans are created in an external system.
Results: The actual dose to the breast tissue was found to be substantially lower than dose to water, since most of the breast tissue of the Intrabeam patient population is adipose tissue. Dose to the ribs in case the lumpectomy cavity is close to the chest wall is very high as expected for the low photon energy of the 50 KeV Intrabeam source.
Conclusion: Pre-operative CT and Radiance treatment planning enable post-operative treatment plans which present the correct doses delivered to the breast and surrounding organs. For lumpectomy cavities close to the chest wall radiation shields protecting the ribs from high radiation doses may be considered.
Funding Support, Disclosures, and Conflict of Interest: The radiance treatment planning system was provided by GMV Innovating Solutions, Inc., Rockville, MD