Room: Exhibit Hall | Forum 3
Purpose: IOERT has proven effective in sterilizing the tumor bed after lumpectomy for early breast cancer. Radiation dose fall off beyond the treatment depth into the heart or lungs is a concern at the high fractional dose used. This study investigated the dosimetry characteristics of chest protectors (CPs) in combination with the electron cones for Mobetron (IntraOp Medical), proposing a general guideline for their use in Partial Breast Irradiation.
Methods: EBT3 films were used to measure the dosimetric characteristics of commonly used stainless steel CPs. The 7cm and 8cm CPs were measured at a depth of 2cm under the 5cm and 6cm cones, respectively. The transmission, backscatter at the central axis, and leakage at the CP edge were analyzed for the available energies (6, 9, and 12MeV). Film measurements were compared with baselines scanned in water phantom (depth dose profiles and transverse profiles).
Results: Transmission at the CP center measured 1.1%, 0.8% and 3.4% for 6, 9, and 12MeV beams, respectively. Backscatter at 2cm depth was 17.4%, 30.0% and 23.7%. Addition of the acrylic cap gave a backscatter of -9.6%, 9.6% and 11.1%, reducing total backscatter dose by 23.0%, 15.7%, and 10.2%. For all available energies, a CP 2cm larger in diameter than the selected cone is sufficient to reduce the edge leakage to <10% prescription at depths up to 3cm. An additional 1cm further reduces the edge leakage to <5% prescription.
Conclusion: Cone size is typically determined by adding a 2 to 3cm margin beyond tumor size. The CP stops >95% of the radiation and should always be used with the acrylic cap to reduce backscatter. A CP of 2 or 3 cm larger than the selected cone is recommended to achieve leakage of <10-5% beyond the CP, based on patient anatomy and location of adjacent organs at risk.