Author: William G. Breen, Dalton Griner, Scott C Lester, Joseph John Lucido, Beth A. Schueler, Jack C. Thull 👨🔬
Affiliation: Mayo Clinic 🌍
Purpose: Designing a total skin electron (TSE) beam therapy protocol requires finding the optimal balance between penetration depth, field uniformity, and x-ray contamination (XC). To better understand these trade-offs, we characterized the XC across the field using the dual-large electron fields at extended distance (Stanford technique), and computed organ and effective doses using reference phantoms.
Methods: Dual-field beams (+/-20 degrees, 40x40 jaws) at 322cm lateral to isocenter were delivered with a 1cm thick acrylic beam spoiler positioned 10cm upstream of the 30 sq. cm SolidWater phantom surface to mimic the TSE delivery technique. A Farmer chamber embedded at a depth of 5cm in the phantom was moved to the lattice points of a 9x3 grid along the mid-sagittal and patient-left lateral positions (patient-right was omitted due to symmetry) in 6- and 9-MeV beams. 27 organ doses, the remainder dose, and whole-body effective dose were computed per ICRP103.
Results: XC dose ranged from a minimum of 0.5% and 1.1% at the center to a maximum of 1.7% and 3.2% at the superior field edge for 6- and 9-MeV, respectively. For a 12Gy prescription (8 fractions), the organs that received the highest dose were the brain (6MeV: 190mSv, 9MeV: 350mSv) and thyroid (6MeV: 170mSv, 9MeV: 310mSv). The effective dose was 120mSv and 210mSv for 6- and 9-MeV, respectively.
Conclusion: With this TSE delivery technique, the XC remains low but does vary across the treatment field. The dual-field approach results in the XC being directed away from the patient’s trunk, and towards the less radiosensitive extremities and head region. The effective dose from XC can be computed and allows a data-driven approach to determining trade-offs with other beam design objectives like uniformity and penetration depth.