SU-E-T-297: Small Field Dosimetry for Superficial Lesions

Journal Article · Sun Jun 01 04:00:00 EDT 2014 · Medical Physics

DOI:https://doi.org/10.1118/1.4888629· OSTI ID:22355857

Ying, J; Casto, B; Wang, S; Talyor, T; Wichman, A; Ku, L ^[1]; Taylor, M ^[2]

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1. Salem Cancer Institute, Salem, Oregon (United States)
2. Kaiser Permanente, Portland, OR (United States)

Purpose: Kilo-voltage (kV) photons and low megavoltage (MeV) electrons are the most common options for treating small superficial lesions, but they present complex dosimetry. Using a tertiary lead shield may protect the surrounding critical structures. Our goal was to quantitatively evaluate the dosimetric impact resulting from applying tertiary shields on superficial lesions. Method: We directly compared the beam characteristics of 80 kV (0.8 mm Al) photon setup abutting the water phantom surface and 6 MeV electron setup at 100 cm SSD. Profiles and depth doses were acquired using a 3D scanning water tank and an ion chamber (active volume 0.01 cm{sup 3}). Beam profiles were scanned at Dmax. Three lead sheets (2 mm thickness) with 2.7, 2.2, and 1.6, cm diameter circular cutouts were fabricated and placed at the water surface for both photon and electron fields. Results: The penumbra (80% – 20%) of the open 4×4 cm{sup 2} electron insert was 10.7 mm, compared to an average of 7.2 mm with the tertiary cutouts. The penumbra of the open kV photon beam was 2.8 mm compared to an average of 1.8 mm with the tertiary cutouts. For field widths 2.7, 2.2, and 1.6 cm, the flatness of the electron beams was 16%, 17.3%, and 21%, respectively, and for the kV photon beams was 1.4%, 2.3%, 3.3%, respectively. The electron depth dose (PDD) shifted shallower and the photon PDD shifted deeper as the field size became smaller. Conclusion: The penumbra of small electron fields can be improved by adding tertiary lead shields. Both modalities are clinically feasible; however, kV photons still offer sharper penumbra and better flatness than that of 6 MeV electrons with tertiary shielding. Thus, kV photons may still be a superior option for small superficial lesions.

• OSTI ID: 22355857
• Journal Information: Medical Physics, Journal Name: Medical Physics Journal Issue: 6 Vol. 41; ISSN 0094-2405; ISSN MPHYA6
• Country of Publication: United States
• Language: English

Related Subjects

07 ISOTOPE AND RADIATION SOURCES
BEAM PROFILES
DEPTH DOSE DISTRIBUTIONS
DOSIMETRY
ELECTRIC POTENTIAL
ELECTRON BEAMS
PHANTOMS
PHOTON BEAMS
SHIELDS
THICKNESS