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Lateral Scattering Penumbra for Radiation Therapy with High Energy Electrons Using Small Cutouts

E Lief

E Lief1*, C Marsden2, E Ruff3, R Gianadda4, H Lee5, R Stevens6, (1) Marsden Medical Physics Associates, Pelham, NY, (2) Marsden Medical Physics Associates, Boonton Township, NJ, (3) Marsden Medical Physics Associates, Rockaway, NJ, (4) Marsden Medical Physics Associates, LLC, White Plains, New York, (5) White Plains Hospital, White Plains, New York, (6) White Plains Hospital, White Plains, New York

SU-E-T-183 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To determine lateral scatter penumbra margins for treatments of small lesions with electrons. These margins could be different for small and normal-sized cutouts due to the lack of lateral scatter equilibrium when the cutout dimension is disproportionally small for a given electron energy.

Methods: Round cutouts with diameters of 2 and 3 cm were compared with a standard electron 4*4 cm² square insert in a 6*6 cm² electron cone on a Varian Trilogy linac. Measurements were done in a scanning M3 water tank and analyzed using Mephysto v.1.5 program from PTW. 6, 9, 12, 15, 18, and 22 MeV electron beams were measured with a small-size semiconductor diode TN60012 from PTW. Depth-dose scanning was performed with the diode in vertical position, while the transverse dose profiles were taken with a horizontal diode.

Results: There was a noticeable reduction in depths of maximum dose and 90% dose for smaller cutouts, especially noticeable for higher electron energies. Depth of 90% dose on the central axis can be twice smaller for 2 cm cutout than for a standard 10*10 cm2 open electron field. The lateral scattering 50%-90% penumbra was higher for smaller electron energies, reaching 8 mm for 6 MeV beams. Although results depend on cutout dimensions, measurement geometry and methods, they give a practical measure of possible lateral scatter penumbra for cutout design in electron treatments of small lesions.

Conclusion: Choice of scatter-based margins is essential for radiation therapy with electrons. Special measurements or calculations should be done for small size cutouts. For off-axis cutout locations care should be taken when light guidance is used for patient positioning because the effective electron source position does not coincide with the light source position which leads to light-radiation field misalignment, especially noticeable at extended distance.

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