Dosimetric characterization of scatter foil-enhanced contact collimation for small superficial electron beam therapy

This study evaluates the dosimetric impact of integrating thin metallic scatter foils with Cerrobend contact skin collimators to improve dose uniformity, conformality, and distal tissue sparing in small superficial electron fields.

Electron beams of 8, 12, and 15 MeV from an Elekta Versa HD LINAC were delivered through a Cerrobend skin collimator with a 2.0 cm aperture at 100.0 cm SSD. Thin aluminum (Al) and lead (Pb) foils (< 2.50 mm) were placed on the collimator. Gafchromic EBT3 film in a solid-water phantom was used to measure depth-dose distributions and isodose profiles following TG-235-consistent calibration.

Scatter foils produced thickness- and Z-dependent modulation of beam characteristics. At 8 MeV, the thickest Pb foil (1.07 mm) reduced the practical range (R_p) by ∼45% and shifted D_{m a x} proximally by ∼0.5 cm, yielding substantial distal tissue sparing. Al foils caused smaller R_p reductions (15%-25%) but improved lateral dose uniformity, producing smoother and more symmetric isodose contours. The 90% isodose diameter decreased with foil thickness for both materials, with Pb showing the largest contraction (∼20%-25%, energy-dependent), enhancing field conformality. Penumbra width increased slightly for thin foils but stabilized or narrowed for larger thicknesses. These effects diminished at 15 MeV, indicating reduced sensitivity of high-energy electrons to thin-foil perturbation.

Thin metallic foils placed on Cerrobend skin collimators enable a controllable balance between dose uniformity (improved with Al) and conformality with distal sparing (enhanced with Pb). This simple, LINAC-independent configuration offers a cost-effective method for modulating small-field electron beam characteristics and may serve as a practical adjunct for treating superficial lesions.