Variations between in-water and in-air root-mean squared spatial width of an electron pencil beam for blocked electron cutouts compared to open standard fields

Increasingly electron treatment fields are varying widely from small ocular ports to large breast square or long neck´s rectangular ports. Their dose distributions are commonly calculated by pencil beam algorithms computing broad dose distributions from summing several elemental electron Pencils. Accurate electron pencil modeling (Pinnacle) requires determining the pencil´s critical parameters including its spatial spread (τ/ρ) typically one value for each electron beam energy. Spreads (σ_RMS-air) are deduced from penumbral decrements (i.e., ICRU-35) of the broadest electron beam profiles in air, which algorithms scale to tissue through water-to-air ratios of electron mass-scattering powers (τ/ρ). It is possible however, to assess pencil spreads (σ_RMS-water) directly by ionization scans at depths in water for narrow blocked as well as broad electron fields at different source-to-skin distances (SSD). This paper presents extensive electron pencil spreads in rising field sizes and SSD between air and water, and assesses the media pencil spreads (σ(RMS-air) σ_RMS-water) ratios changing with typical treatment cutouts versus standard open fields. Gaussian pencil spreads (σ_RMS-air) in air were found in direct constant proportionality (1/[2π]^1/2) to penumbra widths for limited broad fields, but poorly correlated for narrow beams. Water widths and spreads showed limited constant scaling yet their ratios deviated significantly from proportionality for shrinking field ports. Hence for accurate clinical dose commission, an electron pencil that models small as well as large field treatments, may demand RMS values characterized both in air and water, and also scaled empirically with rising electron field diameters or treatment distances