Variations between in-water and in-air root-mean squared spatialwidth of an electron pencil beam for blocked electron cutouts comparedto open standard fields

摘要

Increasingly electron treatment fields are varying widely fromsmall ocular ports to large breast square or long neck's rectangularports. Their dose distributions are commonly calculated by pencil beamalgorithms computing broad dose distributions from summing severalelemental electron Pencils. Accurate electron pencil modeling (Pinnacle)requires determining the pencil's critical parameters including itsspatial spread (τ/ρ) typically one value for each electron beamenergy. Spreads (σ_RMS-air) are deduced from penumbraldecrements (i.e., ICRU-35) of the broadest electron beam profiles inair, which algorithms scale to tissue through water-to-air ratios ofelectron mass-scattering powers (τ/ρ). It is possible however,to assess pencil spreads (σ_RMS-water) directly byionization scans at depths in water for narrow blocked as well as broadelectron fields at different source-to-skin distances (SSD). This paperpresents extensive electron pencil spreads in rising field sizes and SSDbetween air and water, and assesses the media pencil spreads(σ(RMS-air) σ_RMS-water) ratios changing withtypical treatment cutouts versus standard open fields. Gaussian pencilspreads (σ_RMS-air) in air were found in direct constantproportionality (1/[2π]1/2) to penumbra widths for limitedbroad fields, but poorly correlated for narrow beams. Water widths andspreads showed limited constant scaling yet their ratios deviatedsignificantly from proportionality for shrinking field ports. Hence foraccurate clinical dose commission, an electron pencil that models smallas well as large field treatments, may demand RMS values characterizedboth in air and water, and also scaled empirically with rising electronfield diameters or treatment distances