AbstractID: 7649 Title: Lung Inhomogeneity Correction Factors for Intensity Modulated
Radiation Fields
Abstract: Lung Inhomogeneity Correction Factors for Intensity Modulated Radiation
Fields
Intensity Modulated Radiation Therapy (IMRT) delivers dose via a superposition of
multiple radiation fields of varying dimensions. It is well known1 that for radiation fields
having widths less than the ranges of the secondary electrons, as are some used in IMRT,
lateral electronic equilibrium is lost. This can have a significant effect on the dose
distribution inside and beyond low-density media such as air cavities and lung.
Therefore, the accuracy of the algorithms used to calculate the dose for IMRT fields
traversing such inhomogeneities should be investigated. Here dose measurements are
obtained for 6 MV fields in a homogeneous solid-water phantom and in one of identical
dimensions but containing a lung inhomogeneity to determine lung-correction factors. A
0.1 cm3 ionization chambers as well as film are used to investigate dependence on
detector size. Correction factors inside and distal to the lung inhomogeneity are obtained
for typical IMRT fields. In addition, a 10x10 cm2 field of uniform intensity is generated
by scanning 1x1 or 2x2 cm2 pencil beams in the IMRT mode to obtain correction factors
to compare to those for a standard uniform field. Our measurements are compared to
calculations with two independent treatment planning systems2 as well as with a
commercial Monitor Unit verification program.3
1
Metcalfe et. al. The Physics of Radiotherapy X-Rays from Linear Accelerators.
Madison, Med. Phys. Pub. 1997
2
CORVUS and PEREGRINE TPS – Nomos Corp, Pgh., PA
3
RADCALC Lifeline Software Inc., Tyler, TX