| Summary: | 6 MV stereotactic beams, with collimators of size 25 mm and 40 mm, were used
to irradiate many homogeneous phantoms of different densities to quantitatively study
charged particle disequilibrium. In each phantom, the magnitudes of the collisional
KERMA and dose were compared. When the collisional KERMA has almost the same
magnitude as dose, lateral charged particle equilibrium is relatively well established,
which is the case for homogeneous water, aluminum and bone phantoms. However, when
the collisional KERMA has significantly higher magnitude than the dose, there is serious
charged particle disequilibrium, which is the case for homogeneous balsa and cedar
phantoms.
The same beams were used to irradiate many composite phantoms to study the
origins of over-dosage and under-dosage in inhomogeneities. In the composite phantoms
that contain balsa and cedar wood slabs, a reduction of dose in balsa and cedar was
observed. This dose reduction is attributed to the longer lateral range of electrons in low
density materials and therefore rapid increase of lateral charged particle disequilibrium.
In the composite phantom that contains a bone slab, there was no significant change of
dose in the bone slab. This is because there is no change of lateral charged particle
equilibrium from polystyrene slabs to the bone slab.
These observations have immediate relevance to small beam Stereotactic
Radiosurgery in the head and neck region since this region has high density (bones) and
low density (air cavities) material between water equivalent tissue. === Science, Faculty of === Physics and Astronomy, Department of === Graduate
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