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Monte Carlo simulation of radiation transport in inhomogeneous phantoms irradiated using sterotactic radiosurgery beams Cho, Jongmin

Abstract

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.

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