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Dosimetry studies of small fields in homogeneous and inhomogeneous media for high energy photons

University of British Columbia

Dose decreases rapidly for photon field sizes smaller than the range of the laterally scattered electrons. The reduction in dose leads to dose non-uniformity and the degree of dose non-uniformity depends on the shape of the tumor. The dose reduction due to lateral electronic nonequilibrium increases with increasing photon energy. Small tumors are best treated with lower energy photons. We modified the primary and scatter dose model to include the effect of lateral electronic nonequilibrium. The dose model was verified experimentally for various geometries and is in good agreement with the measurements. We developed a new cavity theory which includes secondary electron backscattering from the medium into the cavity. The proposed theory gives better agreement with experiments in aluminium, copper and lead for Co-60 ƴ-rays and 10 MV x-rays than do the Burlin and Kearsley cavity theories. A method for obtaining an ionization chamber correction factor for measuring dose in inhomogeneous media is also given. There is a significant dose reduction in lung as compared to normal density tissue for small fields. The dose reduction in lung increases with decreasing field size and increasing photon energy. Results of the measurements suggest that a tumor in tissue surrounding lung would have better dose uniformity if the direction of the photon beam is such that the tumor resides in the proximal side of the tumor-lung interface. Tumors in lung and surrounding the lung have better dose uniformity if treated with lower energy photons. Significant dose reduction was also observed near the air-tissue interface. The dose perturbation increases with increasing air-cavity thickness, decreasing field size and increasing photon energy. Results of the measurements again suggest that a tumor in tissue surrounding air-cavities, such as the bronchial tube, would have better dose uniformity if the direction of the photon beam is such that the tumor resides in the proximal side of the tumor-air interface. And again, lower energy photons provide better dose uniformity for tumors surrounding the air-cavity. The presence of bone in tissue causes only modest dose perturbation for photon energies between 2 MV and 20 MV x-rays.

Thesis/Dissertation

application/pdf

2009-04-17

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http://hdl.handle.net/2429/7370

Physics

University of British Columbia

UBCV

DSpace