The effects of scan patterns on secondary neutron dose equivalent in a uniform scanning proton therapy beam delivery system
Abstract
Various scanning patterns of both circular and square shapes of proton beams have been tested to determine the resulting secondary neutron dose equivalent to a point representative of a point in a patient. The secondary neutron dose equivalent was measured while varying the scanning patterns' size and the patient collimator size in two separate experiments. Lateral and longitudinal dose profiles have also been measured in a solid water phantom for each alternative scanning pattern to determine whether they meet clinical beam standard requirements for flatness (± 2.5%). Additionally, the secondary neutron dose equivalent was measured within an anthropomorphic phantom while the phantom was treated with a patient specific collimator and compensator inserted into the nozzle. The in-air measurements demonstrated a linear reduction in secondary neutron dose equivalent as the area of the scanning pattern decreased while the area of the treatment field remained constant. A similar linear reduction was consistently observed for different incident proton beam energies having ranges in water of 6 cm, 16 cm, and 27 cm and for changes in the size of the patient collimator. Reducing the square-shaped pattern to fit the patient collimator reduced the secondary neutron dose equivalent by approximately 20%, and circular-shaped patterns scanned in a spiral reduced the secondary neutron dose equivalent by an additional 15% for a total reduction of 35%. In combination these results determine the patient collimator as a major contributor of secondary neutrons to points distant from the target volume. The in-phantom measurements demonstrated secondary neutron dose equivalent could be reduced by optimizing the scanning pattern for the proton beam. For the square and spiral scanning patterns studied, the secondary neutron dose equivalent was decreased by approximately 30%. However a single-ring circular-shaped scanning pattern was able to reduce the neutron dose equivalent by an additional 65%.
Degree
Ph.D.
Advisors
Das, Purdue University.
Subject Area
Medical imaging
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