Patty Oliver and Eric Vandervoort

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Thursday, April 23, 2015
Time:   3:30 - 5:00 pm

Location:   West Foustanellas Auditorium (H-2366) – 2-nd floor – The University of Ottawa Heart Institute, 40 Ruskin Street

 1.       “A study of macroscopic and microscopic dose descriptors for kilovoltage cellular dosimetry using Monte Carlo simulations and cavity theory”
Patty Oliver – Carleton University

Abstract: Monte Carlo (MC) simulations and cavity theory are used to investigate cellular dosimetry for kilovoltage photon sources. Multicellular models of normal and cancerous tissues are developed using data from a literature review; MC simulations are employed to compute doses to cellular targets for a variety of cell morphologies as well as doses to bulk tissues and water. Simulation geometries involve cell clusters, single cells, and single nuclear cavities embedded in various healthy and cancerous bulk tissue phantoms. Cell and nucleus radii range from 5 to 10 microns and 2 to 9 microns, respectively. Variations in cell dose with simulation geometry are most pronounced for lower energy sources: the nuclear dose in a multicell model differs from the dose to a cavity of nuclear medium in an otherwise homogeneous bulk tissue phantom by more than 7% at 20 keV. Bulk tissue and water cavity doses differ from cellular doses by up to 16% so that neither water nor bulk tissue is an appropriate surrogate for subcellular targets in radiation dosimetry. MC results are compared to cavity theory predictions; large and small cavity theories qualitatively predict nuclear doses for energies below and above 50 keV, respectively. Various intermediate cavity theory methods are reviewed. The influence of microscopic inhomogeneities in the surrounding environment on the nuclear dose and the importance of the nucleus as a target for radiation-induced cell death emphasizes the potential importance of cellular dosimetry for understanding radiation effects.
2.       “Sources of uncertainty in composite field delivery for the Cyberknife radiosurgery system”
Eric Vandervoort – The Ottawa Hospital Cancer Centre

Abstract: In recent years, stereotactic ablative radiosurgery (SABR) has moved from using rigid frames fixed to a patient’s skull to the use of non-invasive frameless techniques requiring in-room image guidance. The Cyberknife, consisting of a compact linear accelerator mounted to an industrial robotic arm, is one such SABR system which has been in use at the Ottawa hospital since 2010.  This system delivers highly conformal radiation dose by employing many (typically > 100) small aperture (5 to 60 mm in diameter) radiation fields from many different non-coplanar directions. The central axes of these beams may share a common point of intersection (isocentric) and provide highly-conformal spherically-shaped radiation dose distributions similar to those delivered using arc therapy with cones on a conventional LINAC. The vast bulk of Cyberknife treatments, however, treat arbitrary shaped tumours using hundreds of non-isocentric beams with their central axes directed at points on the exterior surface of a target. The Cyberknife robotic radiosurgery system also employs a complex motion prediction algorithm to compensate for respiratory motion in extracranial treatments. Measurement and simulated results for single detectors and film in a phantom geometry for these isocentric and non-isocentric composite fields will be discussed. The initial calibration of the robot treatment positions, static field commissioning, and the tests employed to monitor and maintain delivery accuracy will be described with a focus on the differences between the Cyberknife and conventional gantry mounted LINAC systems, along with sources of error and opportunities for further investigation.