Eric Christiansen and Dmitry Klokov

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Thursday, March 26, 2015

Time:   3:30 - 5:00 pm

Location:  Hospital Auditorium, 2nd floor, General Campus (escalator from main lobby to 2nd floor, follow signage)


1. "Clinical field ionization chamber correction factors for the CyberKnife radiosurgery system"
Eric Christiansen - McGill University
Abstract: A proposed dosimetry formalism for small and non-standard fields involves the determination of ionization chamber correction factors to convert the dose measured in a clinical plan to the dose associated with either a static machine-specific reference (MSR) field, and/or a plan-class specific reference (PCSR) composite field being as close as possible to a class of clinical plans of interest.  The aim of this work is to determine detector correction factors for a number of representative Cyberknife treatments, for two microchambers and a commercial plastic scintillator.  First, a linac head model was created in BEAMnrc by matching percentage depth dose curves and output factors measured using an A16 microchamber with Monte Carlo calculations performed in egs_chamber to explicitly model detector response.  Using this model, intermediate correction factors related to the non-water components of each detector were calculated for the microchambers and the plastic scintillator in fourteen different composite fields inside a solid water phantom.  Seven of these fields used a 5 mm diameter collimator; the remaining fields employed a 7.5 mm collimator but were otherwise identical to the first seven.  It was determined that large and variable CFs are required for microchambers in small composite fields primarily due to density and volume effects.  Corrections are reduced but not eliminated for a plastic scintillator in the same fields.

2. "Effects of low dose gamma-radiation on cellular aging"
Dmitry Klokov - Canadian Nuclear Laboratories
Abstract: The use of ionizing radiation for diagnostic imaging in medicine has increased tremendously over the past three decades, mostly due to rise of the computed tomography technology. Debates continue whether such increased exposures to low doses of ionizing radiation (LDR) may lead to detrimental health effects, such as cancer, later in life. Cellular aging or senescence, a state of irreversible cell cycle arrest, is a means by which damaged cells are prevented from becoming precancerous cells. However, it has recently been linked to cancer due to proinflammatory  signalling elicited by senescent cells which promotes cancer growth. We, therefore, sought to examine whether LDR can affect cellular aging/senescence, which in turn may affect cancer risk. Primary cultures of normal human diploid fibroblasts were exposed to 10, 100 mGy or left untreated at a young age and were maintained under normal growth conditions for 70 days in which 100% of cells normally reach the state of senescence. Different end-points were measured in the course of aging the cell cultures related to: a) senescence markers; b) gene expression; c) cell proliferation; d) microRNA expression. Our results showed that accumulation of senescent cells with time was delayed in LDR-exposed cultures. This was accompanied by an increased rate of proliferation and changes in the expression of genes involved in various cellular pathways, such as DNA repair, cell cycle, DNA damage signaling and epigenetic gene expression regulation. Profiling of 1008 most abundantly expressed microRNA genes revealed a significant number of changes in LDR-exposed vs. control cells. Our results suggest that LDR is capable of suppressing the rate of cellular aging in vitro and that the mechanisms of such effect include gene expression changes mediated by epigenetic microRNA pathways. It is important to validate these results using in vivo mouse models and such studies are currently underway at Canadian Nuclear Laboratories. Extended to the in vivo conditions, these data may be used for evaluating potential health risks in patients exposed to LDR in the medical setting, as well as to nuclear industry professionals or public exposed to LDR as a result of nuclear accidents.