Student presenter: Meaghen Shiha, PhD Candidate (Supervisors: Dr. Emily Heath, Dr. Eric Vandervoort)
Title: Impact of intra-fraction motion on spine SBRT
Abstract: The spine is one of the most common sites for cancer metastases. These metastases are often painful and can lead to complications such as vertebral fractures and spinal cord compression. Stereotactic Body Radiation Therapy (SBRT), where large doses are delivered in a small number of treatment fractions to a vertebral target, is a very effective way to control pain for these types of metastases. Due to the high doses delivered and proximity to the spinal cord, there are stringent patient immobilization and intra-fraction imaging requirements for SBRT delivery. This presentation will present an evaluation of two different immobilization systems and discuss the development of a tool to model the dosimetric impact of immobilization for VMAT treatments.
Guest speaker: Dr. Devin Hymers, Postdoctoral Fellow, Metrology Research Centre, National Research Council Canada
Title: The fIVI Range Monitoring System
Abstract: Carbon ion radiotherapy delivers a precise and highly conformal dose distribution, which maximizes dose to the target while minimizing the dose to surrounding healthy tissue, and particularly to radiosensitive organs at risk. However, inherent uncertainties in dose delivery limit the extent to which these advantages may be utilized. Range monitoring methods, such as Interaction Vertex Imaging (IVI), provide feedback on the position of the carbon ion beam within the patient, and have the potential to allow more conformal dose distributions which take full advantage of the sharp dose maximum, or Bragg peak, produced when an ion beam stops inside a patient. The prototype filtered IVI (fIVI) Range Monitoring System is the first device to apply large-area (36 cm2 and 72 cm2) silicon sensors to the online monitoring of carbon ion radiotherapy treatments. This system consists of two layers of thin, double-sided strip-segmented silicon detectors, and associated fast readout electronics, to monitor charged particles produced by beam- patient interactions during irradiation. To validate this prototype for the demanding clinical environment, initial commissioning tests were performed using sources and low-energy ion beams. These tests were followed by measurement of clinical beams of varying range in human- sized plastic phantoms, modelling a treatment. In addition to offline analysis, these clinical measurements were saved for real-time playback, to allow assessment of the online acquisition and analysis performance. This promising system meets and exceeds the demands of clinical irradiation. It is capable of accepting count rates beyond 1.0 MHz with negligible losses due to pileup, and extremely low random coincidence rates. During clinical irradiation of a target representing the human head, Bragg peak depth differences on the order of 1.0 mm could be distinguished; with the collection of additional data modelling a larger full-scale sensor array, sub-millimeter precision is possible. These range differences can consistently be determined less than 200 ms after the end of irradiation, as the majority of data processing occurs concurrently with data collection. This setup extends the millimetric precision achieved by prior IVI studies to the more challenging and clinically important case of shallow Bragg peak positions. The speed of online monitoring is comparable to required beam pauses for a change in energy, making it feasible to detect the presence of a range error and abort irradiation prior to delivery of the next beam in the treatment plan. This combination of fast online analysis and high precision enabled by the fIVI Range Monitoring System provides the potential to shrink margins which account for dose delivery uncertainty, by providing an additional safety feature in clinical irradiation. This margin reduction would allow further sparing of healthy tissue and limit the risk of complications in carbon ion radiotherapy while maintaining the same rate of treatment success.