Marc MacKenzie

Area of Study / Keywords

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About

Dr. Marc MacKenzie is currently appointed as Assistant Professor in the Department of Oncology in the Faculty of Medicine & Dentistry.

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Research

My research interests of late have been in the areas of radiation dosimetry, Monte Carlo methods as applied to radiation therapy, and the optimizing of intensity modulated radiation therapy, especially as it pertains to helical tomotherapy.

Radiation Dosimetry
Accurate assessment of the quantity of radiation energy, also denoted as 'radiation dose' or simply 'dose', that is imparted to the patient is key in radiation therapy. Clinical experience dictates that certain levels of radiation dose will achieve the desired tumour control, and that certain levels of radiation to nearby healthy tissues can be tolerated without undue complications. Knowing how much dose both healthy and diseased tissues receive is therefor needed to achieve the desired clinical outcome. As well, quantifying patient response as a function of dose delivered may feed directly into radiobiological modeling, and to ensure that the data collected is of optimal quality requires accurate assessment of dose delivered. Methods which have been developed or refined at our centre involve the use both novel and conventional detectors.

Monte Carlo Methods
The most sophisticated and accurate means of calculating radiation dose and radiation interactions is by direct simulation of individual photons using known interaction probabilities. This technique is known as a Monte Carlo method, and for the accuracy required for most medical physics applications this may involve the simulation of billions of photons. These methods may be used to determine the quantities used in converting measured charge values from ionization chambers to dose, or in determining the response of other novel detectors to therapy beams (e.g. CR plates). Both of these remain active areas of research in medical physics.

Typically, dedicated computers known as treatment planning systems are used to calculate the 3D radiation distribution that will result when a patient radiation treatment plan is implemented. The algorithms used are generally based on radiation dose interactions in water, with correction factors to infer the dose in a patient, and the data is optimized for large radiation field sizes. The calculations that are in wide spread use may break down in certain situations (e.g. small fields, large deviations from water electron density). One may use Monte Carlo methods to simulate the 3D dose imparted to patient from a given treatment plan with greater accuracy than may be achieved by any other means.

Intensity Modulated Radiation Therapy and Inverse Planning
The machines generally used to treat patients with radiation beams have been designed to output broad beams of radiation with 'flat' energy distributions orthogonal to the beam direction. By varying the intensities across the field and by using multiple converging beams with specific non uniform intensity profiles, however, one may achieve superior dose homogeneity to the target structure and improved sparing of nearby structures. This method of delivery is known as Intensity Modulated Radiation Therapy (IMRT), and requires the use of computers and novel algorithms to determine the required profiles to achieve some desired distribution. Inverse planning is the use of these computers in conjunction with optimizations algorithms to iteratively arrive close to the desired dose distribution.

Helical Tomotherapy
Helical tomotherapy (HT) is a novel modality for delivering inverse planned Intensity Modulated Radiation Therapy (IMRT) which enables a highly integrated approach to image guided adaptive radiotherapy. It is 'image guided' because it is a high energy linear accelerator mounted on a CT gantry, which may be run in a mode for imaging and patient set up, and a mode for treatment. HT is also capable of delivering enhanced conformal dose distributions, in part because it delivers a modulated fan beam of radiation continuously while rotating about the patient, instead of from 7-15 fixed gantry angles as in more conventional IMRT. Verification of the accuracy of these highly complex plans must, however, be developed to ensure confidence in treatment delivery within the clinic. We are working to improve the accuracy of our detectors, as well as employing Monte Carlo methods to simulate these treatments. Also, adaptive planning is currently being evaluated, using the daily CT to to delivered on a daily basis.