Medical Physics

 

Research Activities in the Department of Medical Physics, Royal Adelaide Hospital

The Department has two distinct research paths: broad research activities in medical physics as opportunities arise and a focused research activity in conjunction with the Department of Radiation Oncology RAH into Individualized Patient Treatments. A brief outline of each area is given below. More detailed information can be found from the List of Publications.

Individualized Patient Treatment Research

Radiobiological modelling (E Bezak, W Tuckwell, E Yeoh).

Many physical principles are applicable in the analysis of tumour development and regress with therapy. This program is applying Monte Carlo techniques to model the growth of a tumour from a single cell. Similar techniques are then being applied to examine the response of the tumour to radio and chemotherapy. The effect of hypoxia on tumour growth and its response to radiotherapy is of primary interest to us ate present.

Dose verification (E Bezak, M Bhat, R Nelligan, P Reich, T Pham)

Radiotherapy is currently based on a predictive methodology: therapy beam parameters are measured and modelled by the planning computer to predict the dose distribution within the patient. However, as all physicists know, there are experimental uncertainties. Therefore the dose delivered to the patient may not be as precise as the theory predicts. Three basic areas of research in dose verification are being pursued:
a. optical readout of ferrous sulphate doped agarose gels (gel dosimetry);
b. on-line patient entrance and exit dose dosimetry with MOSFETs and,
c. the use of electronic portal imaging devices to record and back project the dose delivered in conformal and intensity modulated radiotherapy.

Normal Tissue Complications and Second Cancer Risk Assessment (E Bezak, R Takam, E Yeoh)

There are various radiotherapy modalities that can be chosen for treatment of prostate cancer, including external beam radiotherapy and brachytherapy. The main therapeutic aim of all radiotherapy procedures is to maximize damage to the tumour and, at the same time, damage to the surrounding organs must be kept as small as possible. In addition to normal tissue complications, there is another factor which should also be taken into consideration when the radiotherapy is used for prostate cancer treatment: the risk of developing a fatal second primary cancer. The risk of severe normal tissue complications as well as estimates of risk of second cancers is being investigated in this project.

Medical Physics Research

Proton Therapy modelling

The Radio-Biological Effectiveness (RBE) per unit dose of high energy protons is almost equal to that of high energy photons and electrons. Clinically, RBE of 1.1 has often been used for protons in SOBP. Protons of low energy, in the region of a few tens MeV are more densely ionizing so the Linear Energy Transfer (LET) increases and as a consequence also RBE. As only a small part of total energy of a high energy proton is deposited with this high LET the clinical importance of this increase near the end of the range is considered to be small. However, the increased RBE (1.25) at the distal fall-off of the SOBP, if this is close to a risk organ, may be of clinical relevance. This has not yet been fully explored and there is with present planning systems, no possibility to of taking these effects into account.  Monte Carlo calculations are used to investigate the dose deposited by protons at the distal end of Bragg peak.

Mammosite Uncertainties (E Bezak, S Bensaleh, M Borg)

MammoSite brachytherapy is an accelerated partial breast irradiation (boost phase) for early stage breast cancer following conservative surgery. Potential advantages of MammoSite brachytherapy are: high localised dose with rapid falloff for normal tissue sparing, minimum delay between surgery and RT, catheter moves with breast, improved local control, no exposure to staff (compared to LDR), likely side-effects reduction and potential cost and time saving (eg for country patients).  However treatment uncertainties involved in MammoSite brachytherapy and their effect on dose distribution and irradiation of normal surrounding healthy tissues have not been fully investigated. These include: balloon contrast concentration, balloon deformation, accuracy of source positioning, patient breathing motion. Monte Carlo code EGS4nrc is being used to model these uncertainties. In addition, TLD LiF chips are used in a Rando phantom to measure the doses delivered directly.

Monte Carlo simulation of photon transport (K Quach, T Pham, S Bensaleh)

Accurate modelling of the dose delivered to a medium from high energy x-ray requires Monte Carlo simulation techniques. As Monte Carlo techniques are highly versatile, the project is generic in nature and addresses broadly based projects requiring the support of accurate dosimetry.