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The use of radiotherapy to treat brain cancer raises serious challenges. The aim of radiotherapy is to deliver a lethal dose of radiation to the cancer while minimising the dose to adjacent healthy (non-cancerous) tissues. The application of radiation to healthy brain tissue has profound neurological implications. Consequently, the reliable and effective use of very small radiation fields, which specifically conform to the tumour area and spare a high proportion of healthy tissue, is a major clinical goal.

A new method for producing such fields, which is currently being adopted in radiation oncology departments in Australia (including Brisbane), is to fit a micro-multileaf collimator (MMLC) to a photon-beam linear accelerator and use this to perform stereotactic radiosurgery. The use of such a novel technique to treat such an important and sensitive organ requires rigorous prior analysis and verification. However, available dose monitoring apparatus are physically limited in the precision with which they can measure small fields. This has restricted the ability of clinical treatments to achieve the theoretical capabilities of the MMLC.

This project, therefore, aims to develop computer simulations of MMLC radiotherapy fields as a means to evaluate the dose delivered in current treatments and test the effects of potential new treatments in a virtual environment. Specifically, this project aims to develop an accurate Monte Carlo computer simulation of the radiotherapy x-ray fields from a Varian linear accelerator collimated by the BrainLab m3 MMLC, which can be combined with patient CT data to calculate the dose delivered in a given treatment. The calculated data will then be used to:

  • Quantify the degree to which existing stereotactic radiosurgical treatments are adhering to their treatment prescriptions;
  • Independently verify the doses predicted in treatment planning calculations;
  • Investigate the potential for improving existing stereotactic radiosurgical treatments or applying the equipment to currently untreatable pathologies; and
  • Retrospectively investigate the causes of adverse outcomes by evaluating the radiation doses actually delivered in stereotactic radiotherapy treatments.

Computer simulations generated using the Monte Carlo technique precisely model the interactions of billions of individual ionising particles to produce results which do not rely on approximations or correction factors. Monte Carlo simulations have therefore become the gold standard for radiotherapy dose calculation. If the radiation beam from the MMLC is calculated from Monte Carlo data, then dose characteristics of very small stereotactic radiotherapy fields can be determined. This will significantly reduce uncertainties in treatment planning, particularly in treatments utilising small fields close to critical anatomy, promoting enhanced utilisation of MMLC capabilities including implementation of new treatment approaches.

The tools and techniques developed through the completion of this project will be applicable to the broader problem of verifying small field doses. This an important issue confronting those Queensland radiation oncology centres which are currently in the process of developing treatment protocols for the future delivery of intensity modulated radiotherapy (IMRT), a form of treatment where all of the beams delivered are divided into small segments.  The Monte Carlo code developed during this project will also constitute a valuable tool for clinical decision making, allowing some cancers to be treated more aggressively, while reducing the risk of impairing neurological or biological function.

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