New approaches to improving the accuracy and outcome of Intensity Modulated Radiotherapy

Cowley, Ian Richard (2006). New approaches to improving the accuracy and outcome of Intensity Modulated Radiotherapy. PhD thesis The Open University.



Several aspects of the IMRT treatment chain are evaluated with a view to improving the accuracy with which the treatment can be delivered and also the radiobiological outcome for the patient. A safe, non-invasive way of evaluating patient repositioning for multi-fraction treatments is developed and presented, with results highlighting the head-and-neck immobilisation system that provides the best results at Addenbrooke's radiotherapy centre. Previously published dose evaluation metrics are studied in depth, identifying ways in which they differ. A new method of calculating the popular Gamma index is proposed, along with a new Kappa index which can highlight areas of dose mismatch more readily than the Gamma index. Using a specially developed dose calculation system, systematic errors found with the IMRT treatment machines at Addenbrooke's are simulated to determine the dose differences created by these errors, which are then evaluated using the Kappa index with typical tolerances used by previous authors. The concept of equivalent uniform dose (EUD) is then used to assess whether the tolerances for distance and dose are equivalent to each other. They are found not to be equivalent, and better relative values of the two quantities are suggested for use in composite indices. The use of the EUD is extended further into analysing the equivalent dose differences of the treatment delivery errors previously calculated. It is found that the equivalent dose to the target volume is changed by more than the composite indices indicate. The EUD is then used to calculate normal tissue complication probabilities, the probability of uncomplicated control and therapeutic gain. It is found that the dose errors predicted can actually be beneficial to the patient outcome, although not always. It is concluded that dose evaluation for complex treatments such as IMRT should be more radiobiologically-based in order to assess patient outcomes properly.

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