Assessment of and improvements to a stereophotogrammetric patient positioning system for proton therapy

Master Thesis


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University of Cape Town

This thesis describes the construction and use of the facemask at the National Accelerator Centre (NAC) as used to both immobilise and position patients for precision proton radiotherapy. The precision achieved using the stereophotogrammetric (SPG) positioning system is measured, and the shortcomings and errors in using the facemask by the SPG system are measured and analysed. The implementation of improvements made to the SPG system is reported upon, and alternative means of both supporting the fiducial markers and immobilising the patient are investigated and evaluated. The accuracy of positioning a facemask using the SPG system is 1.4 mm and of positioning a newly designed frame is 1.6 mm. These measurements were made without using a patient. It is estimated that the total uncertainty of positioning a patient's tumour at the isocentre is 1.6 (1SD) mm using the facemask and it is estimated that the precision using the frame will be less than this value. The largest component of this error (1.39 mm) is due to the error in obtaining the CT scanner co-ordinates. These results are comparable to those obtained by other investigators. The movement of patient bony landmarks within the facemask was measured to be 1.0 ± 0.8 mm. Three main recommendations are that the CT scanner co-ordinating procedure be improved, the SPG computer program be rewritten in parts to achieve greater speed and accuracy, and that the new frame be used. The frame is easier to manufacture than the facemask and allows real time monitoring of the position of the patient's head by the SPG system thus allowing faster throughput of patients and better positioning quality control.

Summary in English.

Bibliography: pages 125-129.