Use of Electromagnetic Technology for Four-Dimensional Radiotherapy Localisation and Realtime Tumour Tracking
Use of Electromagnetic Technology for Four-Dimensional Radiotherapy Localisation and Realtime Tumour Tracking
European Oncology Review 2005 - July 2005
Published: October 2008
Published: October 2008
Conformal Radio therapy and Target Motion
The past decade has seen a quiet revolution in the technology and procedures underlying radiotherapy. Improvements in imaging technology have combined with both three-dimensiononal conformal radiotherapy (3-D CRT) and intensitymodulated radiation therapy (IMRT) techniques to permit very precise planning and delivery of treatment. Such precision has permitted a number of treatment paradigms to flourish. The incidence of radiation-induced complications from treatment has been reduced, leading to the practice of increasing radiation doses in some tumour sites with potential improvements in tumour control, and in other sites to reduced morbidity from radiotherapy. A growing area of interest, spurred in part by this revolution in precision, involves the delivery of large doses of radiation in a few sessions to certain types of tumours. Such hypofractionation has been tested for intracranial applications as well as accelerated partial breast irradiation and in thoracic and intrahepatic tumours. Concomitant with an increase in precision of treatment delivery, very high precision positioning of the patient for treatment is required to maintain potential improvements. The process of radiotherapy treatment requires that a patient be posed and positioned in a reproducible way multiple times over a period ranging from weeks to months. Such accuracy is difficult to achieve. The pursuit of rapid, reliable positioning methods has thus been a major factor driving technological innovation in radiotherapy in recent years.
Positioning Accuracy
The problem of positioning accuracy can be broken down into two major elements immobilisation and localisation. Immobilisation can be considered the process of constraining patient and target motion while treatment is delivered. Localisation describes the processes that ensure the tumour position at treatment matches that intended at the time the treatment was initially planned. A plethora of technology has emerged to aid immobilisation and localisation. A slowly evolving methodology of radiograph-based localisation has recently given way to an explosion of new in-room imaging technology (computed tomography (CT), ultrasound, video). These evolving methodologies improve the ability to visualise the location of soft tissue structures in the body. Their limitation, however, is that they require time to acquire and process the significant volume of information needed to image, analyse and act in order to improve target position. Some target sites are not completely amenable to the long-term (10- to 30- minute) immobilisation that would permit in-room imaging to work effectively. Physiological motions exist on timeframes of seconds (breathing) to minutes (e.g. prostate movement) that would cause the relative position of tumours to deviate from where they were estimated to be on pre-treatment imaging models. Furthermore, such motions generally involve shape and position changes of adjacent organs and tissues, leading to complex deformations of the patient in the region to be treated. Accurately targeting a tumour in the presence of such changes requires use of advanced alignment technology that is not yet mature.
Fiducial-based Target Localisation with X-ray Images
An alternative method of targeting has arisen in parallel with improved imaging. Implanted fiducial markers can identify the target position using 2-D X-ray images. Typically, such markers have been made of metal of sufficient density as to be visualised on megavoltage radiographs. With recent improvements due to the availability of kilovoltage X-ray imaging in treatment rooms, such markers can be visualised fluoroscopically, providing tumour-based positioning and position monitoring during treatment. The cost of such procedures is a significant investment in imaging equipment, as well as potentially high radiation doses to the skin from extended fluoroscopic procedures.
References:
- Balter J M, Wright J N, Newell L J, Friemel B, Dimmer S, Cheng Y, Wong J, Vertatschitsch E, Mate T P, "Accuracy of a wireless localization system for radiotherapy", Int. J. Radiat. Oncol. Biol. Phys. (2005);61 (3): pp. 933-937.
