Image guided radiation therapy: it is only the beginning

Image guided radiation therapy (IGRT) is a hot topic in radiation oncology today but it is not an entirely new concept. Imaging has been part of the set-up verification for radiation therapy patients for many years and there is no hard line in the sand between port films (or electronic portal images (EPI)) and modern IGRT. However, we would argue that there has been a significant qualitative change with the focus on actual structures of interest and very frequent high quality imaging. One of the first integrated radiographic image guidance devices was reported in 1958 in a clinic in Canada. It was a portable x-ray machine that was mounted on the counterweight of a cobalt-60 Theratron machine. In Australia in 1995, an x-ray unit was combined with a linear accelerator such that a 37° rotation would bring the x-ray source into the exact position of the megavoltage (MV) beam. It had a mobile image intensifier with digital storage and processing facilities and thus the prospect of real time imaging. The impetus behind these devices was the poor quality of port films produced both from a cobalt source and MV radiation. It is interesting to note that neither of the innovations was accepted into clinical practice at the time, probably because they were cumbersome and time consuming to operate. Portal imaging and EPI remained the tool of choice; largely to immobilise the patient (according to skin marks and bones) rather than to verify the position of the actual tumour. The resurgence and availability of image guidance devices is probably due to both the high quality of image that is produced and that they have been seamlessly and unobtrusively integrated into our treatment rooms. The onslaught of IGRT is opening many opportunities for radiation therapists and radiographers to collaborate and as such is a very appropriate topic for this journal. The Trans Tasman Radiation Oncology Group (TROG) IGRT quality assurance (QA) working party has adopted the following definition: “IGRT is radiotherapy based on data pertaining to the relationship between beam and patient geometry acquired at the point of treatment delivery with the intent to ensure geometric accuracy of radiation delivery appropriate to the clinical scenario.” The distinguishing factor of IGRT is the transition from conventional verification imaging of the skin and bones infrequently during the course of radiation therapy, to visualising structures of interest (surrogates such as fiducial markers for prostate and lung, surgical clips for breast) every day and/or the actual soft tissue tumour volume (bladder, post-prostatectomy site and lung are examples) as well as other normal tissues surrounding the tumour. There are several key issues that need to be considered when discussing IGRT: 1 The role of imaging within the overall treatment chain particularly in relation to the imaging used for treatment planning and reference image generation 2 Imaging modalities and the associated training requirements for staff 3 Applications ranging from positioning to adaptive radiotherapy and motion management 4 Risks associated with IGRT.