Image-guided radiation therapy

In general, at the time of 'planning' (whether a clinical mark up or a full simulation) the intended area for treatment is outlined by the radiation oncologist. Once the area of treatment was determined, marks were placed on the skin. The purpose of the ink marks was to align and position the patient daily for treatment to improve reproducibility of field placement. By aligning the markings with the radiation field (or its representation) in the radiation therapy treatment room, the correct placement of the treatment field could be identified.[8]

Over time, with improvement in technology – light fields with cross hairs, isocentric lasers – and with the shift to the practice of 'tattooing' – a procedure where ink markings are replaced with a permanent mark by the application of ink just under the first layer of skin using a needle in documented locations - the reproducibility of the patient's setup improved.[9]

Portal imaging is the acquisition of images using a radiation beam that is being used for giving radiation treatment to a patient.[10] If not all of the radiation beam is absorbed or scattered in the patient, the portion that passes through may be measured and used to produce images of the patient.

It is difficult to establish the initial use of portal imaging to define radiation field placement. From the early days of radiation therapy, X-rays or gamma rays were used to develop large format radiographic films for inspection. With the introduction of cobalt-60 machines in the 1950s, radiation went deeper inside the body, but with lower contrast and poor subjective visibility. Today, using advancements in digital imaging devices, the use of electronic portal imaging has developed into both a tool for accurate field placement and as a quality assurance tool for review by radiation oncologists during check film reviews.[8]

Electronic portal imaging is the process of using digital imaging, such as a CCD video camera, liquid ion chamber and amorphous silicon flat panel detectors to create a digital image with improved quality and contrast over traditional portal imaging. The benefit of the system is the ability to capture images, for review and guidance, digitally.[11] These systems are in use throughout clinical practice.[12] Current reviews of Electronic Portal Imaging Devices (EPID) show acceptable results in imaging irradiations and in most clinical practice, provide sufficiently large fields-of-view. kV is not a portal imaging feature.[4]

Fluoroscopy is an imaging technique that uses a fluoroscope, in coordination with either a screen or image-capturing device to create real-time images of patients' internal structures.

Digital X-ray equipment mounted in the radiation treatment device is often used to picture the patient’s internal anatomy at time before or during treatment, which then can be compared to the original planning CT series. Usage of an orthogonal set-up of two radiographic axes is common, to provide means for highly accurate patient position verification.[5]

A medical imaging method employing tomography where digital geometry processing is used to generate a three-dimensional image of the internal structures of an object from a large series of two-dimensional X-ray images taken around a single axis of rotation. CT produces a volume of data, which can be manipulated, through a process known as windowing, in order to demonstrate various structures based on their ability to attenuate and prevent transmission of the incident X-ray beam.

With the growing recognition of the utility of CT imaging in using guidance strategies to match treatment volume position and treatment field placement, several systems have been designed that place an actual conventional 2-D CT machine in the treatment room alongside the treatment linear accelerator. The advantage is that the conventional CT provides accurate measure of tissue attenuation, which is important for dose calculation (e.g. CT on rails).[8]

Cone-beam computed tomography (CBCT) based image guided systems have been integrated with medical linear accelerators to great success. With improvements in flat-panel technology, CBCT has been able to provide volumetric imaging, and allows for radiographic or fluoroscopic monitoring throughout the treatment process. Cone beam CT acquires many projections over the entire volume of interest in each projection. Using reconstruction strategies pioneered by Feldkamp, the 2D projections are reconstructed into a 3D volume analogous to the CT planning dataset.

Megavoltage computed tomography (MVCT) is a medical imaging technique that uses the Megavoltage range of X-rays to create an image of bony structures or surrogate structures within the body. The original rational for MVCT was spurred by the need for accurate density estimates for treatment planning. Both patient and target structure localization were secondary uses. A test unit using a single linear detector, consisting of 75 cadmium tungstate crystals, was mounted on the linear accelerator gantry. The test results indicated a spatial resolution of .5mm, and a contrast resolution of 5% using this method. While another approach could involve integrating the system directly into the MLA, it would limit the number of revolutions to a number prohibitive to regular use.

Optical tracking entails the use of a camera to relay positional information of objects within its inherent coordinate system by means of a subset of the electromagnetic spectrum of wavelengths spanning ultra-violet, visible, and infrared light. Optical navigation has been in use for the last 10 years within image-guided surgery (neurosurgery, ENT, and orthopaedic) and has increased in prevalence within radiotherapy to provide real-time feedback through visual cues on graphical user interfaces (GUIs). For the latter, a method of calibration is used to align the camera's native coordinate system with that of the isocentric reference frame of the radiation treatment delivery room. Optically tracked tools are then used to identify the positions of patient reference set-up points and these are compared to their location within the planning CT coordinate system. A computation based on least-squares methodology is performed using these two sets of coordinates to determine a treatment couch translation that will result in the alignment of the patient's planned isocenter with that of the treatment room. These tools can also be used for intra-fraction monitoring of patient position by placing an optically tracked tool on a region of interest to either initiate radiation delivery (i.e. gating regimes) or action (i.e. repositioning). Alternatively, products such as AlignRT (from Vision RT) allow for real time feedback by imaging the patient directly and tracking the skin surface of the patient.

The first clinically active MRI-guided radiation therapy machine, the ViewRay device, was installed in St. Louis, MO, at the Alvin J. Siteman Cancer Center at Barnes-Jewish Hospital and Washington University School of Medicine. Treatment of the first patients was announced in February 2014.[13] Other radiation therapy machines which incorporate real-time MRI tracking of tumors are currently in development. MRI-guided radiation therapy enables clinicians to see a patient's internal anatomy in real-time using continual soft-tissue imaging and allows them to keep the radiation beams on target when the tumour moves during treatment.[14]

Ultrasound is used for daily patient setup. It is useful for soft tissue such as the breast and prostate. The BAT (Best Nomos) and Clarity (Elekta) systems are the two main systems currently being used. The Clarity system has been further developed to enable intra-fraction prostate motion tracking via trans-perineal imaging.

While not IGRT per se, electromagnetic transponder systems seek to serve exactly the same clinical function as CBCT or kV X-ray, yet provide for more temporally continuous analysis of setup error analogous to that of the optical tracking strategies. Hence, this technology (although entailing the use of no "images") is usually classified as an IGRT approach.

The On-line strategy makes adjustment to patient and beam position during the treatment process, based on continuously updated information throughout the procedure.[8] The on-line approach requires a high-level of integration of both software and hardware. The advantage of this strategy is a reduction in both systematic and random errors. An example is the use of a marker-based program in the treatment of prostate cancer at Princess Margaret Hospital. Gold markers are implanted into the prostate to provide a surrogate position of the gland. Prior to each day's treatment, portal imaging system results are returned. If the center of the mass has moved greater than 3mm, then the couch is readjusted and a subsequent reference image is created.[4] Other clinics correct for any positional errors, never allowing for >1 mm error in any measured axes.

The Off-line strategy determines the best patient position through accumulated data gathered during treatment sessions, almost always initial treatments. Physicians and staff measure the accuracy of treatment and devise treatment guidelines during using information from the images. The strategy requires greater coordination than on-line strategies. However, the use of off-line strategies does reduce the risk of systematic error. The risk of random error may still persist, however.