Image-guided radiation therapy

Image-guided radiation therapy
Image-guided radiation therapy
Other names	IGRT
Specialty	Interventional radiology, oncology
	[ edit on Wikidata]

Last updated November 29, 2024

Image-guided radiation therapy (IGRT) is the process of frequent imaging, during a course of radiation treatment, used to direct the treatment, position the patient, and compare to the pre-therapy imaging from the treatment plan.^[1] Immediately prior to, or during, a treatment fraction, the patient is localized in the treatment room in the same position as planned from the reference imaging dataset. An example of IGRT would include comparison of a cone beam computed tomography (CBCT) dataset, acquired on the treatment machine, with the computed tomography (CT) dataset from planning. IGRT would also include matching planar kilovoltage (kV) radiographs or megavoltage (MV) images with digital reconstructed radiographs (DRRs) from the planning CT.

This process is distinct from the use of imaging to delineate targets and organs in the planning process of radiation therapy. However, there is a connection between the imaging processes as IGRT relies directly on the imaging modalities from planning as the reference coordinates for localizing the patient. The variety of medical imaging technologies used in planning includes x-ray computed tomography (CT), magnetic resonance imaging (MRI), and positron emission tomography (PET) among others.

IGRT can help to reduce errors in set-up and positioning, allow the margins around target tissue when planning to be reduced, and enable treatment to be adapted during its course, with the aim of overall improving outcomes.^[2]^[3]

Goals and clinical benefits

The goal of the IGRT process is to improve the accuracy of the radiation field placement, and to reduce the exposure of healthy tissue during radiation treatments. In years past, larger planning target volume (PTV) margins were used to compensate for localization errors during treatment.^[4] This resulted in healthy human tissues receiving unnecessary doses of radiation during treatment. PTV margins are the most widely used method to account for geometric uncertainties. By improving accuracy through IGRT, radiation is decreased to surrounding healthy tissues, allowing for increased radiation to the tumour for control.^[4]

Currently, certain radiation therapy techniques employ the process of intensity-modulated radiotherapy (IMRT). This form of radiation treatment uses computers and linear accelerators to sculpt a three-dimensional radiation dose map, specific to the target's location, shape and motion characteristics. Because of the level of precision required for IMRT, detailed data must be gathered about tumour locations. The single most important area of innovation in clinical practice is the reduction of the planning target volume margins around the location. The ability to avoid more normal tissue (and thus potentially employ dose escalation strategies) is a direct by-product of the ability to execute therapy with the most accuracy.^[4]

Modern, advanced radiotherapy techniques such as proton and charged particle radiotherapy enable superior precision in the dose delivery and spatial distribution of the effective dose. Today, those possibilities add new challenges to IGRT, concerning required accuracy and reliability.^[5] Suitable approaches are therefore a matter of intense research.

IGRT increases the amount of data collected throughout the course of therapy. Over the course of time, whether for an individual or a population of patients, this information will allow for the continued assessment and further refinement of treatment techniques. The clinical benefit for the patient is the ability to monitor and adapt to changes that may occur during the course of radiation treatment. Such changes can include tumor shrinkage or expansion, or changes in shape of the tumor and surrounding anatomy.^[4]

The precision of IGRT is significantly improved when technologies that were originally developed for image-guided surgery, such as the N-localizer ^[6] and Sturm-Pastyr localizer,^[7] are used in conjunction with these medical imaging technologies. SRT provides a Non-Surgical Alternative for Non-Melanoma Skin Cancer & an Effective Solution for Keloids.

Sensus Healthcare is a manufacturer and distributor of this device With a compact 30”x30” footprint, the mobile unit delivers a precise and calibrated dose of SRT that penetrates only five millimeters below the skin’s surface—making it one of the safest and most effective alternative cancer treatments available. Unlike more powerful radiotherapy devices, the SRT-100™ carefully destroys malignant skin cancer cells while preserving healthy tissue.

Rationale

Radiation therapy is a local treatment that is designed to treat the defined tumour and spare the surrounding normal tissue from receiving doses above specified dose tolerances. There are many factors that may contribute to differences between the planned dose distribution and the delivered dose distribution. One such factor is uncertainty in patient position on the treatment unit. IGRT is a component of the radiation therapy process that incorporates imaging coordinates from the treatment plan to be delivered in order to ensure the patient is properly aligned in the treatment room.^[8]

The localization information provided through IGRT approaches can also be used to facilitate robust treatment planning strategies and enable patient modelling, which is beyond the scope of this article.^{[ citation needed ]}

History of "guidance" for treatment

Surface and skin marks

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

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

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]

Imaging for treatment guidance

Fluoroscopy

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

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]

Computed tomography (CT)

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.

Conventional CT

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

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.

MVCT

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.^{[ citation needed ]} 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^{[ clarification needed ]}, it would limit the number of revolutions to a number prohibitive to regular use.^{[ citation needed ]}

Optical tracking

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.

MRI

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

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.

Electromagnetic transponders

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.

Correction strategies for patient positioning during IGRT

There are two basic correction strategies used while determining the most beneficial patient position and beam structure: on-line and off-line correction. Both serve their purposes in the clinical setting, and have their own merits. Generally, a combination of the both strategies is employed. Often, a patient will receive corrections to their treatment via on-line strategies during their first radiation session, and physicians make subsequent adjustments off-line during check film rounds.^[4]

On-line

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.

Off-line

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.

Future areas of study

The debate between the benefits of on-line versus off-line strategies continues to be contended.
Whether further research into biological functions and movements can create a better understanding of tumor movement in the body before, between and during treatment.
When rules or algorithms are used, large variations in PTV margins can be reduced. Margin "recipes" are being developed that will create linear equations and algorithms that account for "normal" variations. These rules are created from a normal population, and are applied to the treatment plan off-line. Possible side effects include random errors from uniqueness of the target
With a greater amount of data being collected, how systems must will be established for the categorizing and storing of information.

Related Research Articles

Radiation therapy or radiotherapy is a treatment using ionizing radiation, generally provided as part of cancer therapy to either kill or control the growth of malignant cells. It is normally delivered by a linear particle accelerator. Radiation therapy may be curative in a number of types of cancer if they are localized to one area of the body, and have not spread to other parts. It may also be used as part of adjuvant therapy, to prevent tumor recurrence after surgery to remove a primary malignant tumor. Radiation therapy is synergistic with chemotherapy, and has been used before, during, and after chemotherapy in susceptible cancers. The subspecialty of oncology concerned with radiotherapy is called radiation oncology. A physician who practices in this subspecialty is a radiation oncologist.

A brain tumor occurs when abnormal cells form within the brain. There are two main types of tumors: malignant (cancerous) tumors and benign (non-cancerous) tumors. These can be further classified as primary tumors, which start within the brain, and secondary tumors, which most commonly have spread from tumors located outside the brain, known as brain metastasis tumors. All types of brain tumors may produce symptoms that vary depending on the size of the tumor and the part of the brain that is involved. Where symptoms exist, they may include headaches, seizures, problems with vision, vomiting and mental changes. Other symptoms may include difficulty walking, speaking, with sensations, or unconsciousness.

External beam radiation therapy (EBRT) is a form of radiotherapy that utilizes a high-energy collimated beam of ionizing radiation, from a source outside the body, to target and kill cancer cells. A radiotherapy beam is composed of particles which travel in a consistent direction; each radiotherapy beam consists of one type of particle intended for use in treatment, though most beams contain some contamination by other particle types.

A radiation oncologist is a specialist physician who uses ionizing radiation in the treatment of cancer. Radiation oncology is one of the three primary specialties, the other two being surgical and medical oncology, involved in the treatment of cancer. Radiation can be given as a curative modality, either alone or in combination with surgery and/or chemotherapy. It may also be used palliatively, to relieve symptoms in patients with incurable cancers. A radiation oncologist may also use radiation to treat some benign diseases, including benign tumors. In some countries, radiotherapy and chemotherapy are controlled by a single oncologist who is a "clinical oncologist". Radiation oncologists work closely with other physicians such as surgical oncologists, interventional radiologists, internal medicine subspecialists, and medical oncologists, as well as medical physicists and technicians as part of the multi-disciplinary cancer team. Radiation oncologists undergo four years of oncology-specific training whereas oncologists who deliver chemotherapy have two years of additional training in cancer care during fellowship after internal medicine residency in the United States.

<span class="mw-page-title-main">Brachytherapy</span> Type of radiation therapy

Brachytherapy is a form of radiation therapy where a sealed radiation source is placed inside or next to the area requiring treatment. The word "brachytherapy" comes from the Greek word βραχύς, brachys, meaning "short-distance" or "short". Brachytherapy is commonly used as an effective treatment for cervical, prostate, breast, esophageal and skin cancer and can also be used to treat tumours in many other body sites. Treatment results have demonstrated that the cancer-cure rates of brachytherapy are either comparable to surgery and external beam radiotherapy (EBRT) or are improved when used in combination with these techniques. Brachytherapy can be used alone or in combination with other therapies such as surgery, EBRT and chemotherapy.

A radiation therapist, therapeutic radiographer or radiotherapist is an allied health professional who works in the field of radiation oncology. Radiation therapists plan and administer radiation treatments to cancer patients in most Western countries including the United Kingdom, Australia, most European countries, and Canada, where the minimum education requirement is often a baccalaureate degree or postgraduate degrees in radiation therapy. Radiation therapists can also prescribe medications and radiation, interpret tests results, perform follow ups, reviews, and provide consultations to cancer patients in the United Kingdom and Ontario, Canada . In the United States, radiation therapists have a lower educational requirement and often require postgraduate education and certification in order to plan treatments.

<span class="mw-page-title-main">Proton therapy</span> Medical Procedure

In medicine, proton therapy, or proton radiotherapy, is a type of particle therapy that uses a beam of protons to irradiate diseased tissue, most often to treat cancer. The chief advantage of proton therapy over other types of external beam radiotherapy is that the dose of protons is deposited over a narrow range of depth; hence in minimal entry, exit, or scattered radiation dose to healthy nearby tissues.

Radiosurgery is surgery using radiation, that is, the destruction of precisely selected areas of tissue using ionizing radiation rather than excision with a blade. Like other forms of radiation therapy, it is usually used to treat cancer. Radiosurgery was originally defined by the Swedish neurosurgeon Lars Leksell as "a single high dose fraction of radiation, stereotactically directed to an intracranial region of interest".

Stereotactic surgery is a minimally invasive form of surgical intervention that makes use of a three-dimensional coordinate system to locate small targets inside the body and to perform on them some action such as ablation, biopsy, lesion, injection, stimulation, implantation, radiosurgery (SRS), etc.

<span class="mw-page-title-main">Dental radiography</span> X-ray imaging in dentistry

Dental radiographs, commonly known as X-rays, are radiographs used to diagnose hidden dental structures, malignant or benign masses, bone loss, and cavities.

In radiotherapy, radiation treatment planning (RTP) is the process in which a team consisting of radiation oncologists, radiation therapist, medical physicists and medical dosimetrists plan the appropriate external beam radiotherapy or internal brachytherapy treatment technique for a patient with cancer.

<span class="mw-page-title-main">Tomotherapy</span> Type of radiation therapy

Tomotherapy is a type of radiation therapy treatment machine. In tomotherapy a thin radiation beam is modulated as it rotates around the patient, while they are moved through the bore of the machine. The name comes from the use of a strip-shaped beam, so that only one “slice” of the target is exposed at any one time by the radiation. The external appearance of the system and movement of the radiation source and patient can be considered analogous to a CT scanner, which uses lower doses of radiation for imaging. Like a conventional machine used for X-ray external beam radiotherapy, it [the tomotherapy machine] generates the radiation beam, but the external appearance of the machine, patient positioning, and treatment delivery differ. Conventional linacs do not work on a slice-by-slice basis but typically have a large area beam which can also be resized and modulated.

Particle therapy is a form of external beam radiotherapy using beams of energetic neutrons, protons, or other heavier positive ions for cancer treatment. The most common type of particle therapy as of August 2021 is proton therapy.

Cone beam computed tomography is a medical imaging technique consisting of X-ray computed tomography where the X-rays are divergent, forming a cone.

Gel dosimeters, also called Fricke gel dosimeters, are manufactured from radiation sensitive chemicals that, upon irradiation with ionising radiation, undergo a fundamental change in their properties as a function of the absorbed radiation dose.

Four-dimensional computed tomography (4DCT) is a type of CT scanning which records multiple images over time. It allows playback of the scan as a video, so that physiological processes can be observed and internal movement can be tracked. The name is derived from the addition of time to traditional 3D computed tomography. Alternatively, the phase of a particular process, such as respiration, may be considered the fourth dimension.

Jeffrey Harold Siewerdsen is an American physicist and biomedical engineer who is a Professor of Imaging Physics at The University of Texas MD Anderson Cancer Center as well as Biomedical Engineering, Computer Science, Radiology, and Neurosurgery at Johns Hopkins University.He is among the original inventors of cone-beam CT-guided radiotherapy as well as weight-bearing cone-beam CT for musculoskeletal radiology and orthopedic surgery. His work also includes the early development of flat-panel detectors on mobile C-arms for intraoperative cone-beam CT in image-guided surgery. He developed early models for the signal and noise performance of flat-panel detectors and later extended such analysis to dual-energy imaging and 3D imaging performance in cone-beam CT. He founded the ISTAR Lab in the Department of Biomedical Engineering, the Carnegie Center for Surgical Innovation at Johns Hopkins Hospital, and the Surgical Data Science Program at the Institute for Data Science in Oncology at The University of Texas MD Anderson Cancer Center.

A central nervous system tumor is an abnormal growth of cells from the tissues of the brain or spinal cord. CNS tumor is a generic term encompassing over 120 distinct tumor types. Common symptoms of CNS tumors include vomiting, headache, changes in vision, nausea, and seizures. A CNS tumor can be detected and classified via neurological examination, medical imaging, such as x-ray imaging, magnetic resonance imaging (MRI) or computed tomography (CT), or after analysis of a biopsy.

Surface-guided radiation therapy (SGRT) is the process of using 3D imaging to position and track movement of radiation therapy patients during treatment.

Theranostics, also known as theragnostics, is a technique commonly used in personalised medicine. For example in nuclear medicine, one radioactive drug is used to identify (diagnose) and a second radioactive drug is used to treat (therapy) cancerous tumors. In other words, theranostics combines radionuclide imaging and radiation therapy which targets specific biological pathways.

References

↑ "Image-guided Radiation Therapy (IGRT)". RadiologyInfo. Radiological Society of North America. 3 April 2018. Retrieved 21 December 2021.
↑ Korreman, Stine; Rasch, Coen; McNair, Helen; Verellen, Dirk; Oelfke, Uwe; Maingon, Philippe; Mijnheer, Ben; Khoo, Vincent (February 2010). "The European Society of Therapeutic Radiology and Oncology–European Institute of Radiotherapy (ESTRO–EIR) report on 3D CT-based in-room image guidance systems: A practical and technical review and guide". Radiotherapy and Oncology. 94 (2): 129–144. doi:10.1016/j.radonc.2010.01.004. PMID 20153908.
↑ Bujold, Alexis; Craig, Tim; Jaffray, David; Dawson, Laura A. (January 2012). "Image-Guided Radiotherapy: Has It Influenced Patient Outcomes?". Seminars in Radiation Oncology. 22 (1): 50–61. doi:10.1016/j.semradonc.2011.09.001. PMID 22177878.
1 2 3 4 5 6 7 Jaffray, DA; Bissonnette, JP; Craig, T (1999). "X-ray Imaging for Verification and Localization in Radiation Therapy in Modern Technology of Radiation Oncology". The modern technology of radiation oncology : a compendium for medical physicists and radiation oncologists. Madison, Wis.: Medical Physics Pub. ISBN 978-0-944838-38-9.
1 2 Selby, Boris Peter; Walter, Stefan Ottmar; Sakas, Georgios; Wickler, David; Groch, Wolfgang-Dieter; Stilla, Uwe - Full Automatic X-Ray based Patient Positioning and Setup Verification in Practice: Accomplishments and Limitations. Proceedings of the 49th Conference of the Particle Therapy Co-Operative Group (PTCOG). Gunma, Japan, 2010
↑ Galloway, RL Jr. (2015). "Introduction and Historical Perspectives on Image-Guided Surgery". In Golby, AJ (ed.). Image-Guided Neurosurgery. Amsterdam: Elsevier. pp. 2–4. doi:10.1016/B978-0-12-800870-6.00001-7. ISBN 978-0-12-800870-6.
↑ Sturm V, Pastyr O, Schlegel W, Scharfenberg H, Zabel HJ, Netzeband G, Schabbert S, Berberich W (1983). "Stereotactic computer tomography with a modified Riechert-Mundinger device as the basis for integrated stereotactic neuroradiological investigations". Acta Neurochirurgica. 68 (1–2): 11–17. doi:10.1007/BF01406197. PMID 6344559. S2CID 38864553.
1 2 3 4 5 Dawson, Laura A; Sharpe, Michael B (October 2006). "Image-guided radiotherapy: rationale, benefits, and limitations". The Lancet Oncology. 7 (10): 848–858. doi:10.1016/S1470-2045(06)70904-4. PMID 17012047.
↑ Agarwal, Jaiprakash; Munshi, Anusheel; Rathod, Shrinivas (2012). "Skin markings methods and guidelines: A reality in image guidance radiotherapy era". South Asian Journal of Cancer. 1 (1): 27–9. doi: 10.4103/2278-330X.96502 . PMC 3876603 . PMID 24455505.
↑ Langmack, K A (September 2001). "Portal imaging". The British Journal of Radiology. 74 (885): 789–804. doi:10.1259/bjr.74.885.740789. PMID 11560826.
↑ Greer PB, Vial P, Oliver L, Baldock C (2007). "The effect of amorphous silicon EPID spectral response on the dosimetry of IMRT beams". Medical Physics. 34 (11): 4389–4398. doi:10.1118/1.2789406. hdl: 1959.13/33258 . PMID 18072504.
↑ Vial P, Hunt P, Greer PB, Oliver L, Baldock C (2008). "The impact of MLC transmission radiation on EPID dosimetry for dynamic MLC beams". Medical Physics. 35 (4): 1267–1277. doi:10.1118/1.2885368. PMID 18491519.
↑ Imaging Technology News magazine, February 10, 2014, http://www.itnonline.com/article/viewray-mri-guided-radiation-therapy-used-treat-cancer-patients
↑ Siteman Cancer Center News, February 5, 2014 http://www.siteman.wustl.edu/ContentPage.aspx?id=7919