What is imaging guidance

For daily online correction, systematic and random errors may be calculated from the matched data. Post-treatment imaging is required to quantify both intrafraction motion and residual errors. If evaluated for a patient population, these data may help check the PTV margin for that treatment protocol.

In fact, the use of daily online imaging with corrections in conjunction with use of automatic couch with 6 degrees of freedom has obviated the need for invasive frames for SRS treatments [ 13 ].

Depending on the imaging methods used, the IGRT systems may broadly be divided into radiation based and nonradiation based systems [ 14 , 15 ]. These may employ ultrasound, camera-based systems, electromagnetic tracking, and MRI systems integrated into the treatment room.

These systems e. Sites of common application include prostate, lung, and breast radiotherapy. These systems identify the patient reference setup point positions in comparison to their location in the planning CT coordinate system, which aids in computing the treatment couch translation to align the treatment isocenter with plan isocenter.

Optical tracking may also be used for intrafraction position monitoring for either gating treatment delivery only at a certain position of target or repositioning for correction.

Tools such as AlignRT image the patient directly and track the skin surface to give real time feedback for necessary corrections. These systems have found application in treatment of prostate and breast cancer and for respiratory gating using external surrogates.

Beacons need to be placed through a minimally invasive procedure, their presence may introduce artifacts in MR images, and there are limitations to the patient size. However, MRI has certain drawbacks such as motion artifacts, distortion with non-uniform magnetic fields, and cannot be performed for patients with pacemakers or metallic implants.

A wide application potential exists in treatment of prostate, liver, and brain, as well as for brachytherapy. These include static as well as real time tracking, using either kilovoltage KV , megavoltage MV , or hybrid methods.

EPID was developed as a replacement of film dosimetry for treatment field verification and is based on indirect detection active matrix flat panel imagers AMFPIs.

They are offered as standard equipment by nearly all linear accelerator LINAC vendors as both field verification and quality assurance QA tools. Bony landmarks on planar images are used as surrogates for defining positional variations respective to the digital reconstructed radiographs DRRs developed from the planning CT dataset Figure 1. Different systems may use either KV or MV X-rays for imaging, with the image contrast being superior with KV images while there is lesser distortion from metallic implants dental, hip prostheses in MV images.

EPID systems are unable to detect or quantify rotations. Both the field and the bony anatomy are matched sequentially to give an estimate of error. The comparison of live image with reference DRR helps assess and correct translational shifts but does not estimate rotational errors. These systems consist of retractable X-ray tube and amorphous silicon detectors mounted either orthogonal to Elekta Synergy, Varian OBI or along the treatment beam axis Siemens Artiste. The O-ring can be rotated degrees around the isocenter and can be skewed 60 degrees around its vertical axis.

Both interfraction setup changes and anatomical changes related to weight changes or organ filling bladder, rectum may be monitored Figure 2. Repeat scans at the end of treatment may give an estimate of intrafractional changes. For tumors discernible separately from surrounding normal tissue, treatment response may also be monitored and these scans may be used for dose recalculation or treatment plan adaptation after necessary image processing.

KV CT gives better contrast resolution compared to MV CT but may be limited by artifacts from prostheses and scatter from bulky patient anatomy. Both translational and rotational errors may be estimated. Translational errors are easily corrected whereas few systems have provisions for correcting rotational errors with couch rotations. In this particular case, the live image shows negligible bladder filling and treatment was delayed to allow for optimum bladder position for obtaining a reproducible position of prostate as well as moving the bowel out of treatment field.

This includes an on-board imaging system to obtain MV CT images of the patient in treatment position. The Accuray CyberKnife robotic radiosurgery system consists of a compact LINAC mounted on an industrial robotic manipulator arm which directs the radiation beams to the desired target based on inputs from two orthogonal X-ray imaging systems mounted on the room ceiling with flat panel floor detectors on either side of couch, integrated to provide image guidance for the treatment process. Images are acquired throughout the treatment duration at periodic intervals ranging from 5 to 90 seconds, and the couch and robotic head movements are guided through an automatic process.

Several tracking methods may be used depending upon the treatment site Figure 3. Skull, skull base, or brain tumors may be treated using 6D skull tracking, paravertebral tumors whose movement parallels that of spine may be treated with X-Sight spine tracking, and lung tumors that are surrounded by normal lung parenchyma may be tracked with X-Sight lung tracking. Lung tracking may employ automatic generation of internal target volume depending upon visibility of tumor through both, one or none of the X-ray imaging systems in the treatment position.

For all other tumors e. Respiratory motion is also monitored and accounted for when correcting for target position and motion through a synchrony model generated in real time.

The system also has a couch that has 6 degrees of freedom to correct for positional variations. Treatment may be limited by patient position and size, and posterior treatment beams cannot be used. A semi-invasive procedure may be required if fiducial markers are needed for tracking. This system can be employed for both cranial frameless and extracranial radiosurgery or SRT [ 32 , 33 ].

CyberKnife console showing the tumor tracking options in a case of head and neck malignancy. This system is designed for real time tracking of tumors by imaging implanted fiducials and using this information for gating. It consists of four X-ray camera systems mounted on the floor, a ceiling-mounted image intensifier, and a high-voltage X-ray generator. The LINAC is gated to irradiate the tumor only when the marker is within a given tolerance from its planned coordinates relative to the isocenter [ 34 , 35 ].

This system has two X-ray tubes and corresponding flat panel detectors and uses a combination of initial couch motion and a pair of radiographs for patient alignment. The couch is capable of 3D alignment for initial coarse setup and then the on-board imaging subsystem helps fine-tuning. A pair of radiographs is acquired and registered with prior DRRs using bony landmarks to evaluate the translational and rotational shifts.

The system can also compensate for organ motion [ 36 ]. It uses a combination of optical positioning and KV radiographic imaging for online positioning corrections. There are two main subsystems: an infrared-based system for initial patient setup and precise control of couch movement using a robotic couch and a radiographic KV X-ray imaging system for position verification and readjustment based on internal anatomy or implanted markers. Infrared system may also be used for respiratory monitoring and signaling to LINAC for beam tracking and gating.

Similar guidelines have also been proposed by European agencies [ 39 — 41 ]. A summary of the key points is given below. Respective personnel should obtain appropriate certification with specific training in IGRT before performing any stereotactic procedures. Radiation Oncologist. Medical Physicist. Radiation Therapist. Fiducial Markers. These serve as surrogates to soft tissue targets when they are difficult to visualize and their alignment cannot be related to bony anatomy.

These may be tracked in real time to obtain 3D coordinates of the target for subsequent corrections. Moving Targets and Delineation. Intrafraction target motion or interfraction displacement, deformation, or alteration of targets and other tissues should be accounted for during determination of PTVs. Appropriate motion management methods should be chosen depending on available expertise and degree and type of motion.

This process starts at the time of simulation and continues throughout till the end of therapy. Patient Positioning. It is imperative to ensure the accuracy of patient position and its reproducibility for fractionated treatments relative to the chosen IGRT device as well as treatment unit.

Image Acquisition. The IGRT system should be calibrated to ensure high imaging quality with attention to slice thickness uniformity, image contrast, spatial resolution, isocenter alignment between imaging and treatment planning and delivery systems, accuracy of software used for identification, and correction of couch misalignments.

Relevant QA procedures should ensure reliability and reproducibility of the entire process. Treatment Verification. Image review by radiation oncologist at the first fraction and then periodically is necessary to ensure treatment accuracy and reproducibility.

Each department should determine its own threshold of couch positioning changes that would necessitate setup review or change before treatment delivery. Quality Assurance and Documentation. A documentation of all the necessary QA procedures throughout the course of simulation, treatment, and periodic verification should be maintained.

These would help determine departmental thresholds for action as well as serve as guides for modification of the processes involved following review of findings. Use of the IGRT process has improved our awareness and understanding of daily inter- and intrafractional setup variations and motion. Real time tracking has helped quantify interpatient and intrapatient variations in lung and liver tumor motion related to breathing and complexities of such motion have become clearer.

We now understand that even when breath-holds are repeated, the relative position of soft tissue and skeletal structures may vary, rendering use of bony landmarks useless for such endeavors.

Changes in prostate position translation, rotation, and shape have been quantified and we can better correct for these errors as well as tailor PTV margins to these findings, thus allowing more accurate targeting. Understanding of the various IGRT techniques, their applicability, limitations, and additional radiation hazards helps the radiation oncologist take an educated decision on the method best suited to a particular clinical situation for maximizing benefit from radiation therapy. Changes in parotid position relative to the tumor in head and neck cases, change in body contour due to weight loss, seroma, or body fluid collections, change in prostate position relative to bladder or rectal filling and effect of bowel gas, reduction of tumor size during treatment, and changes in spinal position during spinal or head and neck radiotherapy are situations which were never even considered of significance in the pre-IGRT era and their respective roles and solutions are being developed as we are understanding their role during treatment.

With better geometric precision, volume of irradiated healthy normal tissue can be significantly reduced with reduction in toxicity risks. Adaptation to reduction in tumor volume may lead to additional gains in normal tissue toxicity reduction. Results from ongoing and future trials will hopefully demonstrate the net gain in therapeutic ratio from application of IGRT technologies and the onus lies on the radiation oncology community to take up the challenge of demonstrating the benefit of these potentially expensive approaches.

IGRT is most likely to benefit clinical situations where the tumor is in close proximity to sensitive healthy tissues, when doses required for disease control exceed the tolerance levels of adjacent normal tissues or when large organ motion and setup errors may result in severe consequences of positional errors. Thoracic and upper abdominal targets with significant respiratory motion, obese patients, head and neck cancers, paraspinal and retroperitoneal sarcomas, and prostate cancer are situations that are expected to derive maximum benefit with some clinical experience forthcoming.

Clinical situations where even low dose irradiation produces excellent local control, palliative radiotherapy delivered using large fields, and superficial tumors that are amenable to direct visual inspection are likely to derive least benefit from IGRT.

Limited availability of experienced trained staff is a major hurdle in wide application of the technique despite its demonstrable benefits, even with the simplest approaches. Other factors that need consideration include quality control, algorithms that define the decisions whether to change a plan or continue with original plan, and need for commercial development of software as well as hardware to match clinical needs and demands.

Imaging equipment may also be mounted in the treatment room. Sometimes, IGRT is performed by a detector in the room which tracks motion by localizing markers on the surface of a patient, or electromagnetic transponders placed within the patient.

The equipment is operated by a radiation therapist, a highly trained technologist. The overall treatment plan is created and supervised by the radiation oncologist, a highly trained physician specializing in treating cancer with radiotherapy. Women should always inform their physician or technologist if there is any possibility that they are pregnant or if they are breastfeeding their baby. See the Safety page for more information about pregnancy, breastfeeding and imaging.

For some IGRT procedures, very small markers, which are called fiducial markers, or in some cases electromagnetic transponders may be placed inside the body near or in the tumor to help the treatment team identify the area.

They are usually placed at least one week prior to the first radiation therapy treatment. The patient's skin also may be marked or tattooed with colored ink to help align and target the radiation equipment.

Patients with prostate cancer who undergo IGRT using ultrasound must drink enough water about an hour before each treatment to keep their bladder full so that the prostate can be imaged or "seen" by the ultrasound machine. See the Fiducial Marker Placement page for more information. Otherwise, there is no specific preparation for IGRT, other than the preparation for routine radiation therapy, which could be either 3-D conformal radiation therapy, intensity modulated radiation therapy IMRT , proton beam therapy, or stereotactic body radiotherapy SBRT.

At the beginning of each radiation therapy session, the patient is carefully positioned guided by the marks on the skin defining the treatment area. Devices may be used to help the patient maintain the proper position. Images are then taken using imaging equipment that is built into the radiation delivery machine or mounted in the treatment room. Some IGRT techniques require patients to hold their breath for approximately 30 to 60 seconds.

If IGRT requires fiducial markers or electromagnetic transponders inside the body, these will be inserted into the body with a needle about a week prior to the simulation process. On each treatment day, depending on the type of IGRT used, an x-ray, CT scan or ultrasound will be obtained prior to the treatment.

The physicians or a radiation therapist review the images and compare them to the reference images taken during simulation to make position adjustments.

The patient may be repositioned and additional imaging may be performed. After any necessary adjustments are made to match with the patient's reference positioning, radiation therapy is delivered. During the radiation therapy session, you may see or hear equipment moving around you during the imaging procedure. Patients may sometimes smell an odd smell during treatment that is caused by the ozone produced by the linear accelerator.

Some patients may also see a colored light when they receive their treatment; this event is especially true for patients having their brain treated. Radiation treatment can cause side effects. These problems may result from the treatment itself or from radiation damage to healthy cells in the treatment area.

The number and severity of side effects will depend on the type of radiation, dose, and body part under treatment. Radiation can cause early and late side effects. Early side effects happen during or right after treatment.

The new guidance is meant to help clarify imaging documentation for codes that include both a procedure and imaging guidance, explains Melody W. Paul, Minnesota. In addition, you may not use a code that describes imaging to report such non-imaging tracking methods as radar or electromagnetic signals, the AMA advises. However, you may report imaging codes for modalities including radiography, fluoroscopy, ultrasound, MRI, CT or nuclear imaging as appropriate, the guidance states.