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Radiation therapy or radiotherapy, often abbreviated RT, RTx, or XRT, is therapy using ionizing radiation, generally as part of cancer treatment to control or kill malignant cells.
Radiation therapy is commonly applied to the cancerous tumor because of its ability to control cell growth. Ionizing radiation works by damaging the DNA of cancerous tissue leading to cellular death. To spare normal tissues (such as skin or organs which radiation must pass through to treat the tumor), shaped radiation beams are aimed from several angles of exposure to intersect at the tumor, providing a much larger absorbed dose there than in the surrounding, healthy tissue. Besides the tumor itself, the radiation fields may also include the draining lymph nodes if they are clinically or radiologically involved with tumor, or if there is thought to be a risk of subclinical malignant spread. It is necessary to include a margin of normal tissue around the tumor to allow for uncertainties in daily set-up and internal tumor motion. These uncertainties can be caused by internal movement (for example, respiration and bladder filling) and movement of external skin marks relative to the tumor position.
Simulation is a process by which the radiation treatment fields are defined, filmed and marked out on your skin. The simulator is actually a large bore CT scanner that is used to contour your body. It is here that special care is taken to make the patient’s position as comfortable as possible while ensuring reproducibility on a day-to-day basis. All setup information is documented to make your treatment record complete. It is an integral part of the planning process. The CT scan itself is not used as a diagnostic scan but instead used to contour the shape of your body and visualize structures.
The images are then sent to the physics department who, with the doctors, arrange the radiation beams and make a customized plan,
The simulation team will do its best to find a suitable, comfortable position for you. We regret that both the simulation and treatment tables are very hard and very flat. This is done purposely to ensure that your position, relative to the table, is exactly the same during the entire course of treatment.The position chosen will be dependent upon the area treated.
We realize that everyone is nervous and apprehensive at this time. For both simulation and treatment we want you to try to relax. Breathe normally. There is no need to hold your breath during the CT scan. The overall procedure can vary in length, anywhere from thirty minutes to one hour.
Immobilization means "to prevent movement or to keep in place." We use immobilization devices that do just that. They are treatment aids that are pre-made, or that we construct, to help you in maintaining the desired treatment position throughout the entire course of therapy. Some of these aids, to name a few, are:
Recent Advance Treatment Techniques:
Intensity-modulated radiation therapy (IMRT) is an advanced mode of high-precision radiotherapy that uses computer-controlled linear accelerators to deliver precise radiation doses to a malignant tumor or specific areas within the tumor
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A type of three-dimensional radiation therapy that uses computer-generated images to match radiation to the size and shape of a tumor In IMRT, thousands of tiny radiation beamlets enter the body from many angles and intersect the tumor. Since the intensity of each beamlet can be controlled, the radiation dose can wrap around normal tissue, create concave shapes and turn corners. The aim is to deliver a higher radiation dose to a tumor with less damage to nearby healthy tissue. IMRT may be used, for example, to treat a tumor that surrounds the spinal cord and spare the cord itself.
Image-guided radiation therapy (IGRT) is the process of frequent two and three-dimensional imaging, during a course of radiation treatment, used to direct radiation therapy utilizing the imaging coordinates of the actual radiation treatment plan. 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 localization of a cone beam computed tomography (CBCT) dataset with the planning 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. IGRT relies directly on the imaging modalities from planning as the reference coordinates for localizing the patient. Through advancements in imaging technology, combined with a further understanding of human biology at the molecular level, the impact of IGRT on radiotherapy treatment continues to evolve
Volumetric modulated arc therapy (VMAT) is a new radiation technique, which can achieve highly conformal dose distributions on target volume coverage and sparing of normal tissues. The specificity of this technique is to modify the three parameters during the treatment. VMAT delivers radiation by rotating gantry (usually 360° rotating fields with one or more arcs), changing speed and shape of the beam with a multileaf collimator (MLC) ("sliding window" system of moving) and fluence output rate (dose rate) of the medical linear accelerator. VMAT also has the potential to give additional advantages in patient treatment, such as reduced delivery time of radiation, compared with conventional static field intensity modulated radiotherapy (IMRT).
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.
The Aim of stereotactic radiosurgery is to destroy target tissue while preserving adjacent normal tissue, where fractionated radiotherapy relies on a different sensitivity of the target and the surrounding normal tissue to the total accumulated radiation dose
Types of radiosurgery
Stereotactic radiosurgery SRS:
A single high dose fraction of radiation, stereotactically directed to an intracranial region of interest. The word stereotactic refers to a three-dimensional coordinate system that enables accurate correlation of a virtual target seen in the patient's diagnostic images with the actual target position in the patient anatomy.
stereotactic radiosurgery has been redefined as a distinct neurosurgical discipline that utilizes externally generated ionizing radiation to inactivate or eradicate defined targets in the head or spine without the need for a surgical incision.
Principles of radiobiology: repair, reassortment, repopulation, and reoxygenation.
Stereotactic Radiotherapy SRT: Same as SRS but only it is fractionated in 28-30 days of treatment where the tumor size is large and not feasible to SRS.
Fractionated Stereotactic (FSRS): as the name suggest same as SRS BUT fractionated to 3-5 fractions. Dose is higher than SRT per fraction
SBRT:
Stereotactic body radiotherapy (SBRT) is a sophisticated technique that allows the safe delivery of very high doses of radiation to relatively small targets in only one to five treatment sessions.
The major feature that separates SBRT from conventional radiation treatment is
the delivery of large doses in a few fractions, which results in a high biological effective dose
BED. In order to minimize the normal tissue toxicity, conformation of high doses to the
target and rapid fall-off doses away from the target is critical .The practice of SBRT therefore requires a high level of confidence in the accuracy of the entire treatment delivery process.
In SBRT, confidence in this accuracy is accomplished by the integration of modern imaging, simulation, treatment planning, and delivery technologies into all phases of the treatment process; from treatment simulation and planning and continuing throughout beam delivery
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 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.
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
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 intrafraction 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 Siteman Cancer Center at Barnes-Jewish Hospital and Washington University School of Medicine. Treatment of the first patients was announced in February 2014. 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 tumor moves during treatment.
Ultrasound is used for daily patient setup. It is useful for soft tissue such as breast and prostate. The BAT (Best Nomos) and Clarity (Elekta) system 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.
CORRECTION OF PATIENT POSTION:
The On-line strategy makes adjustment to patient and beam position during the treatment process, based on continuously updated information throughout the procedure. 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 error.
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.
