14
IMAGE GUIDANCE
Question 2
What is the difference between accuracy and precision?
Question 3
What are some examples of both inter-fraction and intra-fraction variables that cause uncertainty in treatment targeting during radiation therapy?
Question 4
What are the different systems used for image-guided radiation therapy (IGRT)?
IGRT: image-guided radiation therapy. It is used to ensure that the treatment target is localized and aligned to the radiation beams as in the images that are used to plan the treatment before delivering radiation therapy.
Answer 2
Accuracy is how close the measured value is to the true value. Precision is how close the measured values are to each other. It is possible to be precise without being accurate, and accurate without being precise. The aim of image-guided radiation therapy (IGRT) is accuracy.
Answer 3
Patient positioning/setup errors, weight loss, change in volumes, and bladder/rectum filling are a few examples of changes that can occur from fraction to fraction (inter-fraction). Patient movement during treatment, breathing, gas movement in the bowels, and cardiac motion are a few factors that can lead to uncertainty during treatment (intra-fraction).
Answer 4
Electronic portal imaging device (EPID), two-dimensional kilovoltage imaging pair, stereoscopic kilovoltage images, megavoltage cone beam (MVCT), kilovoltage cone beam (kVCT), CT-on-rails (in-room diagnostic quality CT scan), optical tracking, in-room magnetic resonance imaging, ultrasound (US), electromagnetic transponders.
Question 6
How does an electronic portal imaging device (EPID) work?
Question 7
What is cone beam imaging and how is it different from a regular CT?
Question 8
What are the disadvantages and advantages of megavoltage computed tomography (MVCT) in radiation therapy?
The IGRT process is similar among the different methods. The patient is first positioned on the treatment table in the same way they were simulated, using the room laser to align the machine isocenter with the marked isocenter on the patient body. Images of the patient are acquired using one of the listed IGRT systems. The acquired IGRT images are registered with the reference images, focusing on the specified anatomy. The registration is used to determine the initial patient offset required to align the target with the planned treatment beams. The repositioning software will display the offset as shifts for the treatment couch to align the patient in the same position as planned.
Answer 6
The incoming X-rays interact with the scintillation screen that converts them into optical wavelength photons. The optical photons are detected by the amorphous silicon photodiodes, which then are converted into an electronic signal and image.
Answer 7
Cone beam imaging uses a cone-shaped X-ray beam that transmits onto a detector creating a complete volume image with one rotation. A regular CT images the patient with a narrow collimation creating a fan shaped X-ray beam. The narrow collimated beam images a thin “slice” of the patient. Therefore, while the fan beam rotates around the patient, the couch must slowly move along the longitudinal direction, resulting in a helical (spiral motion) of the X-ray beam, to fully image the patient.
Answer 8
Disadvantages: MVCT uses mega voltage energies where the Compton scattering dominates, so soft tissue is harder to visualize than in kV images where the photoelectric effect dominates. To obtain a good quality image you may need to increase the imaging dose, which causes an increase in radiation exposure to patients. Bony anatomy and tissue with low density (such as air cavity or lung) are visible.
Advantages: The artifacts from large implanted metals are minimized which makes the MV images easier to visualize the anatomy near metal implants than in kV images. The imaging beam and treatment beam also share the same isocenter, preventing potential mis-concordance between the imaging center and treatment center. Linear accelerator (linac)-based kV cone beam computed tomography (CBCT), however, requires a separate kV source that is placed orthogonal to the treatment beam with a discordance between the imaging and treatment center that could cause an offset between imaging and treatment.
Question 10
What is infrared optical tracking and how is it used?
Question 11
What is the Calypso 4D Localization system and how does it work?
Question 12
How is the optical tracking system different from that of the electromagnetic transponder system?
CT-on-rails is a diagnostic quality CT located in the same vault as the linear accelerator (linac). It has the same quality images as the planning CT. This increases the accuracy of registering the treatment verification and planning images. The CT images taken with CT-on-rails can also be used for adaptive replanning purpose as they have accurate electron density information.
Answer 10
Optical tracking uses passive or active infrared markers that are detected by a camera. These markers are placed on the surface of the patient and are useful to detect both the patient position relative to the plan and whether the patient moves during treatment. A calibration method associates the camera’s coordinates with that of the isocenter of the treatment machine.
Answer 11
The Calypso system has been referred to as a global positioning satellite (GPS) for the body, and is a common form of image-guided radiation therapy (IGRT) used for prostate patients receiving external beam therapy. Tiny transponder beacons (8.5 mm long, 1.85 mm diameter) are implanted into the prostate during an outpatient procedure. The locations of these transponders infer the prostate position and rotation. The Calypso tracking system communicates with these transponders via radio waves. Calypso systems can track the prostate position and motion in nearly real time (20 times a second).
During radiation delivery, users can set a prostate position threshold above which the radiation beam will be stopped. Using the Calypso system, high radiation doses can be confidently delivered to the target with tighter margins, sparing normal tissue, and reducing unwanted side effects.
www.varian.com/oncology/products/real-time-tracking/calypso-extracranial-tracking?cat=overview
Answer 12
The optical tracking markers are placed on the surface of the patient’s body. The electromagnetic transponders are typically implanted into an organ (such as the prostate). This will provide a real time tracking method of the organ. The transponders are activated by radiofrequency waves. Both tracking methods are considered a form of image-guided radiation therapy (IGRT) because they are used for the same purpose of aligning the target with the planned treatment beams.
What are the benefits and disadvantages of the ultrasound (US) for image-guided radiation therapy (IGRT)?
Question 14
What is a common imaging modality used for guidance during radioactive prostate seed implants? Briefly explain how it is used.
Question 15
Discuss the ultrasound (US) for external beam therapy.
Question 16
Would megavoltage cone beam computed tomography (MV CBCT) be a good choice for imaging the prostate during image-guided radiation therapy (IGRT)?
US uses sound waves that are reflected back to the transducer and converts the signal to an image. The US is best used for localization of the prostate and the tumor cavity of the breast, because their locations permit for easy access of the US probe. US is noninvasive and does not produce ionizing radiation. It can be used daily to correct for motion and setup error. The disadvantage of using US is that it is operator dependent. Results may vary depending on how the operator applies the pressure to the probe.
Answer 14
Real-time ultrasound (US) imaging is frequently used during the prostate seed implantation procedure. A trans-rectal US probe is used to capture a series of static images for planning purposes. It is also used to visualize the prostate, surrounding tissue, seeds, and needles during the actual implantation.
Answer 15
In external beam, ultrasound provides soft tissue visualization for the prostate and breast cavity. During the simulation phase, US images can be taken and fused with planning CT images to provide better visualization of the target and surrounding structures. During treatment, US images allow target structures to be aligned using shape, size, and position of anatomy. Ultrasound is advantageous as an imaging method using nonionizing radiation.
Answer 16
For visualizing the actual prostate gland, it would be a poor choice since the MV beam suffers from poor soft-tissue contrast. However, MV CBCT is a common form of IGRT for prostate patients when used in conjunction with fiducials implanted into the gland. The fiducials are highly visible on the MV CBCT and can be used to align it to the reference image.
In terms of dose added to the patient’s treatment, arrange the following imaging modalities from least to greatest: megavoltage cone beam computed tomography (MV CBCT), planar kV, ultrasound (US), kilovoltage cone beam computed tomography (kV CBCT)
Question 18
What is the difference between random and systematic errors and how do they affect the dose distribution around the clinical tumor volume (CTV)?
Question 19
Define gross tumor volume (GTV), clinical tumor volume (CTV), internal target volume (ITV), and planning target volume (PTV). What effect does image-guided radiation therapy (IGRT) have on these?
Question 20
What is the van Herk formulation for margins?
Answer 18
Random errors are unpredictable and can vary in magnitude and direction from fraction to fraction. This type of error leads to a blurring of the dose distribution around the CTV over the course of treatment. Systematic errors are deviations that occur during each fraction over the entire course of therapy. The deviation is in a similar size and direction and can lead to a shift in the dose distribution away from the CTV.
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