Respiratory Motion Management
Intrafraction motion, which occurs during treatment delivery, reduces the accuracy of dose calculation, and may reduce tumor dose while increasing normal tissue toxicity. Quantifying and limiting error resulting from this motion is necessary to achieve optimal normal tissue sparing without sacrificing target coverage.
There are many sources of intrafraction motion. Respiratory and cardiac motion as periodic (occur repeatedly over a roughly predictable time frame) impact large portions of the body. There is also organ specific motion such as that of the prostate or rectum which are highly irregular and difficult to quantify even with modern technology. Finally, there is also the simple issue of patient compliance which may require sedation in patients who are very young or have a medical condition the precludes compliance.
Typical Respiratory Motion
Lung tumor motion is highly dependent upon the individual patient’s breathing pattern as well as the location in the lung. Generally, targets that are located anterior (near the breast) and inferior (near the diaphragm) experience the highest amplitude of motion.
Average target motion: <1cm
Maximum target motion: 5cm
Other organs that are impacted by respiratory motion include:
Problems Resulting from Respiratory Motion
Unaddressed intrafraction motion can result in the following dosimetric problems:
Geometric miss occurs when part of the target does not receive adequate dose due to a failure to localize the target.
Occurs due to organ motion moving the target into and out of the beam. This results in poorer dose fall off outside the target and decreased maximum dose.
Interplay effects are complex dosimetric artifacts that occur when intensity modulated radiotherapy (IMRT) is administered to moving anatomy. Since only part of the target is treated at any time during IMRT, the organ motion may cause some areas to be within the beam more often than planned while others can be missed. This results in an inhomogeneous dose distribution that may underdose some areas while overdosing others. This effect can be especially severe for highly modulated therapies, Tomotherapy deliveries which irradiate only a single slice at a time and scanning beam particle therapy.
Key Point: Internal motion can be especially problematic for particle beam therapy which relies on precise knowledge about the radiation path length to place the Bragg peak within the target.
Quantifying Organ Motion
Four-dimensional CT (4DCT)
A 4DCT is essentially a CT video that displays motion throughout the breathing cycle. Because 4DCTs offer reasonable spatial and temporal resolution, they have become the standard method of quantifying respiratory motion.
Motion encompassing via 4DCT is usually accomplished by creating a maximum intensity projection image (MIP). The MIP is created by combining the voxel with the highest attenuation value across all phases of the 4DCT. A dense moving lung target appears as a large bright object in the MIP.
Combined Inhalation and Exhalation CT Scans
When a 4DCT is not available, CTs taken at inhalation and exhalation may be used in conjunction to estimate the extent of tumor motion. This technique, sometimes referred to as “the poor man’s 4DCT” provides less accuracy than a true 4DCT but does not require a specialized CT scanner.
Slow CT Scanning
In a slow CT scan, the projection data is collected over a time much longer than the patient’s respiratory cycle. The resulting image is similar to a MIP with significant motion blurring and increased patient dose.
Motion Management Techniques
Motion encompassing simply means including expected target motion within the Planning Target Volume (PTV). This requires quantifying the extent of target motion which can be accomplished in several ways.
Respiratory gating involves delivering radiation only during a desired portion of the patient’s breathing cycle.
Respiratory Gating Terms
Gate/Window: The portion of the breathing cycle in which radiation is delivered.
Duty cycle: The fraction of the breathing cycle in which treatment is delivered. Increased duty cycle usually results in faster treatments but also greater target motion during delivery.
Surrogate: A marker or structure that is tracked when direct tracking of the target is not possible. This is most commonly the patient’s surface (skin), an IR reflector or internally placed fiducial markers.
Breadth hold is similar to gating in that treatment delivery only occurs while the patient is holding their breath. This may be monitored visually, via surface monitoring or IR reflector positioning, or using a spirometer.
Forced Shallow Breathing
Forced shallow breathing uses chest/abdominal compression to physically limit the extent of respiratory motion. When used, the physical respiratory limitation is applied both during simulation and treatment.
Respiratory tracking uses near real time imaging of the target or fiducial markers to track the target during treatment. The location of the beam is adjusted in near real time to keep the target within the beam during the entire treatment.
The CyberKnife system uses a robotic arm mounted linear accelerator and two orthogonal kV imagers to achieve respiratory tracking.
Key Point: Surrogate tracking techniques, those that rely on external markers to assess internal motion, are generally less spatially and temporally accurate than internal tracking techniques.
AAPM TG-76 Recommendations
- Respiratory motion management techniques are recommended under the following conditions:
- Target motion is a “significant” source of error (>5mm) or motion management would reduce normal tissue dose.
- A motion management technique is available.
- The patient is able to tolerate the available method of motion management.
- Target motion should be used in determination of CTV-to-PTV expansion.
- Motion artifacts reduce accuracy of contour geometry.
- A qualified medical physicist be present at all treatment-simulations in which respiratory motion is used and at least the first treatment.
Strict QA procedures are used for imaging, planning, and delivery of radiotherapy using respiratory management devices.
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