Quality Assurance for Nonradiographic Radiotherapy Localization and Positioning Systems

What TG covers Quality Assurance for Nonradiographic Radiotherapy Localization and Positioning Systems

TG-147

List 2 advantages and disadvantages of non radiographic positioning systems

Advantages:

1. These technologies contribute no additional dose to the patient.

2. They mostly have very quick responses, meaning that they can be used in real time for tracking or gating applications.

   Disadvantages:

1. Blocked views or interferences are possible.

2. Many of these applications require the use of a surrogate or surrogates (such as reflective body markers or RT beacons, for example).

How do RF systems work

1. These devices are unique versus optical and infrared technologies because the RF signals are capable of penetrating the body and, thus, surrogates can be implanted in the patient’s body.

2. An issue with these systems (and particularly implantable beacons) is that markers can migrate over time.

describe in general, how a dual camera optical tracking system works and list a clinically available solution

1. These systems track, in real-time, two-dimensional images obtained in stereo.

2. Current systems use surface imaging technologies to reconstruct surface maps of patients (effectively imaging three-dimensional information of a visible body contour).

3. A drawback of this theory is that further correlation may be necessary to draw inferences from the surface contour to the position of the internal target.

4. Example: alignRT

how do dual camera infrared systems work

IR technologies use the same stereoscopic approach mentioned for optical tracking systems, but oftentimes employee surrogates that reflect infrared light emitted in the direction of the patient.

What are some issues with infrared systems

a direct line of sight is needed between the reflector and the camera.Also, IR systems are only able to address external patient motion.

Thermal Drift:

1. Infrared cameras demonstrate some degree of thermal drift requiring that they include a warm-up period before being used clinically.

2. Thermal drift is the perceived change in the position of a fixed test pattern which is due to components of the IR system warming up (not the observed change in the test pattern).

3. It is vital that an isocenter of the IR system not be defined during this warm-up period because it would introduce an artifact into subsequent uses during the day.

4. Also, it is recommended that this thermal drift be evaluated periodically to evaluate the appropriateness of the warm-up time delay.

How does ExacTrac work

1. The ExacTrac system combines elements of nonradiographic and radiographic components to form a means of initial positioning of patients and monitoring of patient’s positions during treatment using IR.  

2. Also, using stereoscopic x-rays, the system can provide final patient alignment using internal anatomy with a couch capable of translating in 6D (longitudinally, vertically, laterally, pitch, roll, and yaw).

How does Varian Calypso work?

1. The Calypso 4D localization system is a means for localization and tumor tracking during treatment using radiofrequency signals.

2. Transponders (or beacons) are implanted typically in the prostate or prostatic bed under ultrasound guidance in a procedure similar to needle biopsy.

3. The beacons are 8 mm in length and 2 mm in diameter and consist of an alternating current circuit surrounded by non-reactive glass.

4. During treatment, an array of source coils are positioned close to and directly above the patient.  This array produces an oscillating electromagnetic field which induces a resonance inside the transponder.

5. The decay of the resonant signal is then detected by a second array of receiver coils mounted within the same panel as the transmitter coils.

6. Then, by triangulation, the transponder can be localized.  

7. Since the transponders were present during CT simulation, they can be contoured, and an appropriate shift in 3D can be determined for patient alignment.

8. Furthermore, monitoring can continue through the treatment fraction (i.e. intrafractionally).

name an example of a single camera system

Varian RPM for motion management

recommended tolerance for localization accuracy

2mm

how should stability of an isocenter system be evaluated

recording a test pattern for at least 90 minutes (or until stability is fully achieved) to evaluate any start-up drift (such as the kind observed with the perceived thermal drift of IR cameras).

what should regularly be tested for system performance?

1. Calibration

2. System Drift (e.g. thermal drift)

3. Reproducibility

4. Tracking accuracy (e.g. respiratory motion tracking)

5. Localization accuracy

6. Response time (i.e. latency).

Daily recommended tests

1. Check interlocks and field of views.

2. Localization test (move a phantom to an offset position and move to isocenter).

Monthly recommended tests

1. Gating termination.

2. System communication.

3. Daily tests

Annual recommended tests

1. Test emergency off switches.

2. Test backup power supplies.

3. Checking camera mounting.

4. Drift measurements (for at least one hour to quantify any thermal drift and demonstrate an appropriate warm-up period).

5. End-to-end test with reproducibility checks and gating functionality (where applicable).

6. Monthly tests.