Quality Assurance for Nonradiographic Radiotherapy Localization and Positioning Systems

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

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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).

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).