Academic Courses > DOS 512 > Patient Markings During Simulation
DOS 512 - Week 2 Discussion
Writing Prompt
Markings are placed on the patient during simulation. How do these marks relate to the treatment plan and it's implementation in the treatment room? How many marks are generally required? Why?
Initial Post: Patient Markings During Simulation
In conversations with Chris Diaz (December 3, 2014) and Kent McCune (December 3, 2014), I learned that a minimum of three marks on the patient's body are required. These marks are made on the skin at the intersection of positioning laser lines projected onto the top (anterior if supine and posterior if prone) and side surfaces of the patient in the CT simulator room. The crossing points of the lasers define a spatial coordinate system that is duplicated in the treatment room. This common coordinate system creates a reference for positioning that allows patients to be positioned appropriately in space by once again lining the skin marks up with the laser lines projected in the treatment room.
Since the crossing points of the lasers defines a single point in space, one can choose whether that point should be the isocenter of the treatment, or a setup reference point that has a known distance (X, Y, and Z shifts) to the isocenter. At our center, we usually pick a point of anatomy near the treatment zone that is relatively stable (not loose skin, pendulous tissues, or skin near joints) and create a setup reference point. We do this rather than marking the actual isocenter, because it is not always possible to define the isocenter until we have had a chance to evaluate a few beam options, and the patient can not wait on the table until we have decided.
The reason that we need a minimum of three points is that two points are needed to define a line, and three point are required to define a plane. The plane that we define with the two side marks and the center mark is an axial cut that corresponds to a particular slice of the CT image set. By aligning at least that slice of the patient with a particular slice of the CT image set, we can get the patient into a reasonably close approximation of the correct treatment position. In many cases, we will also define additional surface points superior and inferior to those three initial markings, creating a mid-sagittal line and lines on either side of the patient. The mid-sagittal line allows the therapists to quickly determine whether the patient is straight on the table, or if they are tilted off to one side. The line of points down one or both sides can help to determine if the arch of the back is correct or if there are other setup oddities.
By giving the therapists plenty of information for how to set up the patient on the table prior to the first setup verification image, we can reduce the number of radiation exposures as the patient's position is checked, adjusted, and rechecked.
Since the crossing points of the lasers defines a single point in space, one can choose whether that point should be the isocenter of the treatment, or a setup reference point that has a known distance (X, Y, and Z shifts) to the isocenter. At our center, we usually pick a point of anatomy near the treatment zone that is relatively stable (not loose skin, pendulous tissues, or skin near joints) and create a setup reference point. We do this rather than marking the actual isocenter, because it is not always possible to define the isocenter until we have had a chance to evaluate a few beam options, and the patient can not wait on the table until we have decided.
The reason that we need a minimum of three points is that two points are needed to define a line, and three point are required to define a plane. The plane that we define with the two side marks and the center mark is an axial cut that corresponds to a particular slice of the CT image set. By aligning at least that slice of the patient with a particular slice of the CT image set, we can get the patient into a reasonably close approximation of the correct treatment position. In many cases, we will also define additional surface points superior and inferior to those three initial markings, creating a mid-sagittal line and lines on either side of the patient. The mid-sagittal line allows the therapists to quickly determine whether the patient is straight on the table, or if they are tilted off to one side. The line of points down one or both sides can help to determine if the arch of the back is correct or if there are other setup oddities.
By giving the therapists plenty of information for how to set up the patient on the table prior to the first setup verification image, we can reduce the number of radiation exposures as the patient's position is checked, adjusted, and rechecked.
Academic Courses > DOS 512 > Patient Markings During Simulation
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Written December 3, 2014
First Semester, Pre-Internship |