Academic Courses > DOS 522 > Calculations Away from Isocenter
DOS 522 - Week 2 Discussion
Writing Prompt
Give an example of a clinical situation when you had to calculate the dose to a different point(depth) when you were treating parallel opposed SSD or SAD. Be specific.
Initial Post: Moving a Calculation Point Away from Isocenter
At Loyola University Medical Center, we use parallel opposed fields to treat a variety of disease sites and conditions. Whole brains are one example of a case where opposed lateral fields provide good coverage and uniformity without the need for extensive contouring and planning work, but parallel opposed plans are not always straightforward. As I am beginning to learn the process of photon beam planning, I am starting with opposed field plans because they are the simplest to contour, plan, and adjust. They are also the easiest plans to help visualize the interplay of two beams by examining isodose lines.
This week, I observed the planning of a palliative cervical spine bone metastasis case. The planning dosimetrist, Teresa Kent, showed me (oral communication, January 27, 2015) how opposed anterior to posterior (AP) and posterior to anterior (PA) beams could be used to provide appropriate coverage to the spine. We used a fixed source to axis distance (SAD) technique, but she pointed out how calculating dose at the isocenter (the axis) would not provide an appropriate dose distribution for this patient. The treatment site in question was not centrally located, and we wanted to deliver a therapeutic dose to the entire vertebra while keeping high doses out of the airway to the extent possible. Since the airway is located immediately anterior to the spine, this would require a plan that placed more weight on the PA beam than on the AP beam. We could not simply place all of the dose on the PA beam because this would result in unacceptably high doses near the posterior surface of the skin and in the area around the spinous process.
Since the unequal weighting of the beams would cause the overall dose distribution to be asymetrical in the anterior/posterior dimension, we wanted to set the 100% prescription dose point at the front of the vertebral body. This is easily accomplished by simply moving the calculation point to a location immediately anterior to the spine because the planning system was configured to place the 100% prescription dose isodose line through that point. This resulted in everything immediately posterior to the calculation point getting at least the prescription dose, and everthing anterior to the calculation point getting less than the prescription dose. By fixing the 100% isodose line at that point in space, we were free to experiment with different weightings of the beams without the 100% isodose line being a moving target.
This week, I observed the planning of a palliative cervical spine bone metastasis case. The planning dosimetrist, Teresa Kent, showed me (oral communication, January 27, 2015) how opposed anterior to posterior (AP) and posterior to anterior (PA) beams could be used to provide appropriate coverage to the spine. We used a fixed source to axis distance (SAD) technique, but she pointed out how calculating dose at the isocenter (the axis) would not provide an appropriate dose distribution for this patient. The treatment site in question was not centrally located, and we wanted to deliver a therapeutic dose to the entire vertebra while keeping high doses out of the airway to the extent possible. Since the airway is located immediately anterior to the spine, this would require a plan that placed more weight on the PA beam than on the AP beam. We could not simply place all of the dose on the PA beam because this would result in unacceptably high doses near the posterior surface of the skin and in the area around the spinous process.
Since the unequal weighting of the beams would cause the overall dose distribution to be asymetrical in the anterior/posterior dimension, we wanted to set the 100% prescription dose point at the front of the vertebral body. This is easily accomplished by simply moving the calculation point to a location immediately anterior to the spine because the planning system was configured to place the 100% prescription dose isodose line through that point. This resulted in everything immediately posterior to the calculation point getting at least the prescription dose, and everthing anterior to the calculation point getting less than the prescription dose. By fixing the 100% isodose line at that point in space, we were free to experiment with different weightings of the beams without the 100% isodose line being a moving target.
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Academic Courses > DOS 522 > Calculations Away from Isocenter
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Written January 28, 2015
Second Semester, 1 Month into Internship |