Discussion topic:
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. Where is the point of calculation? How did it change your beam weighting? Did you have to use different energies? Etc.
Discussion post:
Today I worked on a sim and treat plan for a whole brain. In my clinic for whole brains we typically use a calc point located off of central axis to achieve optimal dose distribution while reducing the amount of maximum dose. Planning a whole brain in this manner can be tricky and placement of the calc point can vary depending on where the doctor places the beam isocenter, how the fields are drawn, and patient setup geometry. The best starting point is to place your calc point at iso and see how 100% isodose coverage looks. You can move the calc point closer to areas receiving low dose to bring the 100% isodose line closer to covering the desired area.
For this plan the physician prescribed 30Gy to be delivered in ten fractions and then placed the isocenter just posterior to the patient lens approximately mid eye. This is the doctor’s preference as it reduces beam divergence into the contralateral eye. This plan had areas of low dose in the base of skull and between the mastoid air cells just posterior to the clivus. I opted to place my calc point between the mastoid air cells and close to the bony clivus. This is a common place of underdose using our method so it is typical to place a calc point in this area. After placing the calc point my area of under dose in the base of skull was gone but a small area of under dose remained just to the left of my calcpoint which was midline. I moved the point .6cm to the left to receive the coverage needed. As a result, I had to alter beam weighting to reduce my 110% isodose line distribution on the left side. I weighted the left lateral 45.5% and right lateral 50.5% this evened out my isodose lines on both the left and right side. After placing the calc point I used MLCs to produce 4 control points per beam. The control points eliminated all 110% and 107% isodose lines and left a small amount of 105% that could not be eliminated without sacrificing coverage. I used 6x energy for this plan which resulted in a final global max of 31.4Gy. The MD approved the plan and I am one step closer to being ready for a whole brain comp.1
References
1. Khan FM, Gibbons JP. A System of Dosimetric Calculations. In: Khan FM, Gibbons JP, ed. The Physics of Radiation Oncology. 5th Philadelphia, PA: Lippincott Williams & Wilkins; 2014:151-169
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. Where is the point of calculation? How did it change your beam weighting? Did you have to use different energies? Etc.
Discussion post:
Today I worked on a sim and treat plan for a whole brain. In my clinic for whole brains we typically use a calc point located off of central axis to achieve optimal dose distribution while reducing the amount of maximum dose. Planning a whole brain in this manner can be tricky and placement of the calc point can vary depending on where the doctor places the beam isocenter, how the fields are drawn, and patient setup geometry. The best starting point is to place your calc point at iso and see how 100% isodose coverage looks. You can move the calc point closer to areas receiving low dose to bring the 100% isodose line closer to covering the desired area.
For this plan the physician prescribed 30Gy to be delivered in ten fractions and then placed the isocenter just posterior to the patient lens approximately mid eye. This is the doctor’s preference as it reduces beam divergence into the contralateral eye. This plan had areas of low dose in the base of skull and between the mastoid air cells just posterior to the clivus. I opted to place my calc point between the mastoid air cells and close to the bony clivus. This is a common place of underdose using our method so it is typical to place a calc point in this area. After placing the calc point my area of under dose in the base of skull was gone but a small area of under dose remained just to the left of my calcpoint which was midline. I moved the point .6cm to the left to receive the coverage needed. As a result, I had to alter beam weighting to reduce my 110% isodose line distribution on the left side. I weighted the left lateral 45.5% and right lateral 50.5% this evened out my isodose lines on both the left and right side. After placing the calc point I used MLCs to produce 4 control points per beam. The control points eliminated all 110% and 107% isodose lines and left a small amount of 105% that could not be eliminated without sacrificing coverage. I used 6x energy for this plan which resulted in a final global max of 31.4Gy. The MD approved the plan and I am one step closer to being ready for a whole brain comp.1
References
1. Khan FM, Gibbons JP. A System of Dosimetric Calculations. In: Khan FM, Gibbons JP, ed. The Physics of Radiation Oncology. 5th Philadelphia, PA: Lippincott Williams & Wilkins; 2014:151-169