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medphysics
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This is a homework question that is due on Monday and I'm not sure how to proceed. Any help at all would be appreciated. I do not have a background in physics and this is my second quarter as a grad student. please help.
Two 10 MV beams treat a patient in an SAD. The isocenter is given. Determine the dose the patient receives from both beams at points A, and B, and C if the dose to the isocenter is 200 cGy per day. The isocenter is 10 cm deep from the anterior beam and 12 cm deep from the lateral beam. The field size is 12 x 16 for both fields.
A: 2 cm off axis from CAX of AP beam and 12 cm deep (diagonal).
B: depth of dmax from AP beam, on central axis
C: 20 cm deep from lateral beam on central axis
we were told to break up the question into separate parts,
a) dose from AP to point A, b) Dose from AP to point B, Dose from AP to point C, dose from lateral beam to point A, etc
I know that we need to determine the field size by finding the equivalent square and then take the ratio of the TMR's multiplied with the ratio of the Sp's and the ISF...but I am not sure how to calculate the dose from the AP beam to points A and C and I do not even know how to approach the lateral beam.
This is what I did for dose at B taking into account dmax:
Dose from AP to pt B:
AP beam
Depth = 100 – 10 + 2.5 = 92.5cm
Field size = 13.7 x 92.5 = 12.7cm
100
FS at pt B (dmax) = 13 (approx)
Dose = 100 x (ratio of TMRs) x ratio of Sp’s x ISF
= 100 x 1 x 1.009 x (100/92.5)^2
0.847 x 1.012
= 137.6cGy
I do not know if I even did this right. Please someone help me
Two 10 MV beams treat a patient in an SAD. The isocenter is given. Determine the dose the patient receives from both beams at points A, and B, and C if the dose to the isocenter is 200 cGy per day. The isocenter is 10 cm deep from the anterior beam and 12 cm deep from the lateral beam. The field size is 12 x 16 for both fields.
A: 2 cm off axis from CAX of AP beam and 12 cm deep (diagonal).
B: depth of dmax from AP beam, on central axis
C: 20 cm deep from lateral beam on central axis
we were told to break up the question into separate parts,
a) dose from AP to point A, b) Dose from AP to point B, Dose from AP to point C, dose from lateral beam to point A, etc
I know that we need to determine the field size by finding the equivalent square and then take the ratio of the TMR's multiplied with the ratio of the Sp's and the ISF...but I am not sure how to calculate the dose from the AP beam to points A and C and I do not even know how to approach the lateral beam.
This is what I did for dose at B taking into account dmax:
Dose from AP to pt B:
AP beam
Depth = 100 – 10 + 2.5 = 92.5cm
Field size = 13.7 x 92.5 = 12.7cm
100
FS at pt B (dmax) = 13 (approx)
Dose = 100 x (ratio of TMRs) x ratio of Sp’s x ISF
= 100 x 1 x 1.009 x (100/92.5)^2
0.847 x 1.012
= 137.6cGy
I do not know if I even did this right. Please someone help me