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Treatment Planning Matching
Depth of Maximum Equilibrium (Dmax)
The depth at which electronic equilibrium occurs for photon beams. Dmax is the point where the maximum absorbed dose occurs for single field photon beams and depends mainly on the energy of the beam (W/L, pg. 496).
If setting a 10 x 10 field size using an isocentric technique, the field size on the patient’s skin would be?
Smaller.
Depth
The distance beneath the skin surface where the prescribed dose is to be delivered (W/L, pg. 494)
A patient is treated at 100 cm SSD with 6 MV photons. Collimator setting is 15 cm x 15 cm. There is no blocking. 300 cGy per fraction is to be delivered at Dmax. What is the dose delivered at a depth of 5 cm?
(300 x 87.9)/100 = 263.7 cGy. (See Table 24-6 W/L pg. 519)
SSD
The distance from the source of photons to the patient’s skin surface (W/L, pg. 494).
Tumor Dose (Dmax Dose)
TD= (Given dose x PDD at depth of calculation)/100 (W/L, pg. 509).
Inverse Square Law
I1/I2 =(D2/D1 )^2 (W/L, pg. 497).
What is the Dmax for a 1.25 MV beam?
0.5 cm (RT Essentials, pg. 140).
What is the field size on a film if the collimator setting is 7 cm X 19 cm and the magnification factor is 1.33x?
9 X 25; 1.33 x 7 cm = 9.31 cm; 1.33 x 19 cm = 25.27 cm
What are the 80% and 90% isodose lines for a patient treated with a 16 MeV electron beam?
80% = 16/3 = 5.3 cm; 90% = 16/4 = 4 cm.
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