Inverse square, percent depth dose, and the MU you will actually type.
For Planning physicists · RTTs·9 min
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You will be able to
01Move a point source and see dose rate change as 1/r².
02Read a PDD: dmax, the fall, energy and field size.
03Write an SSD MU as dose over output times PDD times modifiers.
Chapter 01
The point that does not move
Isocenter is where gantry, collimator and couch axes meet. Inverse square is the geometry of that point. Double the distance, quarter the intensity — in air, from a point. Inside a patient you still start here, then you add attenuation and scatter.
SSD changes are inverse-square first, scatter second.
Simulation
Inverse square law — Change the numbers; the scene follows.
Variables
I₂ = 156.25 cGy/min
I2=I1(d2d1)2
Keep thisSSD changes are inverse-square first, scatter second.
Percent depth dose is central-axis dose at depth, as a percent of dose at dmax, at a stated SSD. Electrons contaminate the surface; photons build to dmax; then the curve falls. Higher energy pushes dmax deeper. Larger fields lift the tail with scatter. Never mix PDDs from two SSDs without Mayneord or a TMR conversion.
A PDD is married to its SSD, energy, and field.
Simulation
Percent depth dose — Change the numbers; the scene follows.
Variables
PDD = 90 %
PDD(d)=D(dmax)D(d)×100
Keep thisA PDD is married to its SSD, energy, and field.
Monitor units for an SSD setup are prescribed dose divided by calibration output, PDD over 100, and the modifiers Sc, Sp, wedge, tray. If the machine is one centigray per MU at dmax, ten by ten, reference SSD, this formula is the daily workhorse. Always compare with the record-and-verify.
Simulation
Monitor units — SSD technique — Change the numbers; the scene follows.
Variables
MU = 289.76 MU
Keep thisAn MU is not a dose. It is a charge the monitor chamber counted.