Chapter 10 Kahn: The Physics of Radiotherapy
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What is the collimator scatter factor? How do we measure it?
The collimator scatter factor (Sc), commonly called output factor, is defined as the ratio of the output in air for a given field to that for a reference field (like a 10x10 cm2). It may be measured with an ion chamber with a buildup cap large enough to provide maximum dose buildup for the given energy.
What is the phantom scatter factor?
Is the ratio of the dose rate for a given field at a reference depth (like maximum dose depth) to the dose rate at the same depth for the reference field size (like 10x10 cm2) with the same collimator opening. It takes into account the change in scatter radiation originating in the phantom at a reference depth as the field size is changed.
What is TPR and TMR?
TPR is the Tissue Phantom Ratio, defined as the ratio of the dose at a given point in phantom to the dose at the same point at a fixed reference depth, usually 5cm: TPR=D(d)/D(dref)
TMR is the Tissue Maximum Ratio, it is a special case of TPR, defined as the ratio of the dose at a given point in phantom to the dose at the same point at the reference depth of maximum dose dmax:TMR=D(d)/D(dmax)
What is the Scatter-maximum ratio?
Is a quantity designed for the calculation of scattered dose in a medium. Defined as the ratio of the scattered dose at a given point in a phantom to the effective primary dose at the same point at the reference depth of maximum dose.
For a Linac, what are the monitor units necessary to deliver a certain tumor dose (TD) at a depth d for a field size r at the surface at any SSD?
- Monitor Units= [(Tumor Dose)x100]/[Kx(%DD)x(Sc(rc))x(Sp(r))x(SSD factor)]
- Where K for the machines is usually calibrated to deliver 1rad per MU, rc is the collimator field size rc=r*(SAD/SSD) and SSD factor=(SCD/(SSD+t0)]^2
What do Sc and Sp pertain to?
They pertain to the collimator defined field and the field actually irradiating the phantom.
What do we do if the field is not square?
- Block the field with lead and calculate a tray factor. Tray factor is the ratio of the dose rate at a depth with tray to the dose rate at a depth without tray; it is energy and field size dependent.
- Then we calculate the equivalent square of the open portion of the blocked field. We do have to estimate the blocked portion dividing the field in quadrants, though it doesn't have to be precise because the DD doesn't change that much anyway.
Are the percentage depth dose and TMR both dependent on SSD?
No. Whereas PDDs depend on SSD, TMRs are almost independent of SSD.
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