RT Simulator User's Guide
Portal Weight
Portal weight in RT Simulator automatically switches between two definitions:
1. SAD: When the geometrical depth (average depth in the case of an arc) of a portal's isocenter in the patient is greater than Dmax for that portal's energy & field size, beam weight is defined as the dose (cGy) that would be delivered to a point on the central axis of the unblocked beam in the homogeneous calibration phantom at depth equal to the radiological depth (average radiological depth in the case of an arc) in the patient.
2. SSD: If the depth of a portal's isocenter in the patient is less than or equal to Dmax (or is outside of the patient), weight is defined as the dose (cGy) that would be delivered to a point on the central axis of the unblocked beam in the homogeneous calibration phantom at depth Dmax.
The actual dose delivered to the portal isocenter in the patient will differ slightly from the weight depending on the location of blocks and tissue inhomogeneity factors.
Example:
Suppose you want to deliver 180cGy/fraction to the 90% isodose line of a six field prostate setup with a single isocenter, such that 50% of the dose is delivered by the laterals and 50% is from the oblique fields. You might initially set the weight of the right & left lateral fields to 50.0 each, and the oblique fields to 25.0 each for a total of 200.
The display in the Dose Prescription window might look something like this...
WARNING: Be sure to properly normalize the isodose legend! If you have created several 2D and/or 3D dose matrices, you might want to normalize to the global maximum. In this example, the legend was normalized to the maximum dose in a 2D dose matrix on the transverse slice through isocenter.
Now, select the isodose line (or surface) to prescribe to from the Dose Rx buttons (the button opposite the 0.900 line in this example) and then adjust the Rx scaling factor until the dose reads 180 cGy.
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