Question

In beam modelling with Pinnacle RTPS , What are three modelling elements that affect the “out-of-field” dose?

Answer 1

The methodology was initiated by using vendor-provided automodeling software to generate a single set of beam parameters that gives an approximate fit to relative dose distributions for all beams, open and wedged, in a data set. A limited number of beam parameters were adjusted by small amounts to give accurate beam models for four open-beam field sizes and three wedged-beam field sizes. Beam parameters for other field sizes were interpolated and validated against measured beam data. Using this methodology, a complete set of beam parameters for a single energy can be generated and validated in approximately 40 h.

three modelling elements that affect the “out-of-field” dose are,

1) a discrete energy spectrum consisting of a set of energies and corresponding relative photon fluences, which are described by relative weights at each energy

2) a factor, S, that models off-axis beam softening

3) a cone angle and a cone radius for modeling off-axis changes in the in-air photon fluence

4) a transmission factor for photon fluence through the collimators

5) the height and width of the Gaussian distribution of scatter from the flattening filter

6) a measure, MSF, of the scatter from the beam modifiers; ~

7) the dimensions of the photon source; and

8) a set of parameters that model the electron contamination:

dm , a maximum depth of electron contamination.

  K, a factor that describes steepness of the exponential depth dose of electron contamination.

SF, a factor modifying the surface dose.

DF, a depth at which the electron contamination curve becomes linear.

A, a factor that measures the rapidity at which the off-axis component of the electron contamination goes to zero.

C1 , C2 , and C3 , parameters that alter the field-size dependence of the electron contamination.