Prp Correction Factor

 

The Prp correction factor, introduced in the TG-51 addendum (McEwen et al., 2014), is a radial beam profile correction factor that takes into account the non-uniformity of the beam profile in the axial direction. It is particularly important in peaked FFF beams, but can also be on the order of 1-2% for flat beams.

 

Per the TG-51 addendum: “To determine Prp in the clinic, one calculates the average of the radial dose profile over the dimensions of the active part of the chamber. For linacs with a flattening filter, a simple 1D integration (average) along the thimble should be sufficient as this dominates over averaging across the air cavity. The magnitude of Prp can be significantly larger for FFF beams because the unflattened beam is strongly peaked along the central axis and therefore a more detailed 2D measurement and calculation might be required for such beams.”

 

For FFF beams, where the dose is higher at central axis and falls off radially, the correction factor Prp should be greater than 1.000. For flat beams, the correction factor value will depend on the shape of the beam profile at the depth of calibration.