What parameter is used to specify beam quality for electrons in the AAPM TG-51 protocol?

In the context of the AAPM TG-51 protocol, the parameter used to specify beam quality for electrons is R50,D, which is the depth at which the dose is reduced to 50% of the maximum dose in the electron beam. R50,D is a critical measure because it reflects the energy and quality of the electron beam being utilized and ultimately influences the treatment plan.

The reason R50,D is emphasized is that different electron energies have different R50 values, making it a reliable way to characterize and compare the beam qualities of various electron energies. This parameter is essential for ensuring that the correct dose delivery and treatment planning can be achieved by adequately understanding and controlling the energy of the electron beam.

Other options listed, such as field size, PDD(10)x, and PDD(20)y, serve different purposes in dosimetry and treatment planning. Field size relates to the dimensions of the treatment area but does not define beam quality. The percentage depth doses (PDD) measured at 10 cm or 20 cm depths are significant for determining dose distributions but are not specific indicators of beam quality in the same way R50,D is, particularly for electron beams.