Tolerance levels for quality assurance of electron density values generated from CT in radiotherapy treatment planning.

Methods are described which relate the uncertainty in relative electron density derived from CT numbers to the uncertainty in treatment plan calculation for both photon and electron beams. These relationships are used to generate tolerance levels for electron density quality assurance measurements. These tolerance levels are dependent on treatment beam energy and tissue thickness, and are generally broader than current recommendations. The predicted treatment plan errors associated with these tolerance levels are shown to be consistent with calculations made using two commercial treatment planning systems. The tolerance levels are also shown to be practical by comparison against quality assurance measurements made on a conventional CT scanner and a treatment simulator CT system over a 12-month period. The results demonstrate that broader tolerances than are currently recommended can be justified in all situations except for electron beam treatments in which the therapeutic range falls within lung tissue.