Pre-treatment PSQA – Do we still need to measure and what are adequate alternatives?

The most critical failure modes that need to be detected by PSQA are errors in the beam model and algorithm limitations [1]. Patient-specific QA, i.e., measurement of the fluence from the patient plan recalculated on a phantom, is intended to ensure that calculated and delivered dose distributions are consistent. However, these phantoms are usually geometrically regular and homogeneous, and many detector arrays have quite coarse spatial resolution. The tools used for PSQA are therefore unsuitable or inherently unable to probe certain aspects of the beam model and calculation algorithm. This observation is born out in audit results; IMRT and VMAT plans containing serious dosimetric errors were not detected using departmental PSQA. Further, patient anatomy cannot be included in these measurements, so the process should more accurately be termed plan-specific than patient-specific QA.