Independent verification of transferred delivery sinogram between two dosimetrically matched helical tomotherapy machines: A protocol for patient-specific quality assurance

The purpose of this study was to independently verify the transferred delivery sinogram between two dosimetrically matched helical tomotherapy machines with the goal of eliminating redundant quality assurance (QA) measurements on the second machine. The equivalence of the two machines was evaluated based on both geometric and dosimetric beam characteristics, including measuring open field per cent depth doses (PDD), longitudinal and transverse profiles and helical delivery of clinical patient treatment plans measured in phantoms. QA of 56 patient plans was studied. The delivery sinogram on the secondary machine was computed by accounting for the differences in the MLC characteristics of the two machines. Computed sinograms were compared against the transferred sinograms by tomotherapy's data management system for the same 56 patient plans. The PDD, transverse and longitudinal dose profiles agreed within ±1% between the two machines. Ionization chamber and planar dose measurements with the Iba MatriXX device on both machines for the 56 patients were found to be within ±3% of the doses computed by the tomotherapy treatment planning system. For all 56 patients, the differences between computed sinograms and DMS-converted sinograms were within ±2%. The matched tomotherapy machines had similar beam characteristics. The sinogram-based QA was validated using point and planar dose measurements and found to be acceptable for clinical use.