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

Abstract

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.

Measurement‐guided volumetric dose reconstruction for helical tomotherapy

“…A chamber‐based array with a phantom cavity partially compensating for the angular dependence could be successfully compared to the TomoTherapy TPS at the 5%/3 mm level in one publication (31) and 3%/3 mm in another (32) . Another planar chamber array required up to 3% plan‐specific correction factor to achieve acceptable agreement with the TPS, again at the 3% G/3 mm level (33) . This was attributed to the “uncertainties of TomoTherapy delivery”.…”

Implications of leaf fluence opening factors on transfer of plans between matched helical tomotherapy machines

“…The latency is corrected following the methods of (Yaddanapudi et al 2012) to remove the latency from the source machine and insert the latency from the destination machine. It should be noted that in order to reproduce the results of (Yaddanapudi et al 2012) it was necessary to invert the equations from those in their paper.…”

“…It should be noted that in order to reproduce the results of (Yaddanapudi et al 2012) it was necessary to invert the equations from those in their paper. The accuracy of the transfer process between our machines had been confirmed by performing phantom measurements on a large number of transferred plans.…”

“…However when comparing the sinograms for some chest wall plans we noticed some with <95% of LOTs agreeing within 5 ms. On investigation these turned out to result from differences in leaf fluence opening factors (LFOFs) between the two machines (Balog et al 2003). These are mentioned in (Yaddanapudi et al 2012) but no method is given to correct for them. The purpose of this note is to describe an extension to the method to correct for LFOF variation between machines.…”

“…A paper by Yaddanapudi et al (2012) described a method for independent verification of plans that had been transferred between two matched helical tomotherapy machines. The technique involved removing the latency correction for the source machine, and applying the latency correction for the destination machine.…”