Statistical Process Control inTomotherapy pre-treatment QA:Fixing tolerance and action limits for the verification metrics

The different degrees of freedom that IMRT has in helical Tomotherapy lead to increased uncertainties due to the dynamic nature and dosimetric variability of the treatment. Therefore, the verification of treatment plans is a standard and essential part of the workflow in planning patients treated with IMRT in helical Tomotherapy today. This implies a challenge for medical physicists in the development of tools and metrics that ensure and guarantee the quality assurance (QA) of the plans to be delivered to the patient.

One of the most adopted procedure in Tomotherapy pre-treatment verification QA  consists in the evaluation of the deviation between a calculated dose and a measured one (single-point absolute measurement) together with the analysis of the gamma index, after the treatment plan is computed on an appropriate phantom.

For this purpose, Tomotherapy planning system includes a software, better know as DQA (Delivery Quality Assurance), to compute the plan dose in a phantom selected by the user. The phantom is then irradiated under the treatment conditions to obtain the measured dose and the gamma index is evaluated. As we will discuss later, this index includes the comparison of the dose distributions from a geometric point of view (by studying the displacement between the reference and the  measured dose distributions) and from a dosimetric point of view (by punctually evaluating the difference between planned and delivered doses).