[Poster] Root Cause Analysis of In Vivo Dosimetry Controls for Dynamic Arc Treatments

S. Celi^1,2, C. Wessels², P. François PhD³
1 DOSIsoft, Cachan, France
2 Medical Physics Department, Institut Curie, Paris, France
3 Medical Physics Department, CHU de Poitiers, Poitiers, France

Presented at AAPM 2015

Introduction: In-vivo-dosimetry (IVD) is mandatory in France since 2011 for all beams where this control is technically feasible. The most popular method remains the direct dose measurement by means of diodes or MOSFETs.However, these detectors are of limited use in the case of multiple complex fields. So with the broadening use of modern techniques, such as intensity modulated radiotherapy or dynamic arctherapy, the use of diodes and MOSFETs for mandatory IVD is rendered obsolete.

As an alternative, a transit IVD system such as EPIgray® (DOSIsoft S.A.) can reconstruct the delivered dose for IMRT and dynamic arctherapy fields from portal images recorded during the treatment. However, the recorded images of very small, complex, arctherapy fields give little additional information, such as body or bone delineation, for the interpretation of the IVD results and eventual deviations.

In the perspective of developing additional tools for a successful analysis of the results, it is thus important to dig into all technical and clinical parameters influencing the dose outcome. The observations made during a first analysis of the causes often cited for the failure of Quality Assurance controls, such as modulation and measurement point, are here presented.

Conclusion: Several plan parameters were here investigated: the position of the IVD point of measurement in the field/segment (Point Index) , the planned variability of the dose rate during the plan delivery (Dose Rate Variation and Standard Deviation) , and the complexity of the delivered arc (Modulation Complexity Score, Modulation Factor).