[Poster] Exemple d’Utilisation de la Dosimétrie In Vivo de Transit comme Système d’Alerte pour la Radiothérapie Adaptative*

*Using in vivo transit dosimetry as a warning system for adaptive radiation therapy (about  an example)

F. Vincent¹, F. Husson², H. Tournat¹
1 Centre d’Oncologie et de RadioThérapie 37, Chambray-lès-Tours
2 R&D Medical Physics, DOSIsoft, Cachan

Presented at SFPM 2015

ABSTRACT
Introduction: In vivo dosimetry is a powerful decision-making tool since it allows ultimate control of the actually delivered dose to the patient in relation to the planned dose. Commonly used at the initial treatment fractions, it is an essential means of verifying the entire RT-processing, and remains a regulatory requirement in France. In vivo dosimetry based on portal imaging (transit dosimetry) facilitates throughout its simplicity of use, the frequency increase of the dosimetric checks during the whole treatment course, without changing the beam quality. In addition to the clinical follow-up, it helps to provide a dosimetric monitoring as an indicator of the treatment overall quality.

Transit dosimetry is a deviation detection system sensitive to linear accelerator malfunction, reproducibility problems of patient alignment and possible anatomical changes of the patient during the weeks of treatment. The purpose of this study is to illustrate the ability of such a system to issue a relevant warning in a clinical weight loss case.

Methods: The study is based on a cervix treatment case with the following conventional fractionation: 45Gy at 1.8Gy per fraction, 5 times a week, and an additional 20Gy at 2Gy per fraction,5 times a week, for an overall time of 47 days. Initial planning is established from planning CT images using Eclipse-V10 treatment planning system (Varian). Irradiation is carried out with VMAT technique (RapidArc) at 25 MV, with a Clinac 2100C (Varian) equipped with an aS1000 portal imaging device (EPID). The results of transit dosimetry are provided by the Epigray solution (Dosisoft) from portal images acquired in movie mode throughout the gantry course. In vivo dosimetry checks were performed weekly on the basis of three anatomical points specified by the user and an additional automatic volumetric assessment.

Results: Dosimetric controls show a monotonic trend of increased reconstructed dose value for all control points (user and automatic points). This evolution is directly related to a patient weight loss phenomenon. It is compared to onboard imaging (CBCT) acquired during the different fractions and recalculated doses from a second planning CT exam.

Conclusion: In vivo transit dosimetry enables automatic detection of major anatomical changes of the patient when compared with action levels. It constitutes a relevant warning and is an additional argument for an update of the treatment planning (adaptive RT).