ESTRO 2024 - Abstract Book

S4010

Physics - Inter-fraction motion management and offline adaptive radiotherapy

ESTRO 2024

Purpose/Objective:

Highly conformal treatments like VMAT (Volumetric modulated arc therapy), where high dose fall-offs are the norm, may result in significant variation in treated versus planned doses due to daily changes in the volume, position, and contour outline of organs at risk (OAR) in head and neck cases. The purpose of this study is to examine strategies for estimating the effect of these volumetric changes on dose volume histograms (DVH) of serial OARs such as the Mandible, and Spinal cord and parallel OARs Parotids, during VMAT treatment of high-risk hypopharynx patients. The study aimed to determine the most accurate and efficient method for estimating the appropriate dose for daily fractions in adaptive planning. By comparing different modalities and methodologies, including deformable registration, online registration, and transit dosimetry, we sought to identify the optimal approach that would ensure timely adjustments to treatment plans

Material/Methods:

Ten patients with Ca hypopharynx treated in our institute were chosen retrospectively for this study. All treatment plans were made using two full arcs with complementary collimator angles and energy 6MV in Eclipse treatment planning system. Weekly CBCT i.e. every sixth fraction of patients treated were taken for dose estimation using four modalities and PTV (Planned Target Volume) based online matching was done before treatment. The dose volume histogram (DVH) parameters estimated for Serial organs such as spinal cord and mandible and Parallel organ parotid.

Various modalities and methodologies used for DVH parameters estimation were as follows:

Method 1 (CBCT): OARs were contoured on CBCT of the day and propagated onto the planning CT (pCT) based on the online registration during treatment. The doses and volume so obtained are recorded. These are assumed as the reference value/delivered values. Method 2 (PerFRACTION™): The transit fluence through the patient was collected during the treatment using EPID (Electronic Portal Imaging Device) and log files fed as input to PerFRACTION™ (Sun Nuclear Corp, USA), a commercial in-vivo dosimetry system. In this method DVH parameters estimated on CT images created by the system using the CBCT. Method 3(Adaptive Synthetic CT): set of parameters were obtained by recalculating the plan on synthetic CT which is obtained from deformable registration of CBCT with pCT using Velocity™. The contours of OARs are auto created by the system using deformable algorithm.

Method 4 (Dose deformation): set of parameters were obtained by deformation of planned doses according to CBCT using Velocity™. The DVH parameters were recorded using contours of pCT.

Method 5 (Dose and Structure deformation): In this method dose deformation is performed according to CBCT using Velocity™. The contours are directly drawn on CBCT of the day and DVH parameters were recorded way. In this method both OARs deformation and dose deformation were considered

The Wilcoxon signed rank test was used for statistical analysis and p<0.05 considered as Statistically significant.