[On board imaging with cone beam CBCT kV VARIAN: Montauban's radiation therapy department experience].

PURPOSE: To describe our practice day to day with a VARIAN linac "Clinac 2100" fully equipped with an On Board Imager (OBI) for patients with prostate cancer.

MATERIALS AND METHODS: A volumetric and dosimetric study was performed in 2006 using ARTIVIEW software (AQUILAB) for nine patients and 76 Cone Beam CT kV (CBCT kV). We have contoured targets and organs at risk from CBCT kV slides acquisitions. Second, we achieved a dose-volume histogram (DVH) study for a patient treated in 2007 with IMRT technique in comparison with the 2006 study.

RESULTS: 2006 analysis: The study showed a very important variability of organ measurements. Seminal vesicles were strongly influenced by adjacent organs; observed differences for prostate could be explained by contouring uncertainty on the apex. Inter-sessions motions could be observed for bladder, rectum and seminal vesicles (SV). Part of prostate volume not encompassed by PTV is about 2.5%; VS volume outside PTV is about 35%. Tumoral conformation index (TCI) is inferior to 97.5% in 22% of all cases. Anteroposterior displacements of the prostate barycentre is superior to 5mm. From this analysis, we recommended the strict respect of hygienodietetic rules, and we have adapted the system settings for better immobilization, which were applied for the 2007 study. For the 2007 analysis, since April 2007, most of patients are treated with IMRT for prostate cancer, at the second part of the radiation therapy to encompass only the prostate volume. Dose-volume histograms showed a great spreading out for 2006 patients, and not for the 2007 patient.

CONCLUSIONS: IMRT and IGRT should permit a margin reduction for PTV. Strict respect of hygienodietetics rules is necessary to avoid rectal distension and local recurrence.