Evaluation of volumetric modulated arc therapy dose calculation plans based on kV-cone beam CT images for the breast cancer treatment.

The majority of treatment centers currently use Cone Beam Computed Tomography (CBCT) as an effective method of patient repositioning, opening up the prospect of using it in treatment plans calculation. The purpose of this task is to investigate the feasibility of utilizing kV-CBCT images as a potential alternative to CT scans for generating treatment plans during radiation therapy. The images are taken by the imaging system installed on a linear accelerator and implemented in a Treatment Planning System (TPS). This imaging system opens the possibility to proceed to dose calculation using a calibration curve that establishes a link between voxel attenuation in Hounsfield Units (HU) and electron densities relative to water (RED). Furthermore, a comparison of Volumetric Modulated Arc Therapy (VMAT) treatment plans on this imaging modality was done to the Computed Tomography (CT) imaging system using the CIRS phantom. The comparison is also performed the images of 10 patients with right breast cancer. VMAT plans from the two modalities were compared in terms of target coverage, normal tissue sparing, dose distribution parameters, and monitor units (MUs). Then, the gamma index test is employed by using the PTW Verisoft to compare the TPS calculated dose distribution for the two modalities. It is also used to compare the measured dose distribution performed by the Portal Dosimetry. Regarding the volume-dose parameters of the PTV and the Organs at risk (OARs), no differences were found between the VMAT plans of the two imaging modalities, CT and CBCT. Additionally, the gamma analysis results of the patients VMAT plans for 1%-1 mm, 2%-2 mm, 3%-3 mm, 4%-4 mm and 5%-5 mm showed that more than 84%, 90%, 92%, 94%, and 96%, respectively, of the points agreed upon between Eclipse calculated and measured dose distributions for the CT and CBCT VMAT plans. The good dosimetric agreement (gamma index reaches more than 92% for 3%-3 mm) between breast VMAT plans based on CT and CBCT images renders the latter an appealing verification tool and a substitute for CT images, if needed, for dose calculation. CBCT images are an effective option for direct dose calculation or to do adaptive radiotherapy in the case of a breast cancer.