Investigation of Optimal Strategy of Internal Target Volume Generation for Liver Stereotactic Body Radiotherapy (SBRT)

                          http://austinpublishinggroup.com/radiation-oncology-cancer/ 

The success of liver Stereotactic Body Radiation Therapy (SBRT) programmed depends on high precision delivery of hypo fractionated radiation and accurate sparing of adjacent Organs at Risk (OARs). While proximity to OARs is well addressed with the use of highly conformal treatment planning techniques  the intra-fraction target displacement poses challenge in accurate delivery of planned hypo fractionated treatment to the target. While the average liver motion varies from 3-50mm the Centre Of Mass (COM) of the tumor (or target) moves about 9.7mm±5mm. Often anisotropic margin of 1-2cm is added to the Gross Tumor Volume (GTV) to account for the target motion without actual knowledge of patient specific motion which may not be representative of anisotropic tumor trajectory.

This may lead to under dosing of target volume or overdosing the OARs. Different strategies have been adopted to estimate the target motion like ultrasonography/x-ray cine fluoroscopy. In recent years, four dimensional CT (4DCT) has been widely used to generate Internal Target Volume (ITV) for hepatic and pulmonary tumors. ITV generation using 4DCT involves contouring of target volume in each of the respiratory phases, which may possibly be an accurate method of ITV generation. Various commercial systems are presently available to bin the respiration correlated CT into number of respiratory phases. 

Binning the 4DCT data set into 10 respiratory phases is considered optimum. However, target delineation in all the phases is time consuming and labor intensive. Ability to accurately characterize respiratory phase movement in limited phase datasets or maximum/minimum intensity projection images (MIP/MinIP) may provide a time efficient method of encompassing internal target motion Unlike lung tumors, limited information on accuracy of MIP/MinIP is available for liver tumors. Furthermore contouring on MIP/MinIP may not be applicable to all intrahepatic tumors due to variability in enhancement patterns and occasionally presence of I¹³¹ Lipidiol and other artifacts after Trans Arterial Chemo Embolization (TACE) Or Radiofrequency Ablation (RFA).