Recent developments in hypo-fractionated Stereotactic Body Radiation Therapy (SBRT) of liver are allowing for the first time the delivery of extremely high doses of highly conformal radiation for the treatment of liver cancer. The precision and dose conformity with which SBRT treatments can be delivered makes the technique particularly susceptible to normal, respiratory-induced motion of both the targeted lesion and also of the surrounding healthy tissues which the method strives to spare. Failure to appropriately accommodate such motion can lead to unacceptable under-dosing of the target and dangerous overdosing of surrounding healthy tissue. Incorporation of motion into the treatment planning process has the potential to reduce these complications.

Recently, technology known as 4D Respiratory Correlated CT (4D RCCT) has been developed which tracks a patient’s breathing during CT image acquisition by monitoring a chest marker. Slices are tagged according to the patient’s chest wall height, which is used to determine the point in the breathing cycle at which the slice was acquired. These tags are then used to sort slices and create full 3D images which represent the patient’s anatomy at various breathing phases.

Using 4D RCCT imaging and deformable image registration techniques, anatomical images from all phases of breathing are brought into correspondence. This enables direct comparison of dose grids computed for different breathing phases during treatment planning. By summing these doses according to the amount of time the patient spends in each breathing phase, the total energy deposited at each point in the body is predicted. This provides an accurate representation of the end result of the treatment in the presence of patient breathing, allowing for more meaningful evaluation of treatment plans and the development of more effective therapies. Results showing the impact of respiratory-induced organ motion on dose calculation will be presented for patients undergoing SBRT treatment at the Huntsman Cancer Institute.