Session Item

Image-guided radiotherapy (IGRT) is commonly used to minimize the effect of positioning and to control tumor response. It is well known that interfraction variations can significantly influence the dose received by the lesion or organs at risk. These differences are sensitive in treatments such as in head and neck (H&N) region.

An analysis of planning target volumes (PTV) size changes in H&N treatments through image deformation tools is presented.

Seven patients with head and neck pathology treated with VMAT and IGRT by CBCT were retrospectively selected. The total number of PTV was 18, which were distributed among the seven patients with two or three prescribed dose levels ranging from 70 to 54 Gy.

To evaluate PTV volume changes, a synthetic CT (sCT) is generated from the CBCT image for each session. For this purpose, a deformable fusion algorithm implemented in Velocity^TM v4.1 is employed to modify the planning CT and the contours using the online match performed between daily CBCT and the planning CT as a basis.

The size variation across the treatment by the PTVs with respect to the originally contoured ones implies that the volume is smaller, on average. This average shows a decrease of over 3%, with a standard deviation of 4% (Figure 1).  

To illustrate how changes affect during a course, as an example, figure 2 shows daily changes on a patient.

A procedure for monitoring anatomical changes during treatment has been shown, warning about meaningful volume differences and helping us to make clinical decisions on treatment execution.