A study on adaptive IMRT treatment planning using kV cone-beam CT

Background and purpose: Changes in tumor size during the course of radiotherapy warrant performing adaptive radiotherapy (ART). This work investigates the feasibility and usefulness of acquiring on-board cone-beam CT (CBCT) for ART for patients with bulky head and neck tumors treated with IMRT and for prostate patients with potentially significant target position variations during the treatment course. Materials and methods: A phantom designed for CT quality assurance was used to compare the dosimetric and geometric accuracy between conventional CT and CBCT from a linear accelerator's on-board imager. Patient planning CT and CBCT images were acquired before treatment and at mid-course. The IMRT plans made on the CT were applied to the CBCT and dose-volume histograms were calculated. Results: In both phantom and patient studies, the dose-volume histograms (DVHs) based on CBCT images were in excellent agreement with DVHs based on planning CT images. Minimum, maximum and mean doses agreed very well. In a patient study, doses for targets and normal tissues from the same IMRT plans calculated on CBCT images agreed within 1-3% with those calculated on planning CT images. Conclusions: CBCT images can be used to accurately predict dosimetric results. It is feasible to use CBCT to determine dosimetric consequences resulting from tumor shrinkage and patient geometry changes. An additional planning CT may be necessary to perform IMRT re-planning at present in order to accurately delineate tumor and organs. The CBCT has potential to become a very useful tool for on-line ART.