Use of kilovoltage X-ray volume imaging in patient dose calculation for head-and-neck and partial brain radiation therapy

Abstract

Background: To evaluate the accuracy of using kilovoltage x-ray cone-beam computed tomography (kV-CBCT) imaging for in vivo dose calculations.

Methods: A Region-of-Interest (ROI) CT number mapping method was developed to generate the cone-beam CT number vs. relative electron density calibration curve for 3D dose calculations. The stability of the results was validated for three consecutive months. The method was evaluated on three brain tumors and three head-and-neck tumor cases. For each patient, kV-CBCT images were acquired on the first treatment day and two-week intervals on the Elekta XVI system. The delivered dose distributions were calculated by applying the patients' treatment plans to the kV-CBCT images. The resulting dose distributions and dose volume histograms (DVHs) of the tumor and critical structures were compared to the original treatment plan.

Results: The kV-CBCT electron density calibration was stable within 1.5% over a three-month period. The DVH and dose distribution comparison based on the planning CT and the initial kV-CBCT showed good agreements for majority of cases. The doses calculated from the planning CT and kV-CBCT were compared on planes perpendicular to the beam axes and passing through the isocenter. Using gamma analysis with a criterion of 2 mm/2% and a threshold of 10%, more than 99.5% of the points on the iso-planes exhibited gamma <1. For one patient, kV-CBCT images detected 5.8% dose variation in the right parotid due to tumor shrinkage and patient weight loss.

Conclusions: ROI mapping method is an effective method for the creation of kV-CBCT electron density calibration curves for head-and-neck and brain tumor patients. Dose variations as monitored using kV-CBCT imaging suggest that some patients can benefit from adaptive treatment plan re-optimization.

Figure 1

Calibration curves for kV CT and kV CBCT. The calibration curves for kV-CT and kV-CBCT based dose calculations in the treatment planning system.

Figure 2

DVH displays of three clinical cases. The DVHs of three cases: one NK/T lymphoma (a), one NPC (b) and one Brain (c). The solid lines represent the dose based on conventional CT and the dash lines represent the dose based on kV CBCT.

Figure 3

Dose distributions of three clinical cases. The transverse views of dose distributions of the NK/T lymphoma (a), NPC (b) and brain tumor (c). Left: calculated dose based on CBCT images; Right: calculated dose based on planning CT images. They show good agreement on both relative high and low isodoses.

Figure 4

The transverse views of CT and the 2nd CBCT. The transverse views of the reference CT (a) and cone beam CT (b) after two weeks of the treatment. A slight change happened in the external contour and air cavity.