Correction of megavoltage cone-beam CT images of the pelvic region based on phantom measurements for dose calculation purposes

Abstract

Megavoltage cone-beam CT (MVCBCT) is an imaging technology that provides a 3D representation of the patient in treatment position. Because it is a form of x-ray tomography, MVCBCT images give information about the attenuation coefficients of the imaged tissues, and thus could be used for dose calculation. However, the cupping and missing data artifacts seen on MVCBCT images can cause inaccuracies in dose calculations. To eliminate these inaccuracies, a correction method specific to pelvis imaging and based on phantom measurements has been devised. Pelvis-shaped water phantoms of three different sizes were designed and imaged with MVCBCT. Three sets of correction factors were created from the artifacts observed in these MVCBCT images by dividing the measured CT number by the predefined CT number for water. Linear interpolation is performed between the sets of correction factors to take into account the varying size of different patients. To compensate for the missing anatomy due to the limited field of view of the MVCBCT system, the MVCBCT image is complemented with the kilovoltage CT (kVCT) image acquired for treatment planning.When the correction method is applied to an anthropomorphic pelvis phantom, the standard deviation between dose calculations performed with kVCT and MVCBCT images is 0.6%, with 98% of the dose points agreeing within +/- 3%.With uncorrected MVCBCT images this percentage falls to 75%. An example of dose calculation performed with a corrected clinicalMVCBCT image of a prostate cancer patient shows that changes in anatomy of normal tissues result in variation of the dose distribution received by these tissues.This correction method enablesMVCBCT images to be used for the verification of the daily dose distribution for patients treated in the pelvis region.

Figure 1

CT numbers and correction factors profiles. (a) Profiles of CT numbers from MVCBCT images of pelvis water phantoms of different sizes. (b) Profiles of correction factors obtained from the same three water phantoms; blue: phantom A (small size); black: phantom B (medium size); red: phantom C (big size). Solid lines: lateral profiles. Dashed lines: longitudinal profiles.

Figure 2

Example of a custom‐made pelvic water phantom. This picture shows phantom B.

Figure 3

Average thickness of four phantoms and one patient as a function of MVCBCT acquisition angle; blue: phantom A; green: phantom B; red: phantom C; grey: anthropomorphic phantom used for validation; black: prostate cancer patient.

Figure 4

Cross sections of kVCT and MVCBCT images of an anthropomorphic pelvic phantom: (a) the kVCT image; (b) the uncorrected MVCBCT image; (c) the uncorrected MVCBCT+ image; and (d) the corrected MVCBCT+ image for the anthropomorphic phantom.

Figure 5

Percent dose difference histograms. The PDDiffs between the dose distributions obtained with kVCT and the uncorrected CB images are in solid gray, and the kVCT and the corrected MVCBCT+ images are in solid black. The fitted Gaussian function is shown by the black dashed line.

Figure 6

Cross section of the percent dose differences. was obtained with the uncorrected MVCBCT+ image of the anthropomorphic phantom; and 6b was obtained with the corrected MVCBCT+ image of the anthropomorphic phantom. Red shows a PDDiff of 3% or more; green shows a PDDiff of $\pm 3\%$; and blue shows a PDDiff of $-3\%$ or less.

Figure 7

Dose calculation with clinical kVCT and MVCBCT images. (a) Sagittal slice showing the overlaid contours of the prostate (pink), rectum (brown) and bladder (blue) from the planning kVCT and the MVCBCT image. The bigger bladder contour is from the MVCBCT image. (b) Dose‐volume histograms of these structures for treatment planning (dotted lines) and obtained with the MVCBCT image (solid lines). (c) Close up of the prostate and rectum contours on the planning CT image. (d) Close up of the prostate and rectum contours on the MVCBCT image.