Megavoltage cone beam computed tomography: Commissioning and evaluation of patient dose

Abstract

The improvement in conformal radiotherapy techniques enables us to achieve steep dose gradients around the target which allows the delivery of higher doses to a tumor volume while maintaining the sparing of surrounding normal tissue. One of the reasons for this improvement was the implementation of intensity-modulated radio therapy (IMRT) by using linear accelerators fitted with multi-leaf collimator (MLC), Tomo therapy and Rapid arc. In this situation, verification of patient set-up and evaluation of internal organ motion just prior to radiation delivery become important. To this end, several volumetric image-guided techniques have been developed for patient localization, such as Siemens OPTIVUE/MVCB and MVision megavoltage cone beam CT (MV-CBCT) system. Quality assurance for MV-CBCT is important to insure that the performance of the Electronic portal image device (EPID) and MV-CBCT is suitable for the required treatment accuracy. In this work, the commissioning and clinical implementation of the OPTIVUE/MVCB system was presented. The geometry and gain calibration procedures for the system were described. The image quality characteristics of the OPTIVUE/MVCB system were measured and assessed qualitatively and quantitatively, including the image noise and uniformity, low-contrast resolution, and spatial resolution. The image reconstruction and registration software were evaluated. Dose at isocenter from CBCT and the EPID were evaluated using ionization chamber and thermo-luminescent dosimeters; then compared with that calculated by the treatment planning system (TPS- XiO 4.4). The results showed that there are no offsets greater than 1 mm in the flat panel alignment in the lateral and longitudinal direction over 18 months of the study. The image quality tests showed that the image noise and uniformity were within the acceptable range, and that a 2 cm large object with 1% electron density contrast can be detected with the OPTIVUE/MVCB system with 5 monitor units (MU) protocol. The registration software was accurate within 2 mm in the anterior-posterior, left-right, and superior-inferior directions. The additional dose to the patient from MV-CBCT study set with 5 MU at the isocenter of the treatment plan was 5 cGy. For Electronic portal image device (EPID) verification using two orthogonal images with 2 MU per image the additional dose to the patient was 3.8 cGy. These measured dose values were matched with that calculated by the TPS-XiO, where the calculated doses were 5.2 cGy and 3.9 cGy for MVCT and EPID respectively.

Keywords: Dosimetry; EPID; IGRT; IMRT; MV-CBCT; Thermo-luminescent dosimeters; image quality.

Figure 1a,b

The reticule and its electrical radiograph showing the alignment between the EPID isocenter and the linear accelerator isocente

Figure 2a

The geometrical phantom of the MVCB-CT

Figure 2b

The reference calibration phantom on the left top, the calibration phantom image on the right top, the overlapping of the acquired geometrical phantom image with the reference image on the left bottom

Figure 3

(a) The image quality phantom, (b) an axial view for the head slice of the image quality phantom showing the 4 superior beads

Figure 4

(a) Low contrast section of the image quality phantom. Each section has inserts of four different materials: (1) 1% SIG, (2) 3% SIG, (3) brain, and (4) liver. (b) High contrast section of the image quality phantom: (5) inner bone, (6) acrylic, (7) air, and (8) CB2 (bone-50% mineral). For each material, there are five inserts of diameters 2, 1, 0.7, 0.5, and 0.3 cm. (c) Spatial resolution section of the image quality phantom where eleven bar groups with different numbers of line pairs per millimeter are inserted in this section

Figure 5

The center slice of region I in the image QA phantom showing 5 ROIs on the image in the axial view; on the display at the right of the ROI label “Mean/SD are displayed

Figure 6(a, b)

Slices of Section 2 and 4 of the image quality phantom, and (c) spatial resolution for Siemens Mvison MV-CBCT

Figure 7

Calculated dose distribution from the two orthogonal fields of EPID set-up verification for a left breast case