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.

Image Quality Determination of a Novel Low Energy X-ray Detector

“…Lassena was compared to other available CBCTs on the market. The results are illustrated in table 1 [8][9][10][11][12]. It is apparent that Lassena has the lowest noise due to the thick scintillator.…”

Evaluation of mechanical and geometric accuracy of two different image guidance systems in radiotherapy

“…The geometric calibrations of these systems were mentioned elsewhere. [21][22][23] Even though a MV-based imaging system has simple geometry, it lags behind in terms of image quality and imaging dose to the patient. Even if the imaging dose can be accounted in the treatment planning, the volume of tissue irradiated with the low imaging dose may be significant.…”

The risk of secondary cancer in nasopharyngeal carcinoma paediatric patients due to intensity modulated radiotherapy and mega‐voltage cone beam computed tomography

“…Mega‐voltage cone beam computed tomography dose calculations were performed on the CT images of the ten NPC paediatric patients using the arc acquisition offered by the MV‐CBCT system: Angles from 270° to 110° and a field size of (27 × 27 cm 2 ) were used. The number of MUs per fraction was assumed to be five MUs in each verification made for the patient throughout the treatment course …”

“…The number of MUs per fraction was assumed to be five MUs in each verification made for the patient throughout the treatment course. 16 For the cases under investigation, the verification of patients' position using MV-CBCT was acquired for each patient in the first three treatment days and weekly thereafter throughout the rest of the treatment course. 17 So, for all of these cases the total MUs that were taken during the treatment course from imaging using MV-CBCT were 40 MUs.…”