Image-guided radiotherapy using a mobile kilovoltage x-ray device

Monitoring external beam radiotherapy using real-time beam visualization

PURPOSE

To characterize the performance of a novel radiation therapy monitoring technique that utilizes a flexible scintillating film, common optical detectors, and image processing algorithms for real-time beam visualization (RT-BV).

METHODS

Scintillating films were formed by mixing Gd2O2S:Tb (GOS) with silicone and casting the mixture at room temperature. The films were placed in the path of therapeutic beams generated by medical linear accelerators (LINAC). The emitted light was subsequently captured using a CMOS digital camera. Image processing algorithms were used to extract the intensity, shape, and location of the radiation field at various beam energies, dose rates, and collimator locations. The measurement results were compared with known collimator settings to validate the performance of the imaging system.

RESULTS

The RT-BV system achieved a sufficient contrast-to-noise ratio to enable real-time monitoring of the LINAC beam at 20 fps with normal ambient lighting in the LINAC room. The RT-BV system successfully identified collimator movements with sub-millimeter resolution.

CONCLUSIONS

The RT-BV system is capable of localizing radiation therapy beams with sub-millimeter precision and tracking beam movement at video-rate exposure.

Progressive cone beam CT dose control in image-guided radiation therapy

PURPOSE

Cone beam CT (CBCT) in image-guided radiotherapy (IGRT) offers a tremendous advantage for treatment guidance. The associated imaging dose is a clinical concern. One unique feature of CBCT-based IGRT is that the same patient is repeatedly scanned during a treatment course, and the contents of CBCT images at different fractions are similar. The authors propose a progressive dose control (PDC) scheme to utilize this temporal correlation for imaging dose reduction.

METHODS

A dynamic CBCT scan protocol, as opposed to the static one in the current clinical practice, is proposed to gradually reduce the imaging dose in each treatment fraction. The CBCT image from each fraction is processed by a prior-image based nonlocal means (PINLM) module to enhance its quality. The increasing amount of prior information from previous CBCT images prevents degradation of image quality due to the reduced imaging dose. Two proof-of-principle experiments have been conducted using measured phantom data and Monte Carlo simulated patient data with deformation.

RESULTS

In the measured phantom case, utilizing a prior image acquired at 0.4 mAs, PINLM is able to improve the image quality of a CBCT acquired at 0.2 mAs by reducing the noise level from 34.95 to 12.45 HU. In the synthetic patient case, acceptable image quality is maintained at four consecutive fractions with gradually decreasing exposure levels of 0.4, 0.1, 0.07, and 0.05 mAs. When compared with the standard low-dose protocol of 0.4 mAs for each fraction, an overall imaging dose reduction of more than 60% is achieved.

CONCLUSIONS

PINLM-PDC is able to reduce CBCT imaging dose in IGRT utilizing the temporal correlations among the sequence of CBCT images while maintaining the quality.

Low-dose and scatter-free cone-beam CT imaging using a stationary beam blocker in a single scan: phantom studies

Excessive imaging dose from repeated scans and poor image quality mainly due to scatter contamination are the two bottlenecks of cone-beam CT (CBCT) imaging. Compressed sensing (CS) reconstruction algorithms show promises in recovering faithful signals from low-dose projection data but do not serve well the needs of accurate CBCT imaging if effective scatter correction is not in place. Scatter can be accurately measured and removed using measurement-based methods. However, these approaches are considered unpractical in the conventional FDK reconstruction, due to the inevitable primary loss for scatter measurement. We combine measurement-based scatter correction and CS-based iterative reconstruction to generate scatter-free images from low-dose projections. We distribute blocked areas on the detector where primary signals are considered redundant in a full scan. Scatter distribution is estimated by interpolating/extrapolating measured scatter samples inside blocked areas. CS-based iterative reconstruction is finally carried out on the undersampled data to obtain scatter-free and low-dose CBCT images. With only 25% of conventional full-scan dose, our method reduces the average CT number error from 250 HU to 24 HU and increases the contrast by a factor of 2.1 on Catphan 600 phantom. On an anthropomorphic head phantom, the average CT number error is reduced from 224 HU to 10 HU in the central uniform area.

Impact of different CBCT imaging monitor units, reconstruction slice thicknesses, and planning CT slice thicknesses on the positioning accuracy of a MV-CBCT system in head-and-neck patients

The purpose of this study was to investigate the impact of different CBCT imaging monitor units (MUs), reconstruction slice thicknesses, and planning CT slice thicknesses on the positioning accuracy of a megavoltage cone-beam computed tomography (MV-CBCT) system in image-guided radiation therapy (IGRT) in head-and-neck patients. The MV-CBCT system was a Siemens MVision, a commercial system integrated into the Siemens ONCOR linear accelerator. The positioning accuracy of the MV-CBCT system was determined using an anthropomorphic phantom while varying the MV-CBCT imaging MU, reconstruction slice thickness, and planning CT slice thickness. A total of 240 CBCT images from six head-and-neck patients who underwent intensity-modulated radiotherapy (IMRT) treatment were acquired and reconstructed using different MV-CBCT scanning protocols. The interfractional setup errors of the patients were retrospectively analyzed for different imaging MUs, reconstruction slice thicknesses, and planning CT slice thicknesses. Using the anthropomorphic phantom, the largest measured mean deviation component and standard deviation of the MVision in 3D directions were 1.3 and 1.0 mm, respectively, for different CBCT imaging MUs, reconstruction slice thicknesses, and planning CT slice thicknesses. The largest setup group system error (M), system error (Σ), and random error (σ) from six head-and-neck patients were 0.6, 1.2, and 1.7 mm, respectively. No significant difference was found in the positioning accuracy of the MV-CBCT system between the 5 and 8 MUs, and between the 1 and 3 mm reconstruction slice thicknesses. A thin planning CT slice thickness may achieve higher positioning precision using the phantom measurement, but no significant difference was found in clinical setup precision between the 1 and 3 mm planning CT slice thicknesses.

Peripheral dose from megavoltage cone-beam CT imaging for nasopharyngeal carcinoma image-guided radiation therapy

The growing use of cone-beam computed tomography (CBCT) for IGRT has increased concerns over the additional radiation dose to patients. The in-field dose of IGRT and the peripheral dose (PD) from kilovoltage CBCT (KV-CBCT) imaging have been well quantified. The purpose of this work is to evaluate the peripheral dose from megavoltage CBCT (MV-CBCT) imaging for nasopharyngeal carcinoma IGRT, to determine the correlation of peripheral dose with MU protocol and imaging field size, and to estimate out-of-field organ-at-risk (OAR) dose delivered to patients. Measurements of peripheral MV-CBCT doses were made with a 0.65 cm(3) ionization chamber placed inside in a specially designed phantom at various depths and distances from the imaging field edges. The peripheral dose at reference point inside the phantom was measured with the same ionization chamber to investigate the linearity between MUs used for MV-CBCT imaging and the PD. The peripheral surface doses at the anterior, lateral, and posterior of the phantom at various distances from the imaging field edge were also measured with thermoluminescent dosimeters (TLDs). Seven nasopharyngeal carcinoma patients were selected and scanned before treatment with head-neck protocol, and the peripheral surface doses were measured with TLDs placed on the anterior, lateral, and posterior surfaces at the axial plane of 15 cm distance from the field edge. The measured peripheral doses data in the phantom were utilized to estimate the peripheral OAR dose. Peripheral dose from MV-CBCT imaging increased with increasing number of MUs used for imaging protocol and with increasing the imaging field size. The measured peripheral doses in the phantom decreased as distance from the imaging field edges increased. PD also decreased as the depth from the phantom surface increased. For the patient PD measurements, the anterior, lateral, and posterior surface doses of 15 cm distance from the field edge were 2.84 × 10(-2), 1.01 × 10(-2), and 0.78 × 10(-2) cGy/MU, respectively. The lens, thyroid, breast, and ovary and testicle, which are outside the treatment and imaging fields, were estimated to receive peripheral OAR doses from MV-CBCT imaging of 42.4 × 10(-2), 11.9 × 10(-2), 1.4 × 10(-2), 1.0 × 10(-2), and 0.5 × 10(-2) cGy/MU, respectively. In conclusion, MV-CBCT generates a peripheral dose beyond the edge of the MV-CBCT scanning field that is of a similar order of magnitude to the peripheral dose from kV-CBCT imaging. In clinic, using the smallest number of MUs allowable and reducing MV-CBCT scanning field size without compromising acquired image quality is an effective method of reducing the peripheral OAR dose received by patients.

Comparison of cone-beam and conventional multislice computed tomography for image-guided dental implant planning

OBJECTIVES

To compare the accuracy of cone-beam CT (CBCT) and multislice CT (MSCT) with regard to its use in image-guided dental implant surgery in a prospective model based study.

MATERIAL AND METHODS

Ten photopolymer-acrylate mandibula models, each with four precise metal reference markers, were scanned with MSCT and CBCT. The six reference distances between the markers were measured by a three-axis milling machine first. The distances were then measured by (1) navigation with the Medtronic StealthStation® TREON™ image-guided surgery system, (2) with the Medtronic planning-tool and (3) on the PC with the Mimics® software. Mean values were calculated for all three methods for CBCT and MSCT and were compared for statistical significance.

RESULTS

Of all measurements, 83% of the arithmetic mean values were within the ±0.5 mm range (MSCT 88% and CBCT 78%) and 17% within the ±1.0 mm range (MSCT 12% and CBCT 22%). The absolute difference of the arithmetic mean values showed no statistically significant difference between MSCT and CBCT. The difference of the overall mean values to the reference was 0.43 mm for MSCT and 0.46 mm for CBCT.

CONCLUSIONS

The data of our study prove that the application of CBCT for the indicated purpose yielded good results comparable to those of MSCT. All three measuring methods were feasible and accuracy was statistically not different between the data acquired by MSCT and CBCT within the setting of this study.

An image quality comparison study between XVI and OBI CBCT systems

The purpose of this study is to evaluate and compare image quality characteristics for two commonly used and commercially available CBCT systems: the X‐ray Volumetric Imager and the On‐Board Imager. A commonly used CATPHAN image quality phantom was used to measure various image quality parameters, namely, pixel value stability and accuracy, noise, contrast to noise ratio (CNR), high‐contrast resolution, low contrast resolution and image uniformity. For the XVI unit, we evaluated the image quality for four manufacturer‐supplied protocols as a function of mAs. For the OBI unit, we did the same for the full‐fan and half‐fan scanning modes, which were respectively used with the full bow‐tie and half bow‐tie filters. For XVI, the mean pixel values of regions of interest were found to generally decrease with increasing mAs for all protocols, while they were relatively stable with mAs for OBI. Noise was slightly lower on XVI and was seen to decrease with increasing mAs, while CNR increased with mAs for both systems. For XVI and OBI, the high‐contrast resolution was approximately limited by the pixel resolution of the reconstructed image. On OBI images, up to 6 and 5 discs of 1% and 0.5% contrast, respectively, were visible for a high mAs setting using the full‐fan mode, while none of the discs were clearly visible on the XVI images for various mAs settings when the medium resolution reconstruction was used. In conclusion, image quality parameters for XVI and OBI have been quantified and compared for clinical protocols under various mAs settings. These results need to be viewed in the context of a recent study that reported the dose‐mAs relationship for the two systems and found that OBI generally delivered higher imaging doses than XVI.⁽¹⁾

PACS numbers: 85.57.C‐, 85.57.cj, 85.57.cm, 85.57.cf

Method comparison of automated matching software-assisted cone-beam CT and stereoscopic kilovoltage x-ray positional verification image-guided radiation therapy for head and neck cancer: a prospective analysis

We sought to characterize interchangeability and agreement between cone-beam computed tomography (CBCT) and digital stereoscopic kV x-ray (KVX) acquisition, two methods of isocenter positional verification currently used for IGRT of head and neck cancers (HNC). A cohort of 33 patients were near-simultaneously imaged by in-room KVX and CBCT. KVX and CBCT shifts were suggested using manufacturer software for the lateral (X), vertical (Y) and longitudinal (Z) dimensions. Intra-method repeatability, systematic and random error components were calculated for each imaging modality, as were recipe-based PTV expansion margins. Inter-method agreement in each axis was compared using limits of agreement (LOA) methodology, concordance analysis and orthogonal regression. 100 daily positional assessments were performed before daily therapy in 33 patients with head and neck cancer. Systematic error was greater for CBCT in all axes, with larger random error components in the Y- and Z-axis. Repeatability ranged from 9 to 14 mm for all axes, with CBCT showing greater repeatability in 2/3 axes. LOA showed paired shifts to agree 95% of the time within +/-11.3 mm in the X-axis, +/-9.4 mm in the Y-axis and +/-5.5 mm in the Z-axis. Concordance ranged from 'mediocre' to 'satisfactory'. Proportional bias was noted between paired X- and Z-axis measures, with a constant bias component in the Z-axis. Our data suggest non-negligible differences in software-derived CBCT and KVX image-guided directional shifts using formal method comparison statistics.

Dose perturbations from implanted helical gold markers in proton therapy of prostate cancer

Implanted gold fiducial markers are widely used in radiation therapy to improve targeting accuracy. Recent investigations have revealed that metallic fiducial markers can cause severe perturbations in dose distributions for proton therapy, suggesting smaller markers should be considered. The objective of this study was to estimate the dosimetric impact of small gold markers in patients receiving proton therapy for prostate cancer. Small, medium, and large helical wire markers with lengths of 10 mm and helix diameters of 0.35 mm, 0.75 mm, and 1.15 mm, respectively, were implanted in an anthropomorphic phantom. Radiographic visibility was confirmed using a kilovoltage x‐ray imaging system, and dose perturbations were predicted from Monte Carlo simulations and confirmed by measurements. Monte Carlo simulations indicated that size of dose perturbation depended on marker size, orientation, and distance from the beam's end of range. Specifically, the perturbation of proton dose for the lateral‐opposed‐pair treatment technique was 31% for large markers and 23% for medium markers in a typical oblique orientation. Results for perpendicular and parallel orientations were respectively lower and higher. Consequently, these markers are not well suited for use in patients receiving proton therapy for prostate cancer. Dose perturbation was not observed for the small markers, but these markers were deemed too fragile for transrectal implantation in the prostate.

PACS number: 87.53.Pb

Radiation dose from cone beam computed tomography for image-guided radiation therapy

PURPOSE

To perform a comprehensive study on organ absorbed doses and effective doses from cone beam computed tomography (CBCT) for three different treatment sites.

METHODS AND MATERIALS

An extensive set of dosimetric measurements were performed using a widely used CBCT system, the On-Board Imager (OBI). Measurements were performed using a female anthropomorphic phantom with thermoluminescent dosimeters (TLD). The effective doses to the body and the absorbed doses to 26 organs were reported using two different technical settings, namely, the standard mode and the low-dose mode. The measurements were repeated for three different scan sites: head and neck, chest, and pelvis. Comparisons of patient doses as well as image quality were performed among the standard mode CBCT, low-dose mode CBCT, and fan beam CT.

RESULTS

The mean skin doses from standard mode CBCT to head and neck, chest and pelvis were 6.7, 6.4, and 5.4 cGy per scan, respectively. The effective doses to the body from standard mode CBCT for imaging of head and neck, chest, and pelvis were 10.3, 23.7, and 22.7 mSv per scan, respectively. Patient doses from low-dose mode CBCT were approximately one fifth of those from standard mode CBCT.

CONCLUSIONS

Patient position verification by standard mode CBCT acquired by OBI on a daily basis could increase the secondary cancer risk by up to 2% to 4%. Therefore lower mAs settings for daily CBCT should be considered, especially when bony anatomy is the main interest.

Intraoperative navigation in the maxillofacial area based on 3D imaging obtained by a cone-beam device

The aim of this study was to evaluate intraoperative navigation in the maxillofacial area based on three-dimensional imaging obtained by a cone-beam device. Digital volume tomograms (DVT) were obtained by the prototype of GALILEOS (Sirona Dental Systems Inc., Bensheim, Germany), a newly developed, compact size, cone-beam machine with a scan volume of 15 cm x 15 cm x 15 cm. Intraoperative navigation was performed in 12 patients in three selected indications. Target detection error expressing the accuracy of DVT navigation and registration performance of specially developed methods for image-to-patient registration was estimated. Target detection error was maximally 2 mm and depended on the registration method chosen. The automatic detection rate of the fiducial markers ranged between 0.64 and 0.32. The preoperatively defined treatment plan was fully accomplished in 11 out of 12 cases. A favourable surgical outcome was achievable in all cases. Intraoperative complications were not observed. Intraoperative navigation based on DVT imaging can be considered as a valuable alternative to CT-based procedures. Special characteristics of the cone-beam technique, in terms of contrast resolution and the limited field-of-view size of the devices, restrict the indication spectrum and create a demand for modifications of the usual registration methods.

Performance Characteristics and Quality Assurance Aspects of Kilovoltage Cone-Beam CT on Medical Linear Accelerator

A medical linear accelerator equipped with optical position tracking, ultrasound imaging, portal imaging, and radiographic imaging systems was installed at University of Pittsburgh Cancer Institute for the purpose of performing image-guided radiation therapy (IGRT) and image-guided radiosurgery (IGRS) in October 2005. We report the performance characteristics and quality assurance aspects of the kilovoltage cone-beam computed tomography (kV-CBCT) technique. This radiographic imaging system consists of a kilovoltage source and a large-area flat panel amorphous silicon detector mounted on the gantry of the medical linear accelerator via controlled arms. The performance characteristics and quality assurance aspects of this kV-CBCT technique involves alignment of the kilovoltage imaging system to the isocenter of the medical linear accelerator and assessment of (a) image contrast, (b) spatial accuracy of the images, (c) image uniformity, and (d) computed tomography (CT)-to-electron density conversion relationship were investigated. Using the image-guided tools, the alignment of the radiographic imaging system was assessed to be within a millimeter. The low-contrast resolution was found to be a 6-mm diameter hole at 1% contrast level and high-contrast resolution at 9 line pairs per centimeter. The spatial accuracy (50 mm +/- 1%), slice thickness (2.5 mm and 5.0 mm +/- 5%), and image uniformity (+/- 20 HU) were found to be within the manufacturer's specifications. The CT-to-electron density relationship was also determined. By using well-designed procedures and phantom, the number of parameter checks for quality assurance of the IGRT system can be carried out in a relatively short time.

Dose calculation using megavoltage cone-beam CT

PURPOSE

To demonstrate the feasibility of performing dose calculation on megavoltage cone-beam CT (MVCBCT) of head-and-neck patients in order to track the dosimetric errors produced by anatomic changes.

METHODS AND MATERIALS

A simple geometric model was developed using a head-size water cylinder to correct an observed cupping artifact occurring with MVCBCT. The uniformity-corrected MVCBCT was calibrated for physical density. Beam arrangements and weights from the initial treatment plans defined using the conventional CT were applied to the MVCBCT image, and the dose distribution was recalculated. The dosimetric inaccuracies caused by the cupping artifact were evaluated on the water phantom images. An ideal test patient with no observable anatomic changes and a patient imaged with both CT and MVCBCT before and after considerable weight loss were used to clinically validate MVCBCT for dose calculation and to determine the dosimetric impact of large anatomic changes.

RESULTS

The nonuniformity of a head-size water phantom ( approximately 30%) causes a dosimetric error of less than 5%. The uniformity correction method developed greatly reduces the cupping artifact, resulting in dosimetric inaccuracies of less than 1%. For the clinical cases, the agreement between the dose distributions calculated using MVCBCT and CT was better than 3% and 3 mm where all tissue was encompassed within the MVCBCT. Dose-volume histograms from the dose calculations on CT and MVCBCT were in excellent agreement.

CONCLUSION

MVCBCT can be used to estimate the dosimetric impact of changing anatomy on several structures in the head-and-neck region.

Megavoltage cone-beam CT: system description and clinical applications

In this article, we describe a clinical mega-voltage cone-beam computed tomography (MV CBCT) system, present the image acquisition and patient setup procedure, discuss the positioning accuracy and image quality, and illustrate its potential use for image-guided radiation therapy (IGRT) through selected clinical examples. The MV CBCT system consists of a standard linear accelerator equipped with an amorphous-silicon flat panel electronic portal-imaging device adapted for mega-electron volt (MeV) photons. An integrated computer workspace provides automated acquisition of projection images, image reconstruction, CT to CBCT image registration, and couch shift calculation. The system demonstrates submillimeter localization precision and sufficient soft-tissue resolution to visualize structures such as the prostate. In our clinic, we have used the MV CBCT system to detect nonrigid spinal cord distortions, monitor tumor growth and shrinkage, and locate and position stationary tumors in the lung. MV CBCT has also greatly improved the delineation of structures in CT images that suffer from metal artifacts. MV CBCT has undergone significant development in the last few years. Current image quality has already proven sufficient for many IGRT applications. Moreover, we expect the range of clinical applications for MV CBCT to grow as imaging technology continues to improve.

A multi-platform approach to image guided radiation therapy (IGRT)

Siemens Medical Solutions, Oncology Care Systems Group (SMSOCSG) is supporting the development of several technologies that enable image acquisition and decision making processes required for IGRT in various clinical settings. Four such technologies are presented including: (i) the integration of a traditional multi-slice computed tomography (CT) scanner "on rails" with a C-arm gantry linear accelerator; (ii) the development of a high sensitivity, fast, megavoltage (MV) electronic portal imaging device capable of clinical MV Conebeam CT (MVCBCT) reconstruction and fluoroscopy mounted on a C-arm gantry linear accelerator; (iii) the modification of a mobile C-arm with flat panel kilovoltage (kV) diagnostic imager; and (iv) the development of an in-line megavoltage and kilovoltage flat panel imaging system that has the potential to image both anatomical and dosimetric information in "real-time" utilizing the traditional C-arm gantry linear accelerator geometry. Each method of IGRT has unique as well as complementary qualities which are discussed from both a clinical and technical perspective.