CT-based simulation with laser patient marking

A phantom based laser marking workflow to visually assess geometric image distortion in magnetic resonance guided radiotherapy

Magnetic resonance (MR)-only workflows require quality assurance due to potential dosimetric impacts of using geometry distorted MR images in radiotherapy planning. MR-visible silicone-based fiducials were arranged in regular 3D structures to cover extended imaging volumes. The scanner’s patient marking workflow with a 2-axes movable laser bridge allowed to visually check geometric distortions of each MR reconstructed fiducial against its true position in 3D space. A measurement resolution and uncertainty of the order of 0.5 mm in sagittal and coronal, and 1 mm in transversal direction was found. The proposed workflow required 1 min of evaluation time per fiducial position, and a 9 min 3D MR volume acquisition.

Multimodality image registration quality assurance for conformal three-dimensional treatment planning

PURPOSE

We present a quality assurance methodology to determine the accuracy of multimodality image registration and fusion for the purpose of conformal three-dimensional and intensity-modulated radiation therapy treatment planning. Registration and fusion accuracy between any combination of computed tomography (CT), magnetic resonance (MR), and positron emission computed tomography (PET) imaging studies can be evaluated.

METHODS AND MATERIALS

A commercial anthropomorphic head phantom filled with water and containing CT, MR, and PET visible targets was modified to evaluate the accuracy of multimodality image registration and fusion software. For MR and PET imaging, the water inside the phantom was doped with CuNO(3) and 18F-fluorodeoxyglucose (18F-FDG), respectively. Targets consisting of plastic spheres and pins were distributed throughout the cranium section of the phantom. Each target sphere had a conical-shaped bore with its apex at the center of the sphere. The pins had a conical extension or indentation at the free end. The contours of the spheres, sphere centers, and pin tips were used as anatomic landmark models for image registration, which was performed using affine coordinate-transformation tools provided in a commercial multimodality image registration/fusion software package. Four sets of phantom image studies were obtained: primary CT, secondary CT with different phantom immobilization, MR, and PET study. A novel CT, MR, and PET external fiducial marking system was also tested.

RESULTS

The registration of CT/CT, CT/MR, and CT/PET images allowed correlation of anatomic landmarks to within 2 mm, verifying the accuracy of the registration software and spatial fidelity of the four multimodality image sets.

CONCLUSIONS

This straightforward phantom-based quality assurance of the image registration and fusion process can be used in a routine clinical setting or for providing a working image set for development of the image registration and fusion process and new software.

Virtual simulation in patients with breast cancer

BACKGROUND

Investigation of the feasibility and effectiveness of virtual simulation in patients receiving radiotherapy of the breast.

METHODS

Twenty-three patients were included in the study. All of them underwent a course of postoperative tangential breast irradiation. The patients were prospectively randomised into two groups. Group A patients (n=11) received a conventional computed tomography -based treatment planning, group B patients (n=12) a virtual simulation. The results of both treatment planning procedures were compared.

RESULTS

The treatment planning was feasible in all patients. The time expenditure could be reduced from a median of 45.0 to 16.5 min and from 55.0 to 32.0 min for the technician and physician, respectively, using virtual simulation. Furthermore the treatment planning for the patient could be reduced from a median of 45.0 min in two sessions to 16.5 min in one session. The image quality of the digital reconstructed radiographs was satisfying compared to the simulation films. The incidence and extension of set-up corrections for the patients at the first treatment were comparable in both groups. The time interval between the planning CT and the first treatment could be reduced by 31% using virtual simulation due to the omission of the conventional simulation.

CONCLUSION

The virtual simulation is a feasible tool for the treatment planning of patients undergoing tangential irradiation of the breast. Compared with the conventional simulation procedure virtual simulation is superior regarding to the precision of patients marking, the quality of the reference images and, the time expenditure for the patients and medical staff.

Applications of simulator computed tomography number for photon dose calculations during radiotherapy treatment planning

PURPOSE

To study the variation of computed tomography (CT) number from a simulator-based scanner and the effect of this variation on photon-dose calculations.

METHOD AND MATERIALS

CT images of a cylindrical phantom with multiple inserts were obtained using a commercially-available simulator-CT (Ximatron: Varian, Palo Alto, CA). The linear correlation coefficient and Chi-square methods were used to determine the X-ray effective energy in a phantom. CT numbers in Hounsfield units (HU) were measured as a function of phantom size, orientation, field of view (FOV), distance from the center, and time for various inserts. The change of dose calculations due to the CT number variations was then determined using the equivalent path-length (EPL) and collapsed cone convolution methods.

RESULTS AND DISCUSSION

A significant beam-hardening effect was observed for the simulator-CT. Consequently, the CT number from the sim-CT was more sensitive to the size of the phantom than those from a conventional CT. The sim-CT number is not sensitive to the locations within the phantom and is stable over a 6-week period. It is important to use the proper FOV for sim-CT studies; scanning a small polystyrene phantom using a large FOV may result in an increase of l20 HU in CT number at the center of the field. However, the dose-calculation variations, due to the CT number uncertainty, do not exceed 2-3% for 6-18 MV photon beams.

CONCLUSION

The simulator CT images were acquired with patients in the treatment position, and these CT numbers are useful for CT-based dose calculations.

Virtual light field projection for CT-simulation

We report a method to project a virtual light field over the patient during CT-simulations of external beam radiotherapy. It can be used to perform all the tasks associated with the physical light field of a conventional simulator. The system consists of a three-dimensional sonic digitizer interfacing with a window-based software on a personal computer. The digitizer can provide the three-dimensional coordinates of any point in space accessible by the digitizer probe. When these coordinates are transformed into the beam's eye view, the position of the digitized point relative to the beam can readily be displayed. Thus, the system establishes a virtual light field in space, which can be "seen" only by the digitizer probe. For any digitized point, the system can immediately show, by the beam's eye view display, whether the point is inside the field, outside of the field, or on the field border. Moreover, this virtual field projection allows one to evaluate external target coverage (or external normal tissue sparing) conveniently and interactively. By simply digitizing the concerned area and viewing its position in the beam's eye view display, one can immediately assess the coverage and if necessary, modify the treatment field accordingly. The system also provides an efficient and essential procedure in CT-simulations for marking treatment portals on the patient. By cruising the digitizer probe on the patient's skin surface under visual and audio guidances, one can promptly find the projection of the field center, field corners, etc., on the patient. Measurements have been performed to study the accuracy of the GP-12 sonic digitizer using rigid phantoms. Based on the measured data, the overall accuracy of the portal localization system is estimated to be +/-2 mm. The system has been in clinical use for our CT simulator.

[Virtual simulation. First clinical results in patients with prostate cancer]

AIM

Investigation of options of virtual simulation in patients with localized prostate cancer.

PATIENTS AND METHODS

Twenty-four patients suffering from prostate cancer were virtual simulated. The clinical target volume was contoured and the planning target volume was defined after CT scan. The isocenter of the planning target volume was determined and marked at patient's skin. The precision of patients marking was controlled with conventional simulation after physical radiation treatment planning.

RESULTS

Mean differences of the patient's mark revealed between the 2 simulations in all room axes around 1 mm. The organs at risk were visualized in the digital reconstructed radiographs.

CONCLUSIONS

The precise patient's mark of the isocentre by virtual simulation allows to skip the conventional simulation. The visualisation of organs at risk leeds to an unnecessarily of an application of contrast medium and to a further relieve of the patient. The personal requirement is not higher in virtual simulation than in conventional CT based radiation treatment planning.

CT virtual simulation

Advances in computer technology and software design have enabled the concept of virtual simulation, first suggested by George Sherouse, to be realized. This article reviews the hardware and software requirements that provide a system that feels like a simulator and can facilitate the 3D planning process. Fast digital reconstruction of a radiograph from a CT data set provides true verification of treatment field design within the constraints of the virtual method. The clinical application of this technique is discussed in detail in relation to particular treatment sites.

Reduction of radiation-induced xerostomia in nasopharyngeal carcinoma using CT simulation with laser patient marking and three-field irradiation technique

PURPOSE

Tumor control and reduction of postirradiation xerostomia in patients with nasopharyngeal carcinoma (NPC) using the three-field irradiation technique based on the CT-based simulation with laser patient marking was investigated.

METHODS AND MATERIALS

Seventy-eight patients with NPC were consecutively treated between 1983 and 1993. In 33 patients treated before 1987, target volume was determined using a conventional x-ray simulator with a reference of CT images, and the primary site was treated by the conventional parallel-opposed two-field technique (Group I). In 45 patients treated from 1987, target volume was determined using a CT simulator slice by slice, the treatment field was projected onto the patient's skin by a laser beam projector mounted on a C-arm, and the primary site was irradiated by a three-fields (anterior and bilateral) technique (Group II). In Group II, the shape of each field was determined using a beam's eye view to reduce the dose to the bilateral parotid glands. The three-field technique reduced the dose to the superficial lobe of parotid gland to about two-thirds of the dose given by the two-field technique. Radiation-induced xerostomia was evaluated by clinical symptoms and radioisotope sialography.

RESULTS

The 5-year survival rate and disease-free survival rate were 46.6 and 31.2% in Group I, and 46.8 and 46.5% in Group II. A large variation in the volume of parotid glands were demonstrated, ranging from 9 cm3 to 61 cm3 among patients treated with CT simulation. Forty percent of the patients in Group II showed no or mild xerostomia, whereas all of the patients in Group I showed moderate to severe xerostomia (p < 0.01). The radioisotope sialography study showed that the mean secretion ratio by acid stimulation was improved from 3.8% in the Group I to 15.2% in the Group II (p < 0.01).

CONCLUSIONS

CT simulation was useful to determine the size and shape of each field to reduce the dose to the parotid gland, of which size varies largely among individual patients. The three-field technique based on CT simulation with laser patient markings is suggested to result in superior complication-free survival in terms of salivary dysfunction than did the conventional two-field technique with x-ray simulatior for NPC.

Development of an integrated radiotherapy network system

PURPOSE

To introduce the process of developing an integrated radiotherapy network.

METHODS AND MATERIALS

We developed a new radiotherapy treatment-planning system in 1987 that we named the Computer Tomography (CT) simulator. CT images were immediately transported to multiimage monitors and to a planning computer, and treatment planning could be performed with the patient lying on the CT couch. The results of planning were used to guide a laser projector, and radiation fields were projected onto the skin of the patient. Since 1991, an integrated radiotherapy network system has been developed, which consists of a picture archiving and communicating system (PACS), a radiotherapy information database, a CT simulator, and a linear accelerator with a multileaf collimator.

RESULTS

Clinical experience has been accumulated in more than 1,000 patients. Based on our 7 years of experience, we have modified several components of our original CT simulator and have developed a second generation CT simulator. A standard protocol has been developed for communication between the CT scanner, treatment planning computer, and radiotherapy apparatus using the Ethernet network. As a result, treatment planning data can be transported to the linear accelerator within 1 min after completion of treatment planning.

CONCLUSION

This system enables us to make optimal use of CT information and to devise accurate three-dimensional (3D) treatment-planning programs. Our network also allows for the performance of fully computer-controlled dynamic arc conformal therapy.

Picture archiving and communications systems in radiation oncology (PACSRO): tools for a physician-based digital image review system

Digital imaging is becoming more and more important in the diagnosis, staging, and treatment of patients in radiation oncology. In order to facilitate the most efficient interface of this technology to physicians and other users of this information, a medical image display system (MID) has been developed at the Fox Chase Cancer Center (FCCC). The system runs on 20 personal computers situated in physicians offices as well as a modified system located in the radiation oncology conference room. Access to CT, MRI, and EPID information is achieved through an Ethernet connection to the hospital picture archiving and communications system (PACS). Over a 1-year period a total of 503 patients and 3845 images have been stored on the system. Physician approval using the MID system (without conventional films) was performed on 106 patients. Of these, 22%, 16%, 11%, 10%, and 9% consisted of breast, prostate, pelvic, lung, and head and neck patients, respectively. Digital images sent from a variety of image sources to the MID system take up to 15 s to process and format while image access and display can take 2-5 s, dependent upon image size and speed of the host computer.

Transfer errors of planning CT to simulator: a possible source of setup inaccuracies?

The purpose of this study was to analyse whether the intended patient setup, based on a CT scan, was different from the setup at the simulator. Furthermore, we investigated how these possible transfer errors between the planned patient setup and the actual simulator setup affected the resulting overall treatment setup accuracy. Two groups, of 15 prostate patients each, were studied. For one group (group II), the simulation time was about twice as large as for the other (group I), since digitally reconstructed radiographs (DRRs) were used to get a good visual agreement between the intended and the simulator setup. For the purpose of this study DRRs were also calculated for the patients in group I, and for both groups DRRs were matched with the simulator images to obtain quantitative data of the transfer errors. The resulting overall treatment setup accuracy was determined by comparing the DRRs with portal images. For group I, the standard deviations (SD) of the differences between the DRRs and the simulator images ('transfer errors') were 1.5 mm and 4.5 mm in the lateral (x) and cranio-caudal (y) direction, respectively. For group II the SDs were smaller: 1.4 mm and 1.5 mm in the x- and y-direction, respectively. For both groups, the magnitude of the overall mean was less than 1.3 mm. For group I, the SDs of the resulting overall setup deviations during treatment were 1.6 mm and 4.1 mm in the x- and y-direction, respectively. For group II, these figures were 2.4 mm and 2.6 mm, respectively. For both groups, the magnitude of the overall mean was less than 1.0 mm.(ABSTRACT TRUNCATED AT 250 WORDS)