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Biomedical subjects

G Mageras

Publications and source records attributed to G Mageras.

13 recordsLinked to original sources
PubMed2006

Reduction of organ motion effects in IMRT and conformal 3D radiation delivery by using gating and tracking techniques.

Respiration-gated radiotherapy offers a significant potential for improvement in the irradiation of tumour sites affected by respiratory motion such as lung, breast and liver tumours. An increased conformality of irradiation fields leading to decreased complications rates of organs at risk (lung, heart) is expected. Four main strategies are used to reduce respiratory motion effects: integration of respiratory movements into treatment planning, breath-hold techniques, respiratory gating techniques, and tracking techniques. Measurements of respiratory movements can be performed either in a representative sample of the general population, or directly on the patient before irradiation. The measured amplitude could be applied to a geometrical margin or integrated into dosimetry. However, these strategies remain limited for very mobile tumours, in which this approach results in larger irradiated volumes. Reduction of breathing motion can be achieved by using either breath-hold techniques or respiration synchronized gating techniques. Breath-hold can be achieved with active techniques, in which a valve temporarily blocks airflow of the patient, or passive techniques, in which the patient voluntarily breath-holds. Synchronized gating techniques use external devices to predict the phase of the respiration cycle while the patient breaths freely. Another category is tumour tracking, which consists of two major aspects: real-time localization of, and real-time beam adaptation to, a constantly moving tumour. These techniques are presently being investigated in several medical centres worldwide. Although promising, the first results obtained in lung and liver cancer patients require confirmation. This paper describes the most frequently used gating and tracking techniques and the main published clinical reports.

Artifacts↗
PubMed2005

A method for determining the gantry angle for megavoltage cone beam imaging.

Accurate knowledge of gantry angle is essential in megavoltage cone beam imaging (MVCBI) with an electronic portal imager. We present a method for determining the gantry angle by detecting multileaf collimator (MLC) leaf positions in projection images. During image acquisition the gantry moves continuously and the MLC operates in dynamic arc mode. Our algorithm detects the leaf positions in the images and compares them with a stationary reference leaf. Comparison of the algorithm against angles determined from the locations of fiducial markers shows the accuracy (0.26 degrees rms error) to be sufficient for MVCBI.

Algorithms↗
PubMed2005

Developments in megavoltage cone beam CT with an amorphous silicon EPID: reduction of exposure and synchronization with respiratory gating.

We have studied the feasibility of a low-dose megavoltage cone beam computed tomography (MV CBCT) system for visualizing the gross tumor volume in respiratory gated radiation treatments of nonsmall-cell lung cancer. The system consists of a commercially available linear accelerator (LINAC), an amorphous silicon electronic portal imaging device, and a respiratory gating system. The gantry movement and beam delivery are controlled using dynamic beam delivery toolbox, a commercial software package for executing scripts to control the LINAC. A specially designed interface box synchronizes the LINAC, image acquisition electronics, and the respiratory gating system. Images are preprocessed to remove artifacts due to detector sag and LINAC output fluctuations. We report on the output, flatness, and symmetry of the images acquired using different imaging parameters. We also examine the quality of three-dimensional (3D) tomographic reconstruction with projection images of anthropomorphic thorax, contrast detail, and motion phantoms. The results show that, with the proper choice of imaging parameters, the flatness and symmetry are reasonably good with as low as three beam pulses per projection image. Resolution of 5% electron density differences is possible in a contrast detail phantom using 100 projections and 30 MU. Synchronization of image acquisition with simulated respiration also eliminated motion artifacts in a moving phantom, demonstrating the system's capability for imaging patients undergoing gated radiation therapy. The acquisition time is limited by the patient's respiration (only one image per breathing cycle) and is under 10 min for a scan of 100 projections. In conclusion, we have developed a MV CBCT system using commercially available components to produce 3D reconstructions, with sufficient contrast resolution for localizing a simulated lung tumor, using a dose comparable to portal imaging.

Artifacts↗
PubMed2002

Cone-beam CT with megavoltage beams and an amorphous silicon electronic portal imaging device: potential for verification of radiotherapy of lung cancer.

We investigate the potential of megavoltage (MV) cone-beam CT with an amorphous silicon electronic portal imaging device (EPID) as a tool for patient position verification and tumor/organ motion studies in radiation treatment of lung tumors. We acquire 25 to 200 projection images using a 22 x 29 cm EPID. The acquisition is automatic and requires 7 minutes for 100 projections; it can be synchronized with respiratory gating. From these images, volumetric reconstruction is accomplished with a filtered backprojection in the cone-beam geometry. Several important prereconstruction image corrections, such as detector sag, must be applied. Tests with a contrast phantom indicate that differences in electron density of 2% can be detected with 100 projections, 200 cGy total dose. The contrast-to-noise ratio improves as the number of projections is increased. With 50 projections (100 cGy), high contrast objects are visible, and as few as 25 projections yield images with discernible features. We identify a technique of acquiring projection images with conformal beam apertures, shaped by a multileaf collimator, to reduce the dose to surrounding normal tissue. Tests of this technique on an anthropomorphic phantom demonstrate that a gross tumor volume in the lung can be accurately localized in three dimensions with scans using 88 monitor units. As such, conformal megavoltage cone-beam CT can provide three-dimensional imaging of lung tumors and may be used, for example, in verifying respiratory gated treatments.

Algorithms↗
PubMed2000

Technical aspects of the deep inspiration breath-hold technique in the treatment of thoracic cancer.

PURPOSE: The goal of this paper is to describe our initial experience with the deep inspiration breath-hold (DIBH) technique in conformal treatment of non-small-cell lung cancer with particular emphasis on the technical aspects required for implementation. METHODS AND MATERIALS: In the DIBH technique, the patient is verbally coached through a modified slow vital capacity maneuver and brought to a reproducible deep inspiration breath-hold level. The goal is to immobilize the tumor and to expand normal lung out of the high-dose region. A physicist or therapist monitors and records patient breathing during simulation, verification, and treatment using a spirometer with a custom computer interface. Examination of internal anatomy during fluoroscopy over multiple breath holds establishes the reproducibility of the DIBH maneuver for each patient. A reference free-breathing CT scan and DIBH planning scan are obtained. To provide an estimate of tumor motion during normal tidal breathing, additional scan sets are obtained at end inspiration and end expiration. These are also used to set the spirometer action levels for treatment. Patient lung inflation is independently verified over the course of treatment by comparing the distance from the isocenter to the diaphragm measured from the DIBH digitally reconstructed radiographs to the distance measured on the portal films. Patient breathing traces obtained during treatment were examined retrospectively to assess the reproducibility of the technique. RESULTS: Data from the first 7 patients, encompassing over 250 treatments, were analyzed. The inferred displacement of the centroid of gross tumor volume from its position in the planning scan, as calculated from the spirometer records in over 350 breath holds was 0.02 +/- 0.14 cm (mean and standard deviation). These data are consistent with the displacements of the diaphragm (-0.1 +/- 0.4 cm; range, from -1.2 to 1.1 cm) relative to the isocenter, as measured on the (92) portal films. The latter measurements include the patient setup error. The patient averaged displacement of the tumor during free breathing, determined from the tumor displacement between end inspiration and end expiration, was 0.8 +/- 0.5 cm in both the superior-inferior and anterior-posterior directions and 0.1 cm (+/- 0.1 cm) medial-laterally. CONCLUSION: Treatment of patients with the DIBH technique is feasible in a clinical setting. With this technique, consistent lung inflation levels are achieved in patients, as judged by both spirometry and verification films. Breathing-induced tumor motion is significantly reduced using DIBH compared to free breathing, enabling better target coverage.

Carcinoma, Non-Small-Cell Lung↗
PubMed2000

The deep inspiration breath-hold technique in the treatment of inoperable non-small-cell lung cancer.

PURPOSE: Conventional radiotherapeutic techniques are associated with lung toxicity that limits the treatment dose. Motion of the tumor during treatment requires the use of large safety margins that affect the feasibility of treatment. To address the control of tumor motion and decrease the volume of normal lung irradiated, we investigated the use of three-dimensional conformal radiation therapy (3D-CRT) in conjunction with the deep inspiration breath-hold (DIBH) technique. METHODS AND MATERIALS: In the DIBH technique, the patient is initially maintained at quiet tidal breathing, followed by a deep inspiration, a deep expiration, a second deep inspiration, and breath-hold. At this point the patient is at approximately 100% vital capacity, and simulation, verification, and treatment take place during this phase of breath-holding. RESULTS: Seven patients have received a total of 164 treatment sessions and have tolerated the technique well. The estimated normal tissue complication probabilities decreased in all patients at their prescribed dose when compared to free breathing. The dose to which patients could be treated with DIBH increased on average from 69.4 Gy to 87.9 Gy, without increasing the risk of toxicity. CONCLUSIONS: The DIBH technique provides an advantage to conventional free-breathing treatment by decreasing lung density, reducing normal safety margins, and enabling more accurate treatment. These improvements contribute to the effective exclusion of normal lung tissue from the high-dose region and permit the use of higher treatment doses without increased risks of toxicity.

Carcinoma, Non-Small-Cell Lung↗
PubMed1995

Three dimensional conformal treatment planning with multileaf collimators.

PURPOSE: Three dimensional conformal radiation treatments are complex, often involving large numbers of blocked or multileaf collimated fields that shape regions of high dose to conform to the treatment volume. As manual definition and digitization of aperture shapes and their corresponding multileaf configurations can be impractically time consuming, it was necessary to integrate the planning of multileaf fields into an existing three dimensional treatment planning system and improve the efficiency of treatment delivery to make these treatments feasible on a routine basis. METHODS AND MATERIALS: A subfunction of the Beam's Eye View (BEV) component can be used to automatically generate a continuous aperture shape with a margin around the tumor to account for beam penumbra, and excluding any normal structures to be spared (each with its own margin). To convert a continuous aperture shape into one defined by the multileaf collimator (MLC), a leaf coverage mode is chosen to determine how leaves are fitted to aperture shapes. The conversion process also considers parameters of the specific MLC system, e.g., leaf thickness and the number of leaves. If normal structures to be shielded split the target into multiple regions, more than one multileaf aperture can result. An interactive leaf adjustment routine is also provided to allow for modification of individual leaf positions. Dose calculation programs then take into account multileaf apertures for computation of dose distributions using a pencil beam convolution model. Finally, prescription files specifying leaf and jaw configurations are prepared in treatment machine specific formats and downloaded to the computers driving the multileaf collimators and other components of the treatment machines. RESULTS AND CONCLUSION: An example is presented of a prostate treatment plan, with MLC configurations, dose distributions, and treatment delivery description, along with discussion of clinical implementation at Memorial Hospital.

Adenocarcinoma↗
PubMed1993

Perspectives of multidimensional conformal radiation treatment.

We consider the present technological advancement that underlies the implementation of computer-controlled conformal radiotherapy. We also consider the developments in modern biology that may provide input to therapy planning. The concept of multidimensional conformal radiotherapy is advanced, which integrates geometrical precision and biological conformality, to optimize the treatment planning for individual patients, with a view to improve the overall success of radiotherapy.

Humans↗