Biomedical subjects

B G Fallone

Publications and source records attributed to B G Fallone.

At least 37 records · Page 2Linked to original sources

PubMed2002

Derivation of the expressions for gamma50 and D50 for different individual TCP and NTCP models.

This paper presents a complete set of formulae for the position (D50) and the normalized slope (gamma50) of the dose-response relationship based on the most commonly used radiobiological models for tumours as well as for normal tissues. The functional subunit response models (critical element and critical volume) are used in the derivation of the formulae for the normal tissue. Binomial statistics are used to describe the tumour control probability, the functional subunit response as well as the normal tissue complication probability. The formulae are derived for the single hit and linear quadratic models of cell kill in terms of the number of fractions and dose per fraction. It is shown that the functional subunit models predict very steep, almost step-like, normal tissue individual dose-response relationships. Furthermore, the formulae for the normalized gradient depend on the cellular parameters alpha and beta when written in terms of number of fractions, but not when written in terms of dose per fraction.

Computer Simulation↗

PubMed2002

Compensators for intensity-modulated beams.

This study describes the importance of attenuator scatter in the construction of compensators. The attenuator used in this study was Lipowitz metal, commonly referred to as cerrobend. Linear attenuation coefficients of cerrobend were measured in air for different thickness of cerrobend sheets and different field sizes for a 6-MV photon beam. The magnitude of the dose contribution from photons scattered by the attenuator was measured. The variations of beam hardening and the scatter to primary ratio as a function of the thickness of cerrobend and varying field size were investigated. The compensators in this study were produced using a simple exponential attenuation model and the measured linear attenuation coefficients. It was found that the beam hardening effect was significant, and can lead to an error of 6.2% in the transmission, for 6 cm of cerrobend in the beam. The maximum scatter contribution to the measured fluence was 19.8% of the transmitted primary dose for a 20 x 20-cm2 field size, and 6 cm of cerrobend in the beam. For a simple wedge-step compensator; there was a maximum deviation of 6% between the measured and our predicted fluence profile. For simple compensators, this deviation can be attributed to scatter.

Alloys↗

PubMed2002

Modulation transfer function of digitally reconstructed radiographs using helical computed tomography.

We have measured the modulation transfer function (MTF), at a distance from CT isocenter, in digitally reconstructed radiographs (DRR) using a bar pattern phantom for axial and helical data acquisitions. Spatial resolution in DRR increases for thinner slice thicknesses (43% at 1.6 lp/cm for 2 mm versus 16% for 5 mm). For the three slice thicknesses studied, the axial scanning mode provided better spatial resolution in DRR than helical scans performed at pitches > or = 1.5 (41% at 1.6 lp/cm for axial, 3 mm slice versus 18% for pitch = 2), but is similar to helical scans with pitch of 1. The reconstruction of overlapping slices from helical acquisitions of pitch = 1.5 results in spatial resolution of the DRR that is similar to that resulting from axial scans with contiguous reconstruction, but also results in the fine streaks known as "zebra" artifacts in the DRR.

Image Processing, Computer-Assisted↗

PubMed2002

On the implementation of dose-volume objectives in gradient algorithms for inverse treatment planning.

A method that allows a straightforward implementation of dose-volume constraints in gradient algorithms for inverse treatment planning is presented. The method is consistent with the penalty function approach, which requires the formulation of an objective function with penalty terms proportional to the magnitudes of constraint violations. Dose constraints with respect to minimum and maximum target dose levels are incorporated in quadratic, dose-penalty terms. Analogously, quadratic volume-penalty terms in the objective function reflect the violation of dose-volume constraints imposing limits on the fractions of healthy organ volumes that can be irradiated above specified dose levels. It has been demonstrated that within the framework of this formulation neither modified objective functions nor finite difference gradient calculations are necessary for the incorporation of gradient minimization algorithms. As an example, a simple steepest descent algorithm is presented along with its application to illustrate prostate and lung cases.

Algorithms↗

PubMed2002

Technical note: Sinogram merging to compensate for truncation of projection data in tomotherapy imaging.

An advantage of helical tomotherapy radiation therapy systems is that on-line megavoltage computed tomography (CT) images can be reconstructed to verify patient positioning. One limitation of such systems is that the field-of-view (FOV) of the photon fan-beam is limited by the aperture size of the binary multileaf collimator (MLC) used to modulate treatment beams. For patients larger than the FOV the acquired sinograms will be truncated causing artifacts in the resultant megavoltage CT images. Computer simulations are used to demonstrate that such artifacts can be eliminated or at least reduced by merging appropriately acquired truncated fan-beam sinograms to form a nontruncated parallel-beam sinogram. The necessary fan-beam sinograms are acquired with the patient translated to different offset locations within the gantry. The parallel-beam sinogram is then used to reconstruct the final CT image. The increase in patient dose due to the acquisition of more than one fan-beam sinogram can be reduced by using properly designed binary MLC fields to block redundant projection rays.

Artifacts↗

PubMed2002

Dosimetric verification of inverse planned step and shoot multileaf collimator fields from a commercial treatment planning system.

An inverse treatment planning (ITP) module on a commercial treatment planning system (TPS) (Helax AB, Uppsala, Sweden) is being used for an in-house clinical trial for treatment of nasopharyngeal cancer with contralateral parotid sparing. Intensity modulated radiation therapy (IMRT) fields are delivered by step and shoot multileaf collimator (MLC) with a DMLC enabled Varian 2300 CD (Varian Associates, Palo Alto, CA). A series of testing procedures have been devised to quantify the modeling and delivery accuracy of routine clinical inverse planned IMRT using Helax TMS and the Varian step and shoot MLC delivery option. Testing was done on specific aspects of the TPS modeling germane to DMLC. Measured relative dose factors (head scatter plus phantom scatter) for small MLC fields, normalized to a 10x10 cm2 non-MLC field, were found to differ by 2-3% from the TPS values for the smallest of the fields tested. Relative distributions for small off axis fields were found to be in good agreement. A process for the routine clinical verification of IMRT fields has been implemented. Each IMRT field in an inverse plan is imported into a flat water tank plan and a "beam's eye view" (BEV) dose distribution is generated. This is compared to the corresponding measured BEV dose distribution. The IMRT verification process has also been performed using an anthropomorphic phantom. Large clinical fields (i.e., greater than 14.5 cm in the leaf direction) caused difficulties due to a vendor specific machine restriction, and several techniques for dealing with these were examined. These techniques were (i) the use of static stepping of closed junctions, (ii) the use of two separate IMRT fields for a given gantry angle, and (iii) restricting the overall maximum field size used. The overall process has allowed implementation of an in-house protocol for IMRT use on an initial clinical site. Results of the verification measurements for the first ten patients treated at this center reveal an average maximum dose per IMRT field delivered of 71.0 cGy, with a mean local deviation from the planned dose of -1.2 cGy, and a standard deviation of 2.4 cGy.

Algorithms↗

PubMed2001

Cascade analysis for medical imaging detectors with stages involving both amplification and dislocation processes.

Cascade analysis is a powerful tool which can be used to calculate the signal and noise properties of medical imaging detectors. It involves the conceptual separation of the imaging chain into stages which consist of either pure amplification or pure dislocation stages. It is, however, not always possible to break the physical processes down to these elementary stages. In this work we derive a new cascade equation which is applicable to any stage which involves multiple amplifications and dislocations. The equation simplifies to the known equations for pure amplification and pure dislocation stages in the appropriate limits, and can be numerically calculated using Monte Carlo techniques for more complicated situations. We demonstrate the use of this equation with an example: we derive an expression for the DQE of a metal/phosphor detector for megavoltage imaging with our formalism, and evaluate the expression with Monte Carlo techniques. We have found that there is excellent agreement between theory and experimental results, and believe that the formalism could be useful for other applications where the amplification and dislocation processes cannot be divided into elementary stages.

Diagnostic Imaging↗

PubMed2001

Detective quantum efficiency of a direct-detection active matrix flat panel imager at megavoltage energies.

The use of an amorphous selenium (a-Se) based direct-detection active matrix flat-panel imager (AMFPI) is studied for megavoltage imaging. The detector consists of a 1.2 mm copper front plate and 200 microm a-Se layer, and has a 85 microm pixel pitch. The Modulation Transfer Function (MTF), Noise Power Spectrum (NPS), and Detective Quantum Efficiency (DQE) are measured for 6 and 15 MV photon beams. A theoretical expression for the DQE is derived using a recently developed formalism for nonelementary cascade stages. A comparison of theory with experiment is good for the 6 and 15 MV beams. The model is used to explore the DQE for more typical pixel sizes. The results indicate that with proper modifications, such as a larger a-Se thickness, a direct flat-panel AMFPI is a very promising detector for megavoltage imaging.

Electrons↗

PubMed2001

Investigation into the physical characteristics of active matrix flat panel imagers for radiotherapy.

The effect of physical characteristics on active matrix flat panel imagers (AMFPIs) at megavoltage energies is studied. The detective quantum efficiency (DQE) of both direct and indirect AMFPIs is modeled using a modified cascade analysis combined with Monte Carlo simulations. It is found that for a given thickness of the sensitive layer less than about 1 mm, there should be no significant difference between the detection techniques, but for larger mass thicknesses (> or = 1 mm) there should be an advantage to using direct detection if such thick layers can be manufactured. The effect of the front plate on both direct and indirect techniques is also explored in terms of both the DQE and scatter rejection. It is found that for small sensitive layer thicknesses (< or =0.3 mm) a front plate thickness of about 1 mm Cu is optimal, whereas for larger mass thicknesses about 0.4 mm Cu should lead to better image quality.

Computer Simulation↗

PubMed2000

Shielding considerations for tomotherapy.

Tomotherapy presents an evolutionary modality that holds forth the promise of better dose conformation to tumor volumes with a concomitant reduction in radiation-induced damage to surrounding normal structures. This delivery technique also presents a new set of radiation protection challenges that impact upon the design of the shielding vault required to house such a unit. A formalism is presented to determine the requisite amounts of shielding for both the primary beam and leakage radiation associated with a generic tomotherapy unit. A comparison is made with the shielding requirements for a conventional linear accelerator operated in a standard manner. Substantial differences in the amount of both primary and secondary shielding are indicated. A tomotherapy primary beam shield is both reduced in width by a factor of almost 10 and increased in thickness by more than a tenth value layer in comparison to a conventional accelerator. Furthermore, the secondary shielding requirements are enhanced by more than two tenth value layers with respect to conventional shielding demands.

Humans↗

PubMed1999

Accuracy of numerically produced compensators.

A feasibility study is performed to assess the utility of a computer numerically controlled (CNC) mill to produce compensating filters for conventional clinical use and for the delivery of intensity-modulated beams. A computer aided machining (CAM) software is used to assist in the design and construction of such filters. Geometric measurements of stepped and wedged surfaces are made to examine the accuracy of surface milling. Molds are milled and filled with molten alloy to produce filters, and both the molds and filters are examined for consistency and accuracy. Results show that the deviation of the filter surfaces from design does not exceed 1.5%. The effective attenuation coefficient is measured for CadFree, a cadmium-free alloy, in a 6 MV photon beam. The effective attenuation coefficients at the depth of maximum dose (1.5 cm) and at 10 cm in solid water phantom are found to be 0.546 cm-1 and 0.522 cm-1, respectively. Further attenuation measurements are made with Cerrobend to assess the variations of the effective attenuation coefficient with field size and source-surface distance. The ability of the CNC mill to accurately produce surfaces is verified with dose profile measurements in a 6 MV photon beam. The test phantom is composed of a 10 degrees polystyrene wedge and a 30 degrees polystyrene wedge, presenting both a sharp discontinuity and sloped surfaces. Dose profiles, measured at the depth of compensation (10 cm) beneath the test phantom and beneath a flat phantom, are compared to those produced by a commercial treatment planning system. Agreement between measured and predicted profiles is within 2%, indicating the viability of the system for filter production.

Feasibility Studies↗

PubMed1999

Setup verification in linac-based radiosurgery.

A semi-automatic technique for the direct setup alignment of radiosurgical circular fields from an isocentric linac to treatment room laser cross-hairs is described. Alignment is achieved by acquiring images of the treatment room positioning laser cross-hairs superimposed on the radiosurgical circular field image. An alignment algorithm calculates the center of the radiosurgical field image as well as the intersection of the laser cross-hairs. This determines any alignment deviations and the information is then used to translate the radiosurgical collimator to its correct aligned position. Two detectors, each being sensitive to the lasers and ionizing radiation, were used to acquire the radiation/laser images. The first detector consists of a 0.3-mm-thick layer of photoconducting a-Se deposited on a 1.5-mm-thick copper plate and the second is film. The algorithm and detector system can detect deviations with a precision of approximately 0.04 mm. A device with gyroscopic degrees of freedom was built in order to firmly hold the detector at any orientation perpendicular to the radiosurgical beam axis. This device was used in conjunction with our alignment algorithm to quantify the isocentric sphere relative to the treatment room lasers over all gantry and couch angles used in dynamic stereotactic radiosurgery.

Algorithms↗

PubMed1999

Three-dimensional anatomy setup verification by correlation of orthogonal portal images and digitally reconstructed radiographs.

A method for three-dimensional verification of anatomy setup that uses the correlation of portal images and reference megavoltage digitally reconstructed radiographs (MDRRs) is presented. Prior to a treatment, an orthogonal pair of portal images is acquired from which subimages containing anatomical features are selected. These subimages are consequently matched to MDRRs that represent different rotations of the anatomy around axes going through the treatment isocenter. The Pearson correlation coefficient is employed for the matching since it is invariant with respect to global scaling and shifting of the image intensities. Furthermore, it does not require feature extraction or point-pair identification. The greatest value of the correlation coefficient corresponds to the proper rotational alignment of the anatomy and the location of the correlation maximum in each view indicates the translational shifts of the anatomy. The mean accuracy of translation and rotation registrations tests were a fraction of a millimeter and a fraction of a degree, respectively, for MDRR-to-MDRR matching. For portal-to-MDRR matching, the mean translation registration error is on the order of 1 mm and the mean error in radial displacement is of the order of 2.7 mm.

Algorithms↗

PubMed1998

Frame slippage verification in stereotactic radiosurgery.

PURPOSE: To develop a method for detecting frame slippage in stereotactic radiosurgery by interactively matching in three dimensions Digitally Reconstructed Radiographs (DRRs) to portal images. METHODS AND MATERIALS: DRRs are superimposed over orthogonal edge-detected portal image pairs obtained prior to treatment. By interactively manipulating the CT data in three dimensions (rotations and translations) new DRRs are generated and overlaid with the orthogonal portal images. This method of matching is able to account for ambiguities due to rotations and translations outside of the imaging plane. The matching procedure is performed with anatomical structures, and is used in tandem with a fiducial marker array attached to the stereotactic frame. The method is evaluated using portal images simulated from patient CT data and then tested using a radiographic head phantom. RESULTS: For simulation tests a mean radial alignment error of 0.82 mm was obtained with the 3D matching method compared to a mean error of 3.52 mm when using conventional matching techniques. For the head phantom tests the mean alignment displacement error for each of the stereotactic coordinates was found to be delta(x) = 0.95 mm, delta(y) = 1.06 mm, delta(z) = 0.99 mm, with a mean error radial of 1.94 mm (SD = 0.61 mm). CONCLUSION: Results indicate that the accuracy of the system is appropriate for stereotactic radiosurgery, and is therefore an effective tool for verification of frame slippage.

Algorithms↗

PubMed1998

A continuous penalty function method for inverse treatment planning.

Conventional inverse treatment planning attempts to calculate dose distributions that may not be feasible given the specified dose levels to various anatomical structures. A technique for inverse treatment planning has been developed that uses only target dose levels which are easily selectable to be feasible. A nonlinear constrained minimization problem is formulated to reflect the goal of sparing critical organs as much as possible while delivering a certain target dose within specified uniformity. The objective function is the squared dose delivered to critical organs. The constraints require the delivery of certain target dose within specified uniformity and non-negative pencil beam weights. A continuous penalty function method is introduced as a method for solving the large-scale constrained minimization problem. The performance of the continuous penalty function method is optimized by numerical investigation of few numerical integration schemes and a pair of weighting functions which influence the utility of the method. Clinical examples are presented that illustrate several features of the technique. The properties of the continuous penalty function method suggest that it may be a viable alternative to conventional inverse treatment planning.

Biophysical Phenomena↗

PubMed1998

Verification of the correspondence between CT-stimulated and treatment beams.

A single phantom technique has been developed to verify the full CT simulation and treatment-beam delivery procedures. The phantom consists of a target delineated by thin copper strips, affixed to therapy verification film, and inserted securely between two slices of water-equivalent material. The target is defined with the aid of the copper strips, and the position of the isocenter and beam parameters such as field size, and gantry and collimator angle are determined by CT simulation. With these parameters, the phantom is subsequently irradiated by the linear accelerator in the treatment position. Correspondence between the planned and the irradiated region is determined by the position of the copper strips on the film. The technique is a simple and practical method for verifying the entire CT simulation and treatment-beam delivery processes, and provides a permanent record of the correspondence between the planning digitally reconstructed radiographs (DRR) and the actual beam delivered.

Computer Simulation↗

PubMed1998

Preliminary study of a metal/a-Se-based portal detector.

A feasibility study has been performed on metal/amorphous selenium detectors for megavoltage portal imaging. The metal plates of the detectors were positioned facing the incident 6 MV and Co-60 photon spectra. The detectors consist of various thicknesses (0.15 mm, 0.30 mm, and 0.50 mm) of amorphous selenium (a-Se) deposited on metal plates of varying thicknesses: aluminum (2.0 mm), copper (1.0 mm and 1.5 mm), stainless steel (0.9 mm), or glass (1.1 mm). The detectors were charged prior to irradiation by corona methods, and the portal images were subsequently digitized after irradiation with a noncontact electrostatic probe. The sensitivity of the detectors to dose, electric field across the a-Se layer, metal plate type and a-Se thickness, was studied. The electrostatic voltage remaining on the a-Se layer was found, both theoretically and experimentally, to exhibit a cubic relationship with respect to dose. An increase in electric field increases the sensitivity (gradient of the a-Se surface voltage vs dose curve) and dynamic range of the resultant image. An increase in a-Se thickness, however, although also increasing the sensitivity, decreases the dynamic range. The metal plate types and thicknesses within the range studied do not have a significant effect on detector sensitivity. Image quality and contrast resolution of the detector were evaluated with a contrast-detail phantom and compared to commercially available film based and electronic portal imaging devices. Image quality of the metal/a-Se detector as a function of dose was studied by discharging the a-Se to various fractions of its initial charge, and as expected, increases with dose due to a decrease in quantum noise. Contrast-detail images obtained by metal/a-Se detectors are superior to those obtained at higher dose levels by other commercial systems.

Biophysical Phenomena↗

PubMed1998

Monte Carlo detective quantum efficiency and scatter studies of a metal/a-Se portal detector.

Simulation studies of a portal imaging detector based on amorphous Selenium (alpha-Se) are reported. The a-Se is deposited on a metal substrate which is turned toward the megavoltage x-ray beam to act as an effective buildup layer. The thickness and material of the metal layer are studied using Monte Carlo methods by consideration of the effect of dose deposition in the alpha-Se layer for a 6-MV beam hardened by a 20-cm polystyrene phantom. The detective quantum efficiency, DQE(f), due to energy absorption is modeled using the hardened primary beam. It is found that although DQE(0) increases with metal thickness up to dmax, there is a crossover near 1 cycle/mm which indicates that smaller metal thickness are more useful to visualize edges and small objects. Tungsten front plates offer the best DQE of the metals studied. The scatter fraction and scatter-to-primary ratio are also modeled to understand the effect of patient scatter on the various metal/a-Se combinations. The scatter fraction is measured experimentally for four metal plate/a-Se combinations and agrees with the Monte Carlo results within experimental uncertainties.

Humans↗