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Biomedical subjects

J C Yanch

Publications and source records attributed to J C Yanch.

28 records · Page 2Linked to original sources
PubMed1991

Absorbed dose profiles for radionuclides of frequent use in radiation synovectomy.

Absorbed dose profiles are presented for radionuclides of frequent or potential use in radiation synovectomy, an intraarticular technique aimed at treating rheumatoid arthritis through direct and highly selective irradiation of inflamed synovium. The radionuclides investigated were 198Au, 165Dy, 32P, 186Re, 90Y, and 166Ho. These profiles reveal the absorbed dose imparted per unit activity of injected radionuclide (Gy/mCi) as a function of penetration distance (mm) to major components of the arthritic synovial joint. Their usefulness is twofold: they can be employed to select the radionuclide best suited to achieving the proper depth of penetration in tissue (approximating the thickness of the inflamed synovium) and to determine, a priori, the necessary dose of radioactivity (delivering an absorbed dose sufficient to effectively treat all the diseased tissue). The extent of radiation of other synovial joint components, such as bone and articular cartilage, and how the advancing rheumatoid arthritis can be expected to alter the extent and pattern of absorbed dose penetration in the joint are also discussed.

Arthritis, Rheumatoid↗
PubMed1991

A Monte Carlo investigation of the dosimetric properties of monoenergetic neutron beams for neutron capture therapy.

A Monte Carlo simulation study has been carried out to investigate the suitability of neutron beams of various energies for therapeutic efficacy in boron neutron capture therapy. The dosimetric properties of unidirectional, monoenergetic neutron beams of varying diameters in two different phantoms (a right-circular cylinder and an ellipsoid) made of brain-equivalent material were examined. The source diameter was varied from 0.0 to 20.0 cm; neutron energies ranged from 0.025 eV up to 800 keV, the maximum neutron energy generated by a tandem cascade accelerator using 2.5-MeV protons in a 7Li(p,n)7Be reaction. Such a device is currently under investigation for use as a neutron source for boron neutron capture therapy. The simulation studies indicate that the maximum effective treatment depth (advantage depth) in the brain is 10.0 cm and is obtainable with a 10-keV neutron beam. A useful range of energies, defined as those neutron energies capable of effectively treating to a depth of 7 cm in brain tissue, is found to be 4.0 eV to 40.0 keV. Beam size is shown not to affect advantage depth as long as the entire phantom volume is used in determining this depth. Dose distribution in directions parallel to and perpendicular to the beam direction are shown to illustrate this phenomenon graphically as well as to illustrate the differences in advantage depth and advantage ratio and the contribution of individual dose components to tumor dose caused by the geometric differences in phantom shape.

Boron↗
PubMed1990

Monte Carlo based dosimetry and treatment planning for neutron capture therapy of brain tumors.

Monte Carlo based dosimetry and computer-aided treatment planning for neutron capture therapy have been developed to provide the necessary link between physical dosimetric measurements performed on the MITR-II epithermal-neutron beams and the need of the radiation oncologist to synthesize large amounts of dosimetric data into a clinically meaningful treatment plan for each individual patient. Monte Carlo simulation has been employed to characterize the spatial dose distributions within a skull/brain model irradiated by an epithermal-neutron beam designed for neutron capture therapy applications. The geometry and elemental composition employed for the mathematical skull/brain model and the neutron and photon fluence-to-dose conversion formalism are presented. A treatment planning program, NCTPLAN, developed specifically for neutron capture therapy, is described. Examples are presented illustrating both one and two-dimensional dose distributions obtainable within the brain with an experimental epithermal-neutron beam, together with beam quality and treatment plan efficacy criteria which have been formulated for neutron capture therapy. The incorporation of three-dimensional computed tomographic image data into the treatment planning procedure is illustrated. The experimental epithermal-neutron beam has a maximum usable circular diameter of 20 cm, and with 30 ppm of B-10 in tumor and 3 ppm of B-10 in blood, it produces (with RBE weighting) a beam-axis advantage depth of 7.4 cm, a beam-axis advantage ratio of 1.83, a global advantage ratio of 1.70, and an advantage depth RBE-dose rate to tumor of 20.6 RBE-cGy/min (cJ/kg-min). These characteristics make this beam well suited for clinical applications, enabling an RBE-dose of 2,000 RBE-cGy/min (cJ/kg-min) to be delivered to tumor at brain midline in six fractions with a treatment time of approximately 16 minutes per fraction. With parallel-opposed lateral irradiation, the planar advantage depth contour for this beam (with the B-10 distribution defined above) encompasses nearly the whole brain. Experimental calibration techniques for the conversion of normalized to absolute treatment plans are described.

Boron↗
PubMed1990

Monte Carlo methods of neutron beam design for neutron capture therapy at the MIT Research Reactor (MITR-II).

Monte Carlo methods of coupled neutron/photon transport are being used in the design of filtered beams for Neutron Capture Therapy (NCT). This method of beam analysis provides segregation of each individual dose component, and thereby facilitates beam optimization. The Monte Carlo method is discussed in some detail in relation to NCT epithermal beam design. Ideal neutron beams (i.e., plane-wave monoenergetic neutron beams with no primary gamma-ray contamination) have been modeled both for comparison and to establish target conditions for a practical NCT epithermal beam design. Detailed models of the 5 MWt Massachusetts Institute of Technology Research Reactor (MITR-II) together with a polyethylene head phantom have been used to characterize approximately 100 beam filter and moderator configurations. Using the Monte Carlo methodology of beam design and benchmarking/calibrating our computations with measurements, has resulted in an epithermal beam design which is useful for therapy of deep-seated brain tumors. This beam is predicted to be capable of delivering a dose of 2000 RBE-cGy (cJ/kg) to a therapeutic advantage depth of 5.7 cm in polyethylene assuming 30 micrograms/g 10B in tumor with a ten-to-one tumor-to-blood ratio, and a beam diameter of 18.4 cm. The advantage ratio (AR) is predicted to be 2.2 with a total irradiation time of approximately 80 minutes. Further optimization work on the MITR-II epithermal beams is expected to improve the available beams.

Brain Neoplasms↗
PubMed1988

Deconvolution of emission tomographic data: a clinical evaluation.

A method of improving the quality of images in single photon emission computed tomography (SPECT) is demonstrated using transaxial images of the liver and brain. Deconvolution of the nuclear medicine data by a point source response function (PSRF) acquired previously in a scattering medium attempts to compensate for scattered radiation within the patient. The average geometric response of the collimator of the gamma camera is also compensated for with this technique. Three patients with known metastatic lesions in the liver and three with primary lesions in the brain were imaged. Clinical assessment of reconstructed slices both before and after deconvolution demonstrates that compensating for the effects of scatter and of collimator blurring leads to enhanced detail of pathological lesions. In all cases, cold lesions seen prior to deconvolution were enhanced in detail and, in addition, new lesions were seen with this technique.

Adult↗
PubMed1986

Instrumentation and light dosimetry for intra-operative photodynamic therapy (PDT) of malignant brain tumours.

A technique is described for uniform light irradiation of the tumour cavity after radical subtotal malignant brain tumour resection in patients whose tumours have been photosensitized by intravenous injection of hematoporphyrin derivative. The cavity is maintained by an inflatable rubber 'balloon' filled with a light-scattering liquid, within which is placed a single-strand optical fibre coupled to an argon/dye laser operating at 630 nm. Details of the construction of the applicator are given, together with measurements of the uniformity of irradiation and the light loss within the scattering medium. The device has been assessed in nine patients in a phase I trial. In two of these patients, the penetration depth of 630 nm light in brain tissue has also been measured in vivo using an optical-fibre probe, indicating that the penetration depth of 630 nm light in human brain is greater than that found in vitro.

Adolescent↗
PubMedDate not supplied

Improved quantification of radionuclide uptake using deconvolution and windowed subtraction techniques for scatter compensation in single photon emission computed tomography.

A comparison of two methods of scatter compensation in single photon emission computed tomography (SPECT) imaging is made on the basis of improvements in quantification. The methods, scatter-window subtraction and constrained deconvolution of an average point source response function (PSRF), are described; the theoretical basis of each method is also briefly assessed. Improvements in relative quantification offered by each method are measured by examining ratios of counts in hot cylinders to counts in a slightly radioactive background. The cylinder/background concentration ratio was varied by a factor of five; the sizes of the cylinders remained constant. Keeping the diameter of the cylinders constant allowed for an assessment of the effect of concentration differences, without the conflicting effect of variation in the size of the hot source. Results showed that while both scatter-window subtraction and constrained deconvolution offer quantification improvements in the final image, the method of prereconstruction deconvolution of a planar PSRF from each planar projection is substantially more successful than either scatter-window subtraction or other methods of implementing the deconvolution procedure.

Biophysical Phenomena↗
PubMedDate not supplied

Accelerator-based epithermal neutron beam design for neutron capture therapy.

Recent interest in the production of epithermal neutrons for use in boron neutron capture therapy (BNCT) has promoted an investigation into the feasibility of generating such neutrons with a high current proton accelerator. Energetic protons (2.5 MeV) on a 7Li target produce a spectrum of neutrons with maximum energy of roughly 800 keV. A number of combinations of D2O moderator, lead reflector, 6Li thermal neutron filtration, and D2O/6Li shielding will result in a useful epithermal flux of 1.6 x 10(8) n/s at the patient position. The neutron beam is capable of delivering 3000 RBE-cGy to a tumor at a depth of 7.5 cm in a total treatment time of 60-93 min (depending on RBE values used and based on a 24-cm diameter x 19-cm length D2O moderator). Treatment of deeper tumors with therapeutic advantage would also be possible. Maximum advantage depths (RBE weighted) of 8.2-9.2 (again depending on RBE values and precise moderator configuration) are obtained in a right-circular cylindrical phantom composed of brain-equivalent material with an advantage ratio of 4.7-6.3. A tandem cascade accelerator (TCA), designed and constructed at Science Research Laboratory (SRL) in Somerville MA, can provide the required proton beam parameters for BNCT of deep-seated tumors. An optimized configuration of materials required to shift the accelerator neutron spectrum down to therapeutically useful energies has been designed using Monte Carlo simulation in the Whitaker College Biomedical Imaging and Computation Laboratory at MIT. Actual construction of the moderator/reflector assembly is currently underway.

Boron↗
PubMedDate not supplied

Calculation of beta dosimetry in radiation synovectomy using Monte Carlo simulation (EGS4).

Using the EGS4 Monte Carlo code, absorbed dose rate factors were estimated for four radionuclides of interest in radiation synovectomy, an intra-articular radiation therapy to treat rheumatoid arthritis. The treatment consists of the injection of a beta-emitting radionuclide into the joint capsule in order to eliminate diseased synovium through irradiation. The radionuclides investigated are 32P, 90Y, 165Dy, and 198Au. Calculations reveal the absorbed dose factor (cGy cm2/MBq s) as a function of distance (mm) in an EGS4 model of the rheumatic joint. The model incorporates bone, articular cartilage, joint capsule, and tissue (synovium) components found in all synovial joints, with dimensions in the model corresponding to dimensions typically found in larger joints, e.g., the knee, shoulder, or hip. Results are compared with previous, analytical approaches to beta dosimetry in radiation synovectomy. In addition, radiation backscatter due to the presence of bone is investigated and determined to have a negligible enhancement effect on absorbed dose to synovium.

Arthritis, Rheumatoid↗