Biomedical subjects

F T Kuchnir

Publications and source records attributed to F T Kuchnir.

At least 19 recordsLinked to original sources

PubMed2001

Experimental determination of the overall perturbation factor for the NACP chamber in electron beams for dmax < d < or = d80.

In electron beam dosimetry with an ionization chamber, a factor that corrects for the cavity perturbation of the medium, Prepl, and one to account for the disturbance due to the chamber wall material differing from the medium, Pwall, are required. The overall perturbation correction factor, p(q) = PreplPwall, has been introduced because of the difficulty in separately measuring these two components. An advantage of parallel-plate ionization chambers is that p(q) has been shown to be close to unity at dmax. However, many dosimetry applications require knowledge of the overall perturbation factor at depths greater than dmax. We determined p(q) for the NACP chamber at depths beyond dmax by intercomparing percentage depth dose measurements made with it with those obtained with a PTW/diamond detector for which p(q) was taken as unity at all the measurement depths. Data were obtained at depths corresponding to approximately the 90 and 80 per cent of the dose maxima for 20, 16, 12 and 6 MeV incident electrons. The beam energy at depth, Ed, and the percentage depth-dose gradient varied from 1.4 to 14.3 MeV and 0 to 5.8% mm(-1) respectively. Our results show that within the estimated uncertainty of 1.3%, p(q),NACP is unity over the range of energies and dose gradients studied.

Biophysical Phenomena↗

PubMed1999

Measured overall perturbation factors at depths greater than dmax for ionization chambers in electron beams.

In electron beam dosimetry the perturbation effect in the medium by the ionization chamber cavity is accounted for by introducing a replacement correction factor, P(repl). Another perturbation correction factor, denoted as P(wall), is due to the materials of the walls of the parallel-plate chamber differing from the phantom material. Because of the difficulties in separating these two components, we measure the overall perturbation factor, p(q) = P(repl)P(wall). A distinct advantage of parallel-plate ionization chambers over cylindrical chambers is that p(q) has been shown to be close to unity at the standard calibration depth, d(max). However, for many dosimetry applications it is necessary to know the overall perturbation factor at depths greater than d(max). We measured the overall perturbation factor at depths greater than d(max) (approximating the 95%, 90% and 50% depth dose) for a Farmer-type cylindrical ionization chamber and three parallel-plate ionization chambers. We assumed that p(q) for the NACP chamber is unity at these measurement depths. The depth dependence for the other chambers was then measured relative to the NACP chamber. The mean energy at depth, E(d), and percentage depth dose gradient ranges studied were 1.9-18.5 MeV and 0 to 4.5%/mm, respectively. For the other two parallel-plate chambers, we find p(q) to be unity at depths where the percent depth dose is greater than 90%, but it deviates from unity at deeper depths, where the dose gradients exceed about 2.5%/mm. For the cylindrical chamber, p(q) values at depths greater than d(max) were found to be in good agreement with those in TG 21, where the energy at depth, E(d), is used to evaluate p(q).

Calibration↗

PubMed1998

Verification of the omni wedge technique.

The optimal field shape achieved using a multileaf collimator (MLC) often requires collimator rotation to minimize the adverse effects of the scalloped dose distribution the leaf steps produce. However, treatment machines are designed to deliver wedged fields parallel or perpendicular to the direction of the leaves. An analysis of cases from our clinic showed that for 25% of the wedged fields used to treat brain and lung tumors, the wedge direction and optimal MLC orientation differed by 20 degrees or more. The recently published omni wedge technique provides the capability of producing a wedged field with orientation independent of the orientation of the collimator. This paper presents a comparison of the three-dimensional (3D) dose distributions of the omni wedged field with distributions of wedged fields produced using both the universal and dynamic wedge techniques. All measurements were performed using film dosimetry techniques. The omni wedge generated fields closely matched the conventional wedged fields. Throughout 95% of the irradiated volume (excluding the penubra), the dose distribution of the omni wedged field ranged from +5.5 to -3.5 +/- 1.5% of that of the conventionally wedged fields. Calculation of the omni wedged field is as accurate as conventional wedged field calculation when using a 3D treatment planning systems. For two-dimensional treatment planning systems, where one must assume that the omni wedged field is identical to a conventional field, the calculated field and the delivered field differs by a small amount.

Brain Neoplasms↗

PubMed1997

Optimization of relative weights and wedge angles in treatment planning.

An efficient technique to optimize beam weights and wedge angles in radiotherapy treatment planning has been developed. Based on the fact that a wedged field can be regarded as a superposition of an open field and a nominal wedged field, this approach reduces the problem of finding J beam weights and the corresponding wedge angles to optimizing a linear system with 2J unknowns (weights of J open beams and J nominal wedged beams), where J is the total number of incident beam directions. Two iterative algorithms similar to the iterative-least-square technique in image reconstruction are used to optimize the system. Application of the algorithms to two specific examples shows that this technique can reduce treatment planning time and effort, and promises to create a better solution for an arbitrarily complex treatment plan.

Algorithms↗

PubMed1996

Electron-photon field matching in the treatment of paranasal sinus tumors: a case report.

The treatment planning of paranasal sinus tumors is technically demanding due to the compact anatomy of the region and the close proximity of critical structures. In the majority of cases, conventional approaches utilizing 2 or 3 photon fields are adequate. However, in patients with locally advanced disease, these standard techniques may result in the unnecessary treatment of surrounding structures. We present here a case in which, because of the complex tumor geometry, conventional techniques would result in treating through both orbits. A novel treatment approach has been devised in which opposed lateral photon fields are matched to an anterior electron field (both in depth and in profile) to provide a uniform dose distribution to the target volume, while minimizing the dose to certain critical structures. The treatment design, in particular the methodology of electron-photon field matching as well as the specification of tissue compensation and customized blocking, is discussed.

Aged↗

PubMed1996

Repositioning accuracy of a noninvasive head fixation system for stereotactic radiotherapy.

We report on the repositioning accuracy of patient setup achieved with a noninvasive head fixation device for stereotactic radiotherapy. A custom head mask which attaches to our stereotactic radiosurgery head ring assembly is fabricated for each patient. The position and orientation of a patient in the stereotatic space at the time of treatment are determined from analyzing portal films containing images of radio-opaque spheres embedded in a custom mouthpiece. From analysis of 104 setups of 12 patients, we find that the average distance between the treated isocenter and its mean position is 1.8 mm, and that the standard deviations of the position of the treated isocenter in stereotactic coordinate space about its mean position are less than 1.4 mm in translation in any direction and less than 1 degree of rotation about any axis.

Equipment Design↗

PubMed1995

Comparison of static conformal field with multiple noncoplanar arc techniques for stereotactic radiosurgery or stereotactic radiotherapy.

PURPOSE: Compare the use of static conformal fields with the use of multiple noncoplanar arcs for stereotactic radiosurgery or stereotactic radiotherapy treatment of intracranial lesions. Evaluate the efficacy of these treatment techniques to deliver dose distributions comparable to those considered acceptable in current radiotherapy practice. METHODS AND MATERIALS: A previously treated radiosurgery case of a patient presenting with an irregularly shaped intracranial lesion was selected. Using a three-dimensional (3D) treatment-planning system, treatment plans using a single isocenter multiple noncoplanar arc technique and multiple noncoplanar conformal static fields were generated. Isodose distributions and dose volume histograms (DVHs) were computed for each treatment plan. We required that the 80% (of maximum dose) isodose surface enclose the target volume for all treatment plans. The prescription isodose was set equal to the minimum target isodose. The DVHs were analyzed to evaluate and compare the different treatment plans. RESULTS: The dose distribution in the target volume becomes more uniform as the number of conformal fields increases. The volume of normal tissue receiving low doses (> 10% of prescription isodose) increases as the number of static fields increases. The single isocenter multiple arc plan treats the greatest volume of normal tissue to low doses, approximately 1.6 times more volume than that treated by four static fields. The volume of normal tissue receiving high (> 90% of prescription isodose) and intermediate (> 50% of prescription isodose) doses decreases by 29 and 22%, respectively, as the number of static fields is increased from four to eight. Increasing the number of static fields to 12 only further reduces the high and intermediate dose volumes by 10 and 6%, respectively. The volume receiving the prescription dose is more than 3.5 times larger than the target volume for all treatment plans. CONCLUSIONS: Use of a multiple noncoplanar conformal static field treatment technique can significantly reduce the volume of normal tissue receiving high and intermediate doses compared with a single isocenter multiple arc treatment technique, while providing a more uniform dose in the target volume. Close conformation of the prescription isodose to the target volume is not possible using static uniform conformal fields for target shapes lacking an axis of rotational symmetry or plane of mirror symmetry.

Brain Neoplasms↗

PubMed1995

Photon neutron mixed-beam radiotherapy of locally advanced prostate cancer.

PURPOSE: In this article we present the results of mixed-beam, photon/neutron radiation therapy in 45 patients with locally advanced, bulky, or postoperative recurrent prostate cancer treated at the University of Chicago between 1978 and 1991. Survival, disease-free survival, local control, and long-term complications are analyzed in detail. METHODS AND MATERIALS: Between 1978 and 1991, 45 patients with locally advanced (> 5 cm State B2, Stage C, or Stage D1) prostate cancer underwent mixed-beam (photon/neutron) radiation therapy. Forty percent of the treatment was delivered with neutron irradiation at either the University of Chicago or Fermilab. Sixty percent of treatment was delivered with photons at the University of Chicago. Initially, the whole pelvis was irradiated to 50 photon Gy equivalent. This was followed by a boost to the prostate for an additional 20 photon Gy equivalent. RESULTS: The median follow-up for patients in this series is 72 months. The overall 5-year actuarial survival was 72%, and the 5-year disease-free survival was 45%. Thus far, 18 patients have died. Eleven patients have died from prostate cancer and 7 from other medical illness. Twenty-seven patients are alive, and 12 of these patients have recurrent and or metastatic disease. The local control rate was 89% (40 out of 45). Histologic material was available on 18 patients following treatment (i.e., prostate biopsy in 16 patients and autopsy in 2 patients) and was negative for carcinoma in 13 (72%). Significant Grade 3-5 complications occurred in 36% (16 out of 45) of the patients treated with mixed-beam radiation therapy and were related to dose and beam quality. Factors related to survival, disease-free survival, local control, and complications are analyzed. CONCLUSIONS: The survival and local control results of mixed-beam radiation therapy at the University of Chicago appear to be superior to those series using photon radiation in patients with locally advanced prostate carcinoma. Mixed-beam radiation therapy should remain an alternative to studies using dose escalation or implant techniques as a method to increase local control and survival at institutions with this capability. However, appropriate plans with high-energy neutrons are necessary to minimize complications.

Actuarial Analysis↗

PubMed1994

A comparison of methods for calibrating parallel-plate chambers/.

All dosimetry protocols for calibrating the output of electron beams recommend the use of parallel-plate ionization chambers, but the method of determining their value of Ngas is a matter of concern. The AAPM Protocol (TG 21) recommends a direct comparison with a calibrated cylindrical chamber in phantom at dmax with the highest available electron energy beam. This must be done by the user. Since all calibration laboratories traditionally use 60Co for megavoltage chamber calibrations, two alternate procedures based on exposures in-air, or in-phantom, have been proposed. All methods use correction factors in the data reduction. To verify the consistency of the three methods, we have measured Ngas using each of these techniques for six of the most commonly used and commercially-available parallel-plate ionization chambers. The paired cylindrical and parallel-plate ionization chambers, and phantom materials/buildup caps were matched to the wall composition of the plane chambers, as recommended in TG 39. A 22 MeV electron beam was used for the electron irradiations. The ionization chambers were then taken to an Accredited Dosimetry Calibration Laboratory (ADCL), where 60Co calibrations were performed. The results demonstrate that, by using the appropriate correction factors for the chambers described in this work, all three methods yield values for Ngas that are within 1% of each other.

Biophysical Phenomena↗

PubMed1994

Craniospinal axis irradiation: an improved electron technique for irradiation of the spinal axis.

In this work we review the dosimetric features of craniospinal axis irradiation in the areas of matching cranial and spinal fields, with reference to the normal structures within the spinal field. The implications of the use of photon or electron modalities for the spinal port were evaluated. A novel method of matching the cranial photon and the spinal electron fields involving a computer-aided junction design is presented. The technique involves moving the photon beam in three steps to degrade its penumbra to match that of the electron field. Thermoluminescent dosimetry in a Rando phantom and computed tomography-based dose-volume histogram study for an illustrative paediatric case were used to compare the dose to normal structures within the spinal field. Our results show that the use of electrons for the spinal field leads to better sparing of deep seated normal structures. In the case of bone marrow, the use of a customized bolus for the spinal field results in an improved dose distribution, making electrons potentially superior to photons for radiobiological reasons.

Bone Marrow↗

PubMed1992

A dosimetric study comparing three-, four-, and six- field plans for treatment of carcinoma of the prostate.

We present a three-dimensional dosimetric analysis of 3-, 4-, and 6-field plans using 24 MV photon beams for treatment of carcinoma of the prostate. We compare isodose distributions on a transverse plane through the center of the target as well as differential and integral dose volume histograms for the target and critical structures, respectively. An extensive study on a representative case led to the development of a technique where two complementary 3-field daily plans deliver the same daily target dose as the standard 4-field box while affording sparing of the bladder and rectum similar to that achieved with a 6-field plan. This technique was shown to yield the same results on a sample of four additional patients representing a range of target and patient sizes. We conclude that the combined two-day, 3-field method for treatment of the prostate may be a better choice than the standard 4-field box or the 6-field daily plan for dose escalation studies.

Humans↗

PubMed1991

3-D treatment planning and dose delivery verification integrating a variety of state-of-the-art techniques: a case report.

A patient previously treated with radiation for base-of-tongue cancer presented with recurrent disease seven years later. The spinal cord had received tolerance dose. Using state-of-the-art treatment planning techniques, including beam's-eye-view and volumetrics, dose-volume histograms, split field technique, mixed energies, and beam intensity modulation (with a compensator), we achieved uniform dose coverage of the target in 3-D. This was verified in vivo with thermoluminescence dosimeters positioned in the esophagus by means of a nasogastric tube that ran centrally through the target volume. The various techniques applied will be presented with a discussion of the rationale used in each step of plan optimization and verification.

Image Processing, Computer-Assisted↗

PubMed1990

Computer-aided construction and quantitative evaluation of missing-tissue compensators.

We have implemented a system for the design and construction of missing-tissue compensators for Radiation Therapy. The patient topography is obtained by Moire' photography. The thickness of lead required to compensate for a given amount of missing tissue was determined experimentally for three photo-beam energies and a combination of field sizes and geometries. With the aid of a computer, tissue deficit information is converted to isolead-thickness lines. These are used as input to a computer-controlled milling machine to fabricate the compensator. The effectiveness of compensation was evaluated in phantom and in vivo. This work describes the initial effort required to implement a program for compensation of tissue deficit at the patient's surface. It also introduces tools for assessing quantitatively the degree of dose uniformity which can be achieved using compensators in clinical applications.

Humans↗

PubMed1989

Beam's-eye-view aided treatment planning for a nasopharyngeal lesion: a case report.

We report on the application of CT-based multilevel treatment planning to achieve complete and uniform dose distribution over the entire target while sparing critical structures. Treatment strategy and parameters are chosen on the slice containing the isocenter. Target coverage and organ sparing is achieved on all other slices by independently adjusting the asymmetric field width at each level, stimulating the effects of custom blocks. The optimized field borders are back projected using beam's eye views (BEV) from each treatment angle. The BEV printouts are used to assist the physician in the delineation of field blocking on the simulation films.

Humans↗

PubMed1989

Effect of lung-density correction in treatment planning for tangential-fields breast irradiation: a case report.

We report on the effect of lung-density correction on dose distribution in a transverse slice containing the isocenter for tangential-fields breast irradiation. In this case study we analyzed the target coverage as well as hot spots for four types of treatment plans: Plan 1 assumes uniform unit density throughout, Plan 2 utilizes all the treatment parameters of Plan 1, but takes into account the lower lung density. Plan 3 is generated by optimizing the dose distribution in the presence of the lower lung density, and Plan 4 is an improvement on Plan 3 by using custom instead of standard wedges. Our analysis shows that consideration of the lower lung density is important for optimal treatment planning for the breast and that specially designed wedges can improve the dose distribution.

Breast Neoplasms↗

PubMed1980

Comparison of the microdosimetric event-size method and the twin-chamber method of separating dose into neutron and gamma components.

Microdosimetric measurements of event-size spectra, made with a proportional counter, are being used increasingly for separation of dose components in mixed n-gamma fields. Measurements in fields produced by 8.3 MeV deuteron bombardment of thick beryllium and deuterium targets were made in air and at 6 and 12 cm depth in water with a spherical tissue-equivalent (TE) proportional counter and with a pair of calibrated ion chambers (TE-TE and Mg-Ar). The dose results obtained with the two methods agree well for the neutron components, but the gamma components do not demonstrate consistent agreement. An important source of error in the microdosimetric method is the matching of the spectra measured at different gain settings to cover the large range of event sizes. The effect of this and other sources of error is analysed.

Gamma Rays↗

PubMed1979

Energy dependence of the neutron sensitivity of C--CO2, Mg--Ar and TE--TE ionisation chambers.

The neutron sensitivity relative to 60Co of commercially available C--CO2, Mg--Ar and TE--TE ionisation chambers was measured as a function of energy from 1 to 44 MeV. The sensitivity function was obtained by the method of Kuchnir, Vyborny and Skaggs from differences in measurements made at two angles in mixed fields having an isotropic gamma-ray component. Such fields were produced by bombardment of a thick beryllium target with 16 and 28 MeV deuterons, 44 MeV 3He-ions and 35 and 46 MeV protons. The results show that the relative neutron sensitivity of the C--CO2 and Mg--Ar chambers increases continuously with energy, whereas that of the TE--TE chamber is relatively constant.

Argon↗

PubMed1978

Dosimetric properties of neutron beams from the D--D reaction in the energy range from 6.8 to 11.1 MeV.

The tissue kerma in air, the tissue dose at maximum build-up, the relative depth dose on the central axis and the dose build-up characteristics were measured for neutrons produced by 6.8, 8.9 and 11.1 MeV deuterons on deuterium. The neutron beams were produced by a variable-energy cyclotron with a fully stopping deuterium gas target 20 cm long. Measurements were made in a 11.1cm x 11.1 cm field 126 cm from the target entrance window. The dose rate was found to increase rapidly with energy from 0.07 rad min-1 microamperemeter-1 at 6.8MeV to 0.35 rad min-1 muA-1 at 11.1 MeV. The entrance dose is about 50% of the dose maximum for each bombarding energy. The depth of the 95% dose level in the build-up region increased from 50 mg cm-2 at 6.8 MeV to 90 mg cm-2 at 11.1 MeV. The penetration was independent of the bombarding energy in the region investigated. Attenuation of the total dose to 50% of the maximum occurred at 10.2 +/- 0.1 g cm-2 for all three bombarding energies. The dose at the maximum is typically 14% higher than the tissue kerma in air.

Atmosphere↗