Biomedical subjects

J A Fiedler

Publications and source records attributed to J A Fiedler.

8 recordsLinked to original sources

PubMed1997

Dose-volume histograms for bladder and rectum.

PURPOSE: A careful examination of the foundation upon which the concept of the Dose-Volume Histogram (DVH) is built, and the implications of this set of parameters on the clinical application and interpretation of the DVH concept has not been conducted since the introduction of DVHs as a tool for the quantitative evaluation of treatment plans. The purpose of the work presented herein is to illustrate problems with current methods of implementing and interpreting DVHs when applied to hollow anatomic structures such as the bladder and rectum. METHODS AND MATERIALS: A typical treatment plan for external beam irradiation of a patient with prostate cancer was chosen to provide a data set from which DVH curves for both the bladder and rectum were calculated. The two organs share the property of being shells with contents that are of no clinical importance. DVHs for both organs were computed using a solid model and using a shell model. Typical treatment plans for prostate cancer were used to generate DVH curves for both models. The Normal Tissue Complication Probability (NTCP) for these organs is discussed in this context. RESULTS: For an eight-field conformal treatment plan of the prostate, a bladder DVH curve generated using the shell model is higher than the corresponding curve generated using the solid model. The shell model also has a higher NTCP. A six-field conformal treatment plan also results in a higher DVH curve for the shell model. A treatment plan consisting of bilateral 120-degree arcs, results in a higher DVH curve for the shell model, as well as a higher NTCP. CONCLUSION: The DVH concept currently used in evaluation of treatment plans is problematic because current practices of defining exactly what constitutes "bladder" and "rectum." Commonly used methods of tracing the bladder and rectum imply use of a solid structure model for DVHs. In reality, these organs are shells and the critical structure associated with NTCP is obviously and indisputably the shell, as opposed to its contents. Treatment planning algorithms for DVH computation should thus be modified to utilize the shell model for these organs.

Humans↗

PubMed1996

Stereotactic dose computation and plan optimization using the convolution theorem. I. Dose computation.

With Leksell Gamma Knife stereotactic radiosurgery, the dose distribution delivered by a specific helmet can be assumed to remain as a fixed-dose distribution when the shot is moved to different locations within the predefined dose calculation matrix. The convolution theorem may be implemented to take advantage of this fact for fast dose computation and plan construction. Using this technique, the shot spatial arrangement is formulated as a convolution kernel, which is theoretically a three-dimensional multi-delta function. The dose distribution is computed by the convolution of this single-shot dose distribution with the shot convolution kernel. To determine the shot arrangement, an ideal dose distribution is generated based upon the target structure. Deconvolution is then applied to find the convolution kernel which best fits the proposed ideal dose distribution. The primary task of this presentation is to focus on and describe in detail the dose computation using the convolution theorem.

Algorithms↗

PubMed1994

Scattered radiation from linear accelerator and cobalt-60 collimator jaws.

PURPOSE: Solid state diodes and/or thermoluminescent dosimeters (TLDs) are often used to measure scattered radiation doses to critical organs immediately adjacent to radiation field sites. The energy-dependent response of these commonly used in vivo dosimeters sometimes makes the interpretation of measured values uncertain. This study investigates scattered radiation arising from the collimator jaws of linear accelerators and the treatment head of a cobalt-60 teletherapy unit. METHODS AND MATERIALS: A thin window Markus-type parallel-plate ionization chamber placed in a polystyrene phantom was employed to document the magnitude, energy composition, and sources of scattered radiation at surfaces near radiation fields. Measurements were taken both with and without additional phantom material covering the ionization chamber, as well as with various distances between the ionization chamber and edges of the radiation fields tested. RESULTS: Data was collected, analyzed and compared for treatment units produced by different manufacturers. It was found that the magnitude of scattered radiation to surfaces immediately adjacent to radiation fields ranged from 1% to 15% of the maximum dose along the beam central axis. These values showed a strong dependence upon distance from the edge of the radiation field, beam energy, collimator setting (field size), and the presence of externally mounted accessories. Teletherapy unit differences due to manufacturing firm origins were found to only slightly affect scattered radiation magnitude, while the orientation of upper and lower collimator jaws had absolutely no effect. CONCLUSIONS: Percent depth dose curves of scattered radiation were obtained and analyzed. The shapes of these depth dose curves suggest the presence of complex energy spectra from secondary electrons and scattered x-rays. Because of the presence of these complex energy spectra in areas immediately adjacent to radiation fields, caution should be observed when interpreting patient doses near radiation fields, if dose values have been measured in vivo using thermoluminescent dosimeters (TLDs) or solid state diodes. Many of these on-patient dosimetry devices are strongly energy dependent and may demonstrate large over- or under-responses in areas dominated by scattered radiation. The results of this study, thus, suggest that ionization chambers are preferred for determination of scattered radiation doses in such regions.

Humans↗

PubMed1993

Use of a clinical MR scanner for imaging the rat brain.

Magnetic resonance imaging (MRI) and magnetic resonance spectroscopy are established techniques that enable noninvasive anatomic and functional tissue characterization in vivo. These tools have been employed to probe experimental models of neoplasia, cerebrovascular disease, brain injury, and neurotransplantation in small animals. To date, these studies have been executed primarily on research-dedicated instruments of limited availability or resolution. Using relatively straightforward software and hardware modifications of a widely used clinical MRI unit, we were able to image numerous structures within the living rat brain including the neostriatum, hippocampus, periaqueductal gray, and the ventricular system. Illustrative applications of this imaging technique in two intracerebral infusion models involving rats are presented. Such adaptation of clinical MRI scanners has the potential to significantly expand the availability of high resolution in vivo imaging of small animals for a variety of experimental protocols.

Animals↗

PubMed1992

Dose determination in high dose-rate brachytherapy.

Although high dose-rate brachytherapy with a single, rapidly moving radiation source is becoming a common treatment modality, a suitable formalism for determination of the dose delivered by a moving radiation source has not yet been developed. At present, brachytherapy software simulates high dose-rate treatments using only a series of stationary sources, and consequently fails to account for the dose component delivered while the source is in motion. We now describe a practical model for determination of the true, total dose administered. The algorithm calculates both the dose delivered while the source is in motion within and outside of the implanted volume (dynamic component), and the dose delivered while the source is stationary at a series of fixed dwell points. It is shown that the dynamic dose element cannot be ignored because it always increases the dose at the prescription points and, in addition, distorts the dose distribution within and outside of the irradiated volume. Failure to account for the dynamic dose component results in dosimetric errors that range from significant (> 10%) to negligible (< 1%), depending on the prescribed dose, source activity, and source speed as defined by the implant geometry.

Brachytherapy↗

PubMedDate not supplied

Dosimetric aspects of the therapeutic photon beams from a dual-energy linear accelerator.

Parameters of the photon beams (6 and 20 MV) from a dual-energy linear accelerator (Mevatron-KD, Siemens Medical Laboratories, CA) are presented. The depth dose characteristics of the photon beams are dmax of 1.8 and 3.8 cm and percentage depth dose of 68% and 80% at 10-cm depth and 100-cm source-surface distance for a field size of 10 X 10 cm2 for 6 and 20 MV, respectively. The 6 and 20 MV beams were found to correspond to nominal accelerating potentials of 4.7 and 17 MV, respectively. The stability of output is within +/- 1% and flatness and symmetry are within +/- 3%. These figures compare favorably with the manufacturer's specifications.

Humans↗

PubMedDate not supplied

A rapid method for electron beam energy check.

Assessment of electron beam energy and its long term stability is part of standard quality assurance practice in radiation oncology. Conventional depth-ionization or depth-film density measurements are time consuming both in terms of data acquisition and analysis. A procedure is described utilizing ionization measurements at two energy specific depths. It is based on a linear relationship between electron beam energy and its practical range. Energy shifts within the range covered by the two measurement depths are easily resolved. Within a range of +/- 0.50 MeV (+/- 1.30 MeV) around the established mean incident energy of 5.48 MeV (20.39 MeV), the method accuracy is better than 0.10 MeV.

Electrons↗

PubMedDate not supplied

Radiation-induced brachial plexopathy: MR and clinical findings.

A 54-year-old man had a slowly progressive bilateral brachial plexopathy 17 months after surgery and radiation therapy for a stage IV supraglottic carcinoma. MR imaging at presentation showed a symmetric pattern of parascalene and interscalene hyperintense signal on T2-weighted images and after contrast enhancement. Although hyperintense signal has been more often associated with recurrent tumor than with delayed radiation injury or fibrosis, the location and pattern of the signal abnormalities suggested a diagnosis of radiation-induced plexopathy. This diagnosis was confirmed by the relative stability of the neurologic and MR findings 30 months after treatment.

Brachial Plexus↗