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Biomedical subjects

L Papiez

Publications and source records attributed to L Papiez.

15 recordsLinked to original sources
PubMed2003

Contamination dose from photoneutron processes in bodily tissues during therapeutic radiation delivery.

Dose to the total body from induced radiation resulting from primary exposure to radiotherapeutic beams is not detailed in routine treatment planning though this information is potentially important for better estimates of health risks including secondary cancers. This information can also allow better management of patient treatment logistics, suggesting better timing, sequencing, and conduct of treatment. Monte Carlo simulations capable of taking into account all interactions contributing to the dose to the total body, including neutron scattering and induced radioactivity, provide the most versatile and accurate tool for investigating these effects. MCNPX code version 2.2.6 with full IAEA library of photoneutron cross sections is particularly suited to trace not only photoneutrons but also protons and heavy ion particles that result from photoneutron interactions. Specifically, the MCNPX code is applied here to the problem of dose calculations in traditional (non-IMRT) photon beam therapy. Points of calculation are located in the head, where the primary irradiation has been directed, but also in the superior portion of the torso of the ORNL Mathematical Human Phantom. We calculated dose contributions from neutrons, protons, deutrons, tritons and He-3 that are produced at the time of photoneutron interactions in the body and that would not have been accounted for by conventional radiation oncology dosimetry.

Humans↗
PubMed2001

Improved leaf sequencing reduces segments or monitor units needed to deliver IMRT using multileaf collimators.

Leaf sequencing algorithms may use an unnecessary number of monitor units or segments to generate intensity maps for delivery of intensity modulated radiotherapy (IMRT) using multiple static fields. An integer algorithm was devised to generate a sequence with the fewest possible segments when the minimum number of monitor units are used. Special hardware related restrictions on leaf motion can be incorporated. The algorithm was tested using a benchmark map from the literature and clinical examples. Results were compared to sequences given by the routine of Bortfeld that minimizes monitor units by treating each row independently, and the areal or reducing routines that use fewer segments at the price of more monitor units. The Bortfeld algorithm used on average 58% more segments than provided by the integer algorithm with bidirectional motion and 32% more segments than did an integer algorithm admitting only unidirectional sequences. The areal algorithm used 48% more monitor units and the reducing algorithm used 23% more monitor units than did the bidirectional integer algorithm, while the areal and reducing algorithms used 23% more segments than did the integer algorithm. Improved leaf sequencing algorithms can allow more efficient delivery of static field IMRT. The integer algorithm demonstrates the efficiencies possible with an improved routine and opens a new avenue for development.

Algorithms↗
PubMed2000

On the equivalence of rotational and concentric therapy.

The aim of this work is to analyse the equivalence of two classes of radiation therapy. One class of therapy is characteristic of Gamma Knife type irradiations and is defined by pencil beam concentric irradiation converging on multiple centres throughout the patient's body. The other class of treatment is characteristic of accelerator based, beam intensity modulated type irradiation defined by a rotation of wide beams around a single centre. We focus our attention on deriving formulae that relate treatments in these two classes and characterize conditions under which they are valid.

Models, Theoretical↗
PubMed2000

150-250 meV electron beams in radiation therapy.

High-energy electron beams in the range 150-250 MeV are studied to evaluate the feasibility for radiotherapy. Monte Carlo simulation results from the PENELOPE code are presented and used to determine lateral spread and penetration of these beams. It is shown that the penumbra is comparable to photon beams at depths less than 10 cm and the practical range (Rp) of these beams is greater than 40 cm. The depth dose distribution of electron beams compares favourably with photon beams. Effects caused by nuclear reactions are evaluated, including increased dose due to neutron production and induced radioactivity resulting in an increased relative biological effectiveness (RBE) factor of < 1.03.

Algorithms↗
PubMed2000

CPP calculation of multiple scattering distributions for charged particles penetrating compounds or mixtures.

Charged particle multiple scattering distributions may be constructed from individual atomic scattering events on the basis of compound Poisson process (CPP) theory. We present a CPP method for computing multiple scattering transition probability densities from charged particles penetrating compounds and mixtures. Water as a scattering medium provides here an example of the calculation method which is applicable to compounds or mixtures. Electrons are chosen as examples of charged particle beams. The Rutherford single scattering cross section and a partial wave analysis single scattering cross section are chosen as example cross sections. Transition probability densities predicted on the basis of CPP theory can be calculated with great accuracy for the improvement of radiation dose calculations. The advantages of the CPP method are (a) an effective atomic number need not be defined for the scattering medium, (b) it can be applied in both spherical and planar coordinate systems, and (c) it does not require any specific form for the single scattering cross section.

Algorithms↗
PubMed1999

The clinical application of dynamic shielding and imaging in moving table total body irradiation.

The moving table technique for total body irradiation (MTT TBI) has some advantages in regard to dose homogeneity, patient positioning and comfort. However, divergence of the radiation field coupled with patient motion necessitates corresponding motion of the shielding blocks and verification film so that penumbra is minimized. MTT TBI system is presented, together with dose calculations, incorporating moving trays for shields and film to ensure dose delivery with minimal penumbra of the blocked field.

Algorithms↗
PubMed1997

Contamination of the pleural surfaces in childhood sarcoma. Use of colloidal P-32 to reduce radiation dose to the whole lung.

Children with pulmonary sarcomas who have diffuse contamination of the pleural cavity present a difficult management problem for the radiation oncologist. Doses required to control even microscopic disease exceed lung tolerance. We report on the use of intracavity colloid P-32 in an attempt to treat the pleural surface and spare normal lung parenchyma and tissues of the chest wall. Three children--18 months, 12 years, and 3 years of age--had spillage of pulmonary sarcomas into the chest cavity. All children were treated with systemic chemotherapy. Initially, 0.5 mCi of technetium sulfur colloid (99mTc-sulfur colloid) was instilled into the pleural space to ascertain even distribution of isotope. This was then followed by installation of 5.0 mCi of colloidal P-32. Uniform distribution was then confirmed by bremsstrahlung scanning. All three patients are in complete remission 3.5 years, 3 years, and 1 year after treatment, respectively. The major toxicity was asymptomatic pleural thickening, which could be confused with disease. This was confirmed histologically to be fibrous in the first patient. The process diminished or stabilized with time in all 3 patients over the period of observation. In this small series, intrapleural colloidal P-32 appeared to be safe and well tolerated and would be expected to be less toxic than wide-field external beam in the treatment of spilled pulmonary sarcomas.

Child↗
PubMed1995

Optimization of rotational radiotherapy treatment planning.

PURPOSE: Rotational therapy treatment planning for rotationally symmetric geometry of tumor and healthy tissue provides an important example of testing various approaches to optimizing dose distributions for therapeutic x-ray irradiations. In this article, dose distribution optimization is formulated as a variational problem. This problem is solved analytically and numerically. METHODS AND MATERIALS: The classical Lagrange method is used to derive equations and inequalities that give necessary conditions for minimizing the mean-square deviation between the ideal dose distribution and the achievable dose distribution. The solution of the resulting integral equation with Cauchy kernel is used to derive analytical formulas for the minimizing irradiation intensity function. RESULTS: The solutions are evaluated numerically and the graphs of the minimizing intensity functions and the corresponding dose distributions are presented. CONCLUSIONS: The optimal solutions obtained using the mean-square criterion lead to significant underdosage in some areas of the tumor volume. Possible solutions to this shortcoming are investigated and medically more appropriate criteria for optimization are proposed for future investigations.

Humans↗
PubMed1994

Radiance and particle fluence.

The concepts of radiance and fluence are fundamental to the description of a radiation field. The International Commission on Radiological Units and Measurements (ICRU) has defined fluence in terms of the number of the radiation particles crossing a small sampling sphere. A second definition has been proposed in which the length of track segments contained within any sampling volume are used to calculate the incident fluence. This approach is often used in Monte Carlo simulations of individual particle tracks, allowing the fluence to be scored in small volumes of any shape. In this paper we stress that the second definition generalizes the classical (ICRU) concept of fluence. We also identify the assumptions inherent in the two definitions of fluence and prove their equivalence for the case of straight-line particle trajectories.

Algorithms↗
PubMed1994

Inclusion of energy straggling in a numerical method for electron dose calculation.

Energy straggling along electron trajectories has been incorporated into a numerical algorithm for electron beam dose calculations. Landau's theory is used to predict, at any point in the absorber, the broadening of the primary electron energy spectrum due to energy loss straggling. Numerical calculations have been performed for electron beams with energies of 10-30 MeV incident upon water in order to determine the variation of dose with depth and variation of energy spectra with pathlength. These calculations are compared with the results of Monte Carlo simulations performed with the EGS4 code. The inclusion of energy loss straggling in the numerical calculations leads to predictions of energy spectra and dose deposition that are in good agreement with modified Monte Carlo simulations in which bremsstrahlung is ignored and the energy given to knock-on electrons is deposited at the site of their creation. Less satisfactory agreement was achieved when these calculations were compared to full Monte Carlo simulations that included the bremsstrahlung events and transported the knock-on electrons. It is concluded that bremsstrahlung energy loss must also be included into this algorithm, if an acceptable dose computation accuracy is to be achieved for clinical applications.

Computer Simulation↗
PubMed1992

A numerical method for electron transport calculations.

A numerical algorithm for calculating the penetration of electrons in dense media is presented. The numerical algorithm is intended for future application to radiotherapy dose calculations. The method is generic in the sense that it may be used with different theoretical models describing the angular scattering of electrons with depth. It is also general enough that it may be applied to electron dose calculations in heterogeneous as well as homogeneous media. The assumptions used in the algorithm are examined and equations describing the evolution of the distribution of electrons with depth are presented. Calculations have been performed for 10 MeV broad beams and pencil beams incident on water. It is shown that the Fermi-Eyges analytical solutions are recovered if the angular scattering process is assumed to be a Gaussian Markov process and the cumulative angle of electron travel remains small. In the case where the small angle approximation is not imposed, the numerical method qualitatively reproduces, at large depths, the wide angle scattering 'tails' seen in Monte Carlo generated profiles.

Algorithms↗
PubMedDate not supplied

High-dose-rate brachytherapy for vaginal cancer: learning from treatment complications.

Historically, early stage vaginal cancer has been treated with low-dose-rate (LDR) brachytherapy with or without external beam radiation therapy (EBRT). Complication rates have been low and treatment efficacious. Although high-dose-rate (HDR) brachytherapy has been used for cervical cancer in many countries for over a decade, only more recently has it been integrated into treatment plans for vaginal cancer. This paper describes three patients treated with HDR brachytherapy who experienced significant late effects. Given the very limited amount of literature regarding the use of HDR brachytherapy in vaginal cancer, this analysis potentially contributes to an understanding of treatment-related risk factors for complications among patients treated with this modality.A focused review of hospital and departmental treatment records was done on three patients treated with HDR brachytherapy. Abstracted information included clinical data, treatment parameters (technique, doses, volume, combinations with other treatments) and outcomes (local control, survival, early and late effects). A review of the available literature was also undertaken. All patients had significant complications. Although statistical correlations between treatment parameters and complications are impossible given the limited number of patients, this descriptive analysis suggests that vaginal length treated with HDR brachytherapy is a risk factor for early and late effects, that the distal vagina has a lower radiation tolerance than the upper vagina with HDR as in LDR, and that combining HDR with LDR as done in our experience carries a high risk of late toxicity. Integration of HDR brachytherapy techniques into treatment plans for early stage vaginal cancers must be done cautiously. The etiology of the significant side effects seen here is likely to be multifactorial. For users of HDR brachytherapy in vaginal cancer, there is a need to further refine and standardize treatment concepts and treatment delivery. Ideally this will be based on continued careful observation and reporting of both favorable and unfavorable outcomes and experiences.

Adult↗
PubMedDate not supplied

A restricted angular scattering model for electron penetration in dense media.

A restricted angular scattering model for electron penetration in dense media is presented. In the model, the Fermi-Eyges transport equation is modified through the addition of an extra term which may be interpreted as representing an apparent force opposing the scattering of electrons into wider angles. The introduction of this extra term allows the modeling of the measured saturation in the mean square angular spread of electrons with depth. The restricted scattering model retains the Gaussian features of the Fermi-Eyges model and, therefore, may be readily incorporated into existing dose computation algorithms. Good agreement is obtained with measured angular electron distribution data for a point monodirectional beam over a wide range of incident electron energies (5-20 MeV) and scattering media (atomic numbers of 6 to 82). Also, a comparison of the restricted scattering model predictions with measurements of the lateral pencil beam spread shows an improvement over the predictions of Fermi-Eyges model close to the end of the electron range. Broad beam profiles were generated using both the Fermi-Eyges and restricted scattering models. A comparison of predicted and measured beam profiles shows that the restricted scattering model is a significant improvement over the Fermi-Eyges model for the prediction of beam penumbra shape in homogeneous media.

Aluminum↗