Biomedical subjects

P L Petti

Publications and source records attributed to P L Petti.

33 records · Page 2Linked to original sources

PubMed1988

Description of the scatter component in photon-beam data.

Kerma scatter-to-primary ratios (SPR) were calculated, using a Monte Carlo code, for water irradiated by photon beams (200 keV monoenergetic, 60Co gamma, 4 and 15 MV x-rays). The SPR at depth d on the centreline of a circular field of radius r can be described by SPR = aZ + bZ2, with Z = rd/(r + d). The proportionality constant a is determined by the linear attenuation coefficient mu and the ratio r/d. The coefficient b for the second-order term depends on r/d and energy, the latter by being proportional to mu3. This relation between SPR and Z can facilitate analysis of beam data in terms of the contributions from primary and scattered photons, in particular for small depths and small fields where measurements are difficult. This is illustrated by examining measured TAR data for 60Co gamma beams. While the relation between SPR and Z is not fully explained, it is demonstrated qualitatively that SPR approximately equal to aZ if only first-order scatter is to be accounted for.

Humans↗

PubMed1986

A multiplicative correction factor for tissue heterogeneities.

A new multiplicative correction factor for tissue heterogeneities based on an exact expression for homogeneous non-unit-density media is proposed. O'Connor's density scaling theorem is invoked to evaluate the medium-specific tissue-maximum ratios used in the exact formulation. Monte Carlo calculations were performed at energies ranging from 60Co to 15 MV to test the technique for single- and multiple-slab geometries as well as for the clinical benchmark problem described recently by Orton et al consisting of a six-field lung plan. Within the level of statistical uncertainty (less than 2%) in the Monte Carlo calculations, the new correction factor agrees with the calculated data for single slabs of non-unit-density material. For the multiply layered geometries, the correction factor accurately represents the data beyond the build-up region at any interface, but is less reliable close to a boundary. For high energies, however, it is more accurate in the build-up region than other commonly used correction techniques such as the ratio-of-TMR or Batho methods. For all energies considered, the new correction factor agrees with the data measured by Orton et al to within 1.5%. It is more accurate than other correction factors considered by Orton et al at high energies, and is competitive with the Batho and equivalent TAR methods at low energies.

Cobalt Radioisotopes↗

PubMed1986

A measurement of w for 150 MeV protons in nitrogen and argon.

Ionisation chamber dosimetry for proton radiation therapy requires accurate measurements of w, the average energy necessary to produce an ion pair for protons in the therapeutic energy range of 30-240 MeV. In this investigation, such measurements were made at the Harvard Cyclotron Laboratory for 150 MeV protons in nitrogen and argon with an accuracy of 2%. Ionisation and energy loss were measured simultaneously in a gas cell 1 m in length. A parallel plate ionisation chamber situated inside the gas cell was used to measure ionisation, and the energy loss was determined from time-of-flight and range measurements. The data indicate that w is 26.5 +/- 0.6 eV/(ion pair) for argon and 36.3 +/- 0.8 eV/(ion pair) for nitrogen. These results are consistent with measurements at lower energies.

Argon↗

PubMed1985

Effective wedge angles with a universal wedge.

Some recently designed x-ray-producing accelerators are equipped with a single built-in wedge, and different 'effective' wedge angles are obtained by combining an open (unwedged) and a wedged field in the appropriate proportions. This paper describes a technique for determining these proportions from measured isodose distributions of the two component fields. Our data for the Philips SL/75 6 MV accelerator are compared with two existing theoretical models. One model, in which the beams are weighted by the ratio of the tangents of the effective and nominal wedge angles, agrees with the data to within 3 degrees over the range of effective wedge angles and square field sizes examined. The second and simpler model, in which the beams are weighted by the ratio of the wedge angles directly, results in errors of as much as 11 degrees. It is shown that both of these models are approximations to an exact theoretical solution which may be formulated in terms of one free parameter. This parameter may be interpreted physically as the ratio of the slopes of the central-axis depth-dose curves for the open and wedged fields.

Humans↗

PubMedDate not supplied

Investigation of buildup dose from electron contamination of clinical photon beams.

The contribution made by contaminating electrons present in a clinical photon beam to the buildup dose in a polystyrene phantom has been calculated and compared to measurements. A Monte Carlo technique was employed. The calculation was divided into two parts. First, the accelerator treatment head was simulated in detail using the EGS-PEGS electromagnetic shower code. Then, information obtained from these calculations was used to compute dose curves in a polystyrene phantom. Two cases were considered, one in which both electrons and photons were incident upon the phantom, and another in which electrons were eliminated from the incident beam. Results of these calculations agree with recent experimental findings. A decrease in buildup dose as well as a shift in dmax was observed when electrons were eliminated from the beam.

Electrons↗

PubMedDate not supplied

Sources of electron contamination for the Clinac-35 25-MV photon beam.

A detailed Monte Carlo approach has been employed to investigate the sources of electron contamination for the 25-MV photon beam generated by Varian's Clinac-35. Three sources of contamination were examined: (a) the flattening filter and beam monitor chamber, (b) the fixed primary collimators downstream from the monitor chamber and the adjustable photon jaws, and (c) the air volume separating the treatment head from the observation point. Five source-to-surface distances (SSDs) were considered for a single field size, 28 cm in diameter at 80 cm SSD. It was found that for small SSDs (80-100 cm), the dominant sources of electron contamination were the flattening filter and the beam monitor chamber which accounted for 70% of the unwanted electrons. Thirteen percent of the remaining electrons originated in the downstream primary collimators and the photon jaws, and 17% were produced in air. At larger SSDs, the fraction of unwanted electrons originating in air increased. At 400 cm SSD, 61% of the contaminating electrons present in the beam were produced in air, 34% originated in the flattening filter and beam monitor chamber, and 5% were due to interactions in the fixed collimators downstream from the monitor chamber and the adjustable photon jaws. These calculated results are substantiated by recent experiments.

Electrons↗

PubMedDate not supplied

Monte Carlo calculations of scatter dose for small field sizes in a 60Co beam.

Photon dose calculations are frequently based on the separation of primary and scatter dose components. For small field sizes it is difficult to make this separation from measured data. Such problems lend themselves to solution by calculational methods, which permit rigorous definition of primary and scatter dose. Using Monte Carlo methods, we verify an equation derived from O'Connor's scaling theorem relating scatter-primary ratios at small depths and field sizes to those at larger depths and fields. This equation may prove to be useful when separating primary and scatter dose components, and consequently in dose calculations involving irregular fields or inhomogeneities.

Cobalt Radioisotopes↗

PubMedDate not supplied

Heterogeneity model for photon beams incorporating electron transport.

A method of calculating photon doses in heterogeneous media incorporating electron transport is studied. The dose is represented as the convolution of kerma with an exponential longitudinal electron spread function which describes the penetration of electrons from one medium to another. At large distances from an interface, the dose approaches an asymptotic value equal to the kerma multiplied by a constant describing the degree of longitudinal and lateral electron equilibrium. For the simple situations studied, this asymptotic dose is adequately described by O'Connor's scaling theorem. The method is compared with both Monte Carlo calculations and measurements for a 15-MV photon beam for various geometries and field sizes. It predicts the dose in regions of electron disequilibrium to within 2% in most cases. In situations of extreme electron disequilibrium, such as within low-density regions at high energies and small field sizes, this represents a significant improvement over many existing techniques.

Electrons↗

PubMedDate not supplied

The effect of patient motion on dose uncertainty in charged particle irradiation for lesions encircling the brain stem or spinal cord.

A specialized charged-particle radiotherapy technique developed at Lawrence Berkeley Laboratory (LBL) is applied to patients with lesions abutting or surrounding the spinal cord or brain stem. This technique divides the target into two parts, one partially surrounding the critical structure (brain stem or spinal cord) and a second excluding the critical structure and abutting the first portion of the target. Compensators are used to conform the dose distribution to the distal surface of the target. This technique represents a novel approach in treating unresectable or residual tumors surrounding the spinal cord or brain stem. Since the placement of the patient with respect to beam-shaping devices is critical for divided-target treatments, a method for calculating dose distributions reflecting random patient motion is proposed, and the effects of random patient motion are studied for two divided-target patient examples. Dose-volume histograms and a normal-tissue complication probability model are used in this analysis. For the patients considered in this study, the normal-tissue-complication probability model predicts that random patient motion less than or equal to 0.2 cm is tolerable in terms of spinal cord complications.

Central Nervous System Neoplasms↗

PubMedDate not supplied

Sensitivity of helium beam-modulator design to uncertainties in biological data.

The goal in designing beam-modulating devices for heavy charged-particle therapy is to achieve uniform biological effects across the spread-peak region of the beam. To accomplish this, the linear-quadratic model for cell survival has been used to describe the biological response of the target cells to charged-particle radiation. In this paper, the sensitivity of the beam-modulator design in the high-dose region to the values of the linear-quadratic variables alpha and beta has been investigated for a 215-MeV/u helium beam, and implications for higher LET beams are discussed. The major conclusions of this work are that, for helium over the LET range of 2 to 16 keV/mu, uncertainties in measuring alpha and beta for a given cell type which are of the order of 20% or less have a negligible effect on the beam-modulator design (i.e., on the slope of the spread Bragg peak); uncertainties less than or equal to 10% in the dose-averaged LET at each depth are unimportant; and, if the linear-quadratic variables for the tumor differ from those used in the beam-modulator design by a constant factor between about 0.5 and 3, then the resultant nonuniformity in the photon-equivalent dose delivered to the tumor is within +/- 25%. It is also shown that for any ion, if the nominal values of alpha or beta used by the beam-modulator design program differ from their actual values by a constant factor, then the maximum errors possible in the beam-modulator design may be characterized by two limiting depth-dose curves such that the ratio of the dose at the proximal end of the spread Bragg curve to the dose at the distal end of the spread peak is given by alpha distal/alpha prox for the steepest curve, and square root of beta distal/beta prox for the flattest curve.

Cell Survival↗

PubMedDate not supplied

Design of beam-modulating devices for charged-particle therapy.

The computer modeling program used to design beam-modulating devices for charged-particle therapy at Lawrence Berkeley Laboratory has been improved to allow a more realistic description of the beam. The original program used a single calculated Bragg peak to design the spread Bragg peak. The range of this curve was shifted so that Bragg curves of varying ranges could be superimposed. The new version of the program allows several measured Bragg curves with different ranges to be used as input, and interpolates between them to obtain the required data for the superposition calculation. The experimental configuration for measuring these input curves simulated therapy conditions. Seven beam-modulating propellers with spread Bragg-peak widths ranging from 2.2 to 14.4 cm were designed and constructed for a 215-MeV/u helium beam using this new design program. Depth-dose distributions produced by these new propellers were in good agreement with predicted distributions, and these propellers are currently being used clinically.

Equipment Design↗

PubMedDate not supplied

Differential-pencil-beam dose calculations for charged particles.

The use of a convolution or differential-pencil-beam (DPB) algorithm has been studied for charged-particle dose calculations as a means of more accurately modeling the effects of multiple scattering. Such effects are not reflected in current charged-particle dose calculations since these calculations rely on depth-dose data measured in homogeneous water-equivalent phantoms and use ray-tracing techniques to calculate the water-equivalent pathlength from patient CT data. In this study, isodose plots were generated from three-dimensional dose calculations using Monte Carlo, DPB, and standard ray-tracing methods for a 4-cm modulated 150-MeV proton beam incident on both homogenous and heterogeneous phantoms. To simulate therapy conditions with charged particles, these studies included cases where compensating boluses were introduced to modify the particle range across the treatment field. Results indicate that multiple-scattering effects, including increased penumbral width as a function of beam penetration and the "smearing" of isodose distributions downstream from complex heterogeneities, are well modeled by the DPB algorithm. The DPB algoirthm may also be used to obtain more useful estimates of the dose uncertainty in regions near the end of the beam's range downstream from complex heterogeneities than can be derived from standard ray-tracing calculations.

Algorithms↗

PubMedDate not supplied

Evaluation of fixed- versus variable-modulation treatment modes for charged-particle irradiation of the gastrointestinal tract.

The clinical usefulness of variable-modulation dose delivery of neon ion and proton beams over fixed-modulation beams is evaluated for several patients with tumors in the gastrointestinal tract by comparing dose distributions, dose volume histograms, and predictions of normal tissue complication probabilities calculated with the two methods. Both techniques provide excellent coverage of the target volume with neon ion and proton beams. The advantage of variable modulation is that less dose is delivered proximal to the target volume. For tumors in the gastrointestinal tract, this implies that less dose is given to the liver, gut, kidneys, and lungs. For the ten patients considered in this study, variable-modulation reduced the total integral dose by an average of 17% for neon ion beams and by 18% for protons as compared to fixed-modulation. If the tumor volume is excluded, the reduction in the integral dose to normal tissues ranged from 15% to 32% for neon ions and from 18% to 34% for proton beams. These gains are larger than those anticipated on the basis of an analytic study by Goitein and Chen [Med. Phys. 10, 831-840 (1983)], which predicted integral dose reductions of the order of 10% for protons and 14% for neon ions. They are also larger than those reported in a similar study by Urie and Goitein [Med. Phys. 16, 593-601 (1989)] for proton irradiation of skull-base tumors. This is probably because the tumors in the GI tract considered in this study were more irregularly shaped than Goitein and Chen's analytic model assumes. The results of this study also suggest that due to increased sparing of normal tissues, the number of different portal directions required to achieve a satisfactory treatment plan will be reduced for variable-modulation beam delivery systems. This implies that variable-modulation treatment plans will be easier to execute than current fixed-modulation plans.

Adenocarcinoma↗

PubMedDate not supplied

Factors influencing survival after gamma knife radiosurgery for patients with single and multiple brain metastases.

PURPOSE: Radiosurgery has been reported to yield high local control rates for brain metastases. However, further work is needed to define which subgroups of patients may benefit from this treatment modality. PATIENTS AND METHODS: We reviewed 116 patients who underwent stereotactic radiosurgery for initial management or recurrence of solitary or multiple brain metastases from September 1991 through December 1994 at the University of California, San Francisco. Survival time and time to local-regional failure were calculated using the Kaplan-Meier method. Univariate and multivariate analyses were performed using the Cox proportional hazards model. RESULTS: Median survival was 40 weeks from radiosurgery. In multivariate analysis, smaller total tumor volume, absence of extracranial metastases, higher Karnofsky score, and age < or = 70 had a positive effect on survival. In patients initially managed for brain metastases, the addition of whole brain radiotherapy to radiosurgery had no significant effect on survival. Although the presence of multiple metastases was associated with a significantly worse survival rate in patients initially managed with radiosurgery in univariate analysis, it was not as a significant factor in multivariate analysis. An analysis of patients within this series treated with radiosurgery who would have been eligible for Patchell's study on the role of surgery in the treatment of solitary brain metastasis revealed a favorable median survival of 70 weeks. CONCLUSIONS: We conclude that radiosurgical treatment of brain metastases results in survival times that compare favorably with the historic experience in patients treated with whole brain radiotherapy alone or with surgical resection. In patients presenting initially with brain metastases, radiosurgery alone may yield survival results equivalent to radiosurgery with whole brain radiotherapy, but intracranial control and quality of life also need to be evaluated. Also, the presence of multiple brain metastases should not be a contraindication for the use of radiosurgery given the good survival achieved with such patients in this series. Each such case should therefore be evaluated based on other factors such as patient's age, Karnofsky score and systemic disease.

Adolescent↗

PubMedDate not supplied

Gamma knife radiosurgery for malignant melanoma brain metastases.

PURPOSE: To evaluate the efficacy and toxicity of gamma knife radiosurgery in the treatment of melanoma metastases to the brain. PATIENTS AND METHODS: We retrospectively reviewed 55 patients with single or multiple intracranial melanoma metastases treated at the University of California, San Francisco, with gamma knife radiosurgery from 1991 through 1995. Sixteen patients were treated with gamma knife radiosurgery for recurrence following previous radiation therapy, 11 received radiosurgery as a boost to whole-brain radiation therapy, and 28 had radiosurgery alone for initial management of brain metastases. The median minimum radiosurgery tumor dose for 140 treated lesions was 19 Gy (range, 10-22 Gy) prescribed at the 35% to 90% isodose contour (median, 50%). The median total target volume per patient was 6.1 cc (range, 0.25-28.3 cc). RESULTS: With a median follow-up of 75 weeks in living patients, the median survival times were 35 weeks overall: 35 weeks for patients with solitary metastases versus 33 weeks for those with multiple metastases. A factor that was significant in univariate analysis of survival was total target volume treated. This parameter remained significant on multivariate analysis. The actuarial median freedom from progression analyzed by lesion for 113 lesions in 46 patients with imaging follow-up was 89 weeks with 6-month and 1-year actuarial freedom from progression rates of 89% (95% confidence interval, 80%-95%) and 77% (95% confidence interval, 62%-87%). In univariate analysis, improved freedom from progression was associated with smaller target volume treated, smaller maximum diameter, or higher prescribed dose. Four patients (7%) developed acute Radiation Therapy Oncology Group grade > or = 2 morbidity, and five patients (9%) developed late grade > or = 2 morbidity. DISCUSSION: Median survival and freedom from progression in patients treated with radiosurgery for melanoma metastatic to the brain are comparable to results in published radiosurgery series of grouped histologies. For melanoma patients, total intracranial tumor volume appears to be of greater prognostic significance than the absolute number of metastases treated. We conclude that gamma knife radiosurgery is effective and should be considered among various management strategies.

Adolescent↗