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.

A monitor unit verification calculation in intensity modulated radiotherapy as a dosimetry quality assurance.

In standard teletherapy, a treatment plan is generated with the aid of a treatment planning system, but it is common to perform an independent monitor unit verification calculation (MUVC). In exact analogy, we propose and demonstrate that a simple and accurate MUVC in intensity modulated radiotherapy (IMRT) is possible. We introduce the concept of modified Clarkson integration (MCI).

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).

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.

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.

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.

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.

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.

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.

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.

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.

Experimental determination of fluence perturbation factors for five parallel-plate ionization chambers.

The calibration of parallel-plate chambers for absolute dosimetry is an unsettled matter. The medical physics community has not yet agreed on a practical method of obtaining Ngas, although several researchers are working on this problem. If the photon and electron fluence perturbation factors, KwallKcomp, were known for chambers of standard construction with full buildup provision, then an in-air Co-60 calibration could be applied to these, as is done with cylindrical chambers.

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.

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.

Measurement of the replacement correction factor for parallel-plate chambers in electron fields.

When parallel-plate chambers are used for dosimetry in electron fields, the AAPM dosimetry protocol recommends a value of 1.0 for the replacement correction factor, P(repl),pp,E, until further data become available. Here, P(repl),pp,E for five commercially available parallel-plate chambers was measured as a function of electron energy from a nominal value of 5.

Dosimetry of Sr-90 ophthalmic applicators.

Sr-90 ophthalmic applicators are commonly used for the treatment of superficial eye disorders. Although a variety of dosimetric devices such as film, thermoluminescent dosimeters (TLD's), ion chambers, and radiochromic foils have been used to measure the peak dose at the applicator surface, there is no internationally agreed upon calibration procedure. Recently, large discrepancies among calibrations of the same applicator at three institutions have been reported.

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.

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.

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.

Radiotherapy of the resected mandible following stainless steel plate fixation.

There is general concern among otolaryngologists that irradiation of a stainless steel prosthesis used in mandibular reconstruction may cause irradiation overdosage to adjacent tissues. A tissue-equivalent plastic/steel model, simulating the characteristics of a stainless steel, reconstructed mandible, was irradiated and measurements were made with a parallel plate ionization chamber. The results of our measurements show that irradiation of an implanted steel plate results in an overdosage (120%) "in front" and an underdosage (80%) "behind" the steel plate.

Characteristic parameters of 6-22 MeV electron beams from a 25-MeV linear accelerator.

Depth-ionization measurements were performed using a thin wall parallel plate chamber in water at nominal electron energies of 6, 9, 12, 15, 18, and 22 MeV for the standard available square field sizes. The characteristic parameters of the central axis depth-dose distributions were derived and compared to corresponding values for other accelerators. Vacuum packed therapy-verification films were used in water to obtain isodose distributions in a plane containing the central axis of the beam.

Determination of the source position for the electron beams from a high-energy linear accelerator.

We have investigated the energy and field-size dependence of the source position of the electron beams from a Varian Clinac-2,500 accelerator. Three independent experimental methods were used: (1) multipinhole camera (MPC), (2) back projection of the full width at half maximum (FWHM), and (3) the inverse square law (ISL). The positions of the virtual and effective sources were calculated using the multiple Coulomb scattering (MCS) formalism.

Correlation of microdosimetric measurements with relative biological effectiveness from clinical experience for two neutron therapy beams.

Microdosimetric measurements were made for the neutron therapy beams at the University of Chicago and at the Cleveland Clinic with the same geometry and phantom material using the same tissue-equivalent spherical proportional counter and standard techniques. The energy deposition spectra (dose distributions in lineal energy) are compared for these beams and for their scattered components (direct beam blocked). The model of dual radiation action (DRA) of Kellerer and Rossi is employed to interpret these data in terms of biological effectiveness over this limited range of radiation qualities.

Neutron dose equivalent next to the target shield of a neutron therapy facility using an LET counter.

The use of a spherical tissue-equivalent proportional counter for measurements of the lineal energy (y) and derivations of the linear energy transfer (LET) for fast neutrons has the advantage of giving distributions of dose and dose equivalent as functions of either LET or y. A measurement next to the target shielding of the neutron therapy facility at the University of Chicago Hospitals and Clinics (UCHC) is described, and the data processing is outlined. The distributions are presented and compared to those from measurements in the neutron beam.