Assessment of different IMRT boost delivery methods on target coverage and normal-tissue sparing.

Purpose: Because of biologic, medical, and sometimes logistic reasons, patients may be treated with 3D conformal therapy or intensity-modulated radiation therapy (IMRT) for the initial treatment volume (PTV(1)) followed by a sequential IMRT boost dose delivered to the boost volume (PTV(2)). In some patients, both PTV(1) and PTV(2) may be simultaneously treated by IMRT (simultaneous integrated boost technique). The purpose of this work was to assess the sequential and simultaneous integrated boost IMRT delivery techniques on target coverage and normal-tissue sparing.

Effectiveness of compensating filters in the presence of tissue inhomogeneities.

CT based 3D treatment planning systems (3DTPS) can be used to design compensating filters that, in addition to missing tissue compensation, can account for tissue inhomogeneities. The use of computer-driven systems provides a practical, convenient, and accurate method of fabricating compensating filters. In this work, we have evaluated a commercially available PAR Scientific DIGIMILL milling machine linked with FOCUS 3DTPS.

Automatic feathering of split fields for step-and-shoot intensity modulated radiation therapy.

Due to leaf travel range limitations of the Varian Dynamic Multileaf Collimator (DMLC) system, an IMRT field width exceeding 14.5 cm is split into two or more adjacent abutting sub-fields. The abutting sub-fields are then delivered as separate treatment fields.

Comparison of ionization chambers of various volumes for IMRT absolute dose verification.

IMRT plans are usually verified by phantom measurements: dose distributions are measured using film and the absolute dose using an ionization chamber. The measured and calculated doses are compared and planned MUs are modified if necessary. To achieve a conformal dose distribution, IMRT fields are composed of small subfields, or "beamlets.

Role of IMRT in reducing penile doses in dose escalation for prostate cancer.

Purpose: In three-dimensional conformal radiotherapy (3D-CRT), penile tissues adjacent to the prostate are exposed to significant doses of radiation. This is likely to be a factor in development of posttreatment erectile dysfunction. In this study, we investigate whether intensity-modulated radiation therapy (IMRT) leads to lower radiation exposure to proximal penile tissues (PPT) when compared with 3D-CRT.

Comparative evaluation of Kodak EDR2 and XV2 films for verification of intensity modulated radiation therapy.

Film dosimetry provides a convenient tool to determine dose distributions, especially for verification of IMRT plans. However, the film response to radiation shows a significant dependence on depth, energy and field size that compromise the accuracy of measurements. Kodak's XV2 film has a low saturation dose (approximately 100 cGy) and, consequently, a relatively short region of linear dose-response.

An immobilization and localization technique for SRT and IMRT of intracranial tumors.

A noninvasive localization and immobilization technique that facilitates planning and accurate delivery of both intensity modulated radiotherapy (IMRT) and linac based stereotactic radiotherapy (SRT) of intracranial tumors has been developed and clinically tested. Immobilization of a patient was based on a commercially available Gill-Thomas-Cossman (GTC) relocatable frame. A stereotactic localization frame (LF) with the attached NOMOS localization device (CT pointer) was used for CT scanning of patients.

Improvement of treatment plans developed with intensity-modulated radiation therapy for concave-shaped head and neck tumors.

Purpose: To improve dose conformity and normal tissue sparing in patients with concave-shaped head and neck cancers by using tomotherapy and static step-and-shoot intensity-modulated radiation therapy (IMRT) and by comparing results with those of three-dimensional (3D) conformal radiation therapy (CRT) and two-dimensional (2D) radiation therapy.

Materials And Methods: Treatment planning in 10 patients with concave-shaped head and neck tumors was performed by using tomotherapy and step-and-shoot IMRT, 3D CRT, and 2D techniques. IMRT plans were modified by placing "virtual critical structures" in regions outside the target where hot spots occurred.

Improvement of dose distributions in abutment regions of intensity modulated radiation therapy and electron fields.

In recent years, intensity modulated radiation therapy (IMRT) is used to radiate tumors that are in close proximity to vital organs. Targets consisting of a deep-seated region followed by a superficial one may be treated with abutting photon and electron fields. However, no systematic study regarding matching of IMRT and electron beams was reported.

Matching tomographic IMRT fields with static photon fields.

The matching of abutting radiation fields presents a challenging problem in radiation therapy. Due to sharp penumbra of linear accelerator beams, small (1-2 mm) errors in field positioning can lead to large (>30%) hot or cold spots in the abutment region. With head and neck immobilization devices (thermoplastic mask/aquaplast) an average setup error of 3 mm has been reported.

Elimination of field size dependence of enhanced dynamic wedge factors.

Enhanced dynamic wedge factors (EDWF) are characterized by a strong field size dependence. In contrast to physical wedge factors, the EDWF decrease as the field size is increased: for 6 MV 60 degrees wedge, the EDWF decreases by 50% when the field size is increased from 4 x 4 cm2 to 20 x 20 cm2. A method that eliminates the field size dependence of EDWF was developed and investigated in this work.

A modified technique for RF-LCF interstitial hyperthermia.

To provide uniform heating of a tumour, it is necessary to establish sufficient volumetric control of power deposition. The interstitial Radio-Frequency Localized Current Field (RF-LCF) technique may provide such control when segmented electrodes are used. The length of segments is equal to 1-1.

Mix-mode technique of feeding arrays of dipole microwave antennae.

A 4-element array of coherently driven dipole microwave antennae produces a hot spot in the central region of the implanted volume and cold spots in the peripheral regions of the implant. Conversely, an incoherently driven array of antennae predominantly heats peripheral (along the antennae) regions of the implant. These two modes of feeding the antennae are complementary in a sense that the cold zones obtained with the coherently driven antennae coincide with the hot zones obtained with the incoherently driven antennae and vice versa.

Improvement of tomographic intensity modulated radiotherapy dose distributions using periodic shifting of arc abutment regions.

Based on the study of treatment arc positioning versus target length, a method that allowed periodic shift of arc abutment regions through the course of intensity modulated radiotherapy (IMRT) was developed. In this method, two treatment plans were developed for the same tumor. The first plan contained the original target (Planning Target Volume as defined by radiation oncologist) and the second one contained a modified target.

A modified method of planning and delivery for dynamic multileaf collimator intensity-modulated radiation therapy.

Purpose: To develop a modified planning and delivery technique that reduces dose nonuniformity for tomographic delivery of intensity-modulated radiation therapy (IMRT).

Methods And Materials: The NOMOS-CORVUS system delivers IMRT in a tomographic paradigm. This type of delivery is prone to create multiple dose nonuniformity regions at the arc abutment regions.

Dosimetry of very-small (5-10 mm) and small (12.5-40 mm) diameter cones and dose verification for radiosurgery with 6-MV X-ray beams.

The dosimetry and dose verification for 6-MV X-rays were performed for radiosurgery cones of 5- to 40-mm diameter. The total scatter factors decrease slowly from 0.936 (40-mm cone) to 0.

Output factors for irregularly shaped electron fields.

It is necessary to know the output factors (dose per monitor unit at depth of maximum) for irregularly shaped electron beam fields to accurately deliver the prescribed dose to the target. Measuring the output factors for individually shaped electron beam fields for each patient is inconvenient. Using the measured output factors for two square fields, one can obtain the output factor for an irregular shaped electron portal with area intermediate between the areas of the two square fields, by obtaining the equivalent square area (as with photons) of the irregularly shaped field, and then interpolating between the output factors of the two square field areas to obtain the output factor for the irregularly shaped field.

Theoretical limits of SAR distributions of a four-element square array of dipole-type antennas.

Four-element dipole microwave antenna arrays with square insertion patterns are commonly used clinically for interstitial hyperthermia. One major disadvantage with this type of antenna array is the presence of a large dead length at the tips because the current gradually decreases from maximum at the junctions to zero at the tips. This dead length is usually 1.

Phantom evaluation of heating of superficial tissues located above interstitial microwave antennas.

Purpose: A technique that improves heating of superficial tissues above an implant of microwave interstitial antennas is presented.

Methods And Materials: Adequate heating of tumor margins is achieved by extending an implant of microwave antennas beyond the tumor boundary by 1-2 cm. When the tumor infiltrates the superficial tissues including the skin, the implant cannot even reach the superficial margin of the tumor since it requires tissue to support the catheters.

Evaluation of microwave interstitial antennas in the phantom with varying cross-section.

Dipole-regular microwave interstitial antennas are characterized with a "dead" space located along the tip segment of the antenna. The length of the "dead" space is on the order of 2 cm or larger, depending on the antenna's insertion depth. If the insertion depth is smaller than 4 cm, then coupling of the antennas to tissue becomes a problem.

Equilibrium temperature distributions in uniform phantoms for superficial microwave applicators: implications for temperature-based standards of applicator adequacy.

Equilibrium temperature distributions are computed using measured SAR distributions for five different superficial microwave (915 MHz) applicators. We assume a model with uniform conduction and blood flow. A Green's function approach is used to calculate equilibrium solutions which identically obey boundary conditions at the surface of the phantom and at infinite depth.

Combined hyperthermia and irradiation in the treatment of superficial tumors: results of a prospective randomized trial of hyperthermia fractionation (1/wk vs 2/wk).

From December 1984 to December 1989, 240 superficially located recurrent/metastatic malignant lesions (173 patients) were enrolled in a prospective randomized study of one versus two hyperthermia fractions per week. In the majority of patients, the dose of radiation therapy was less than 4000 cGy over 4 to 5 weeks. Stratification was by tumor size, site, and histology.

Phantom studies and preliminary clinical experience with the BSD 2000.

The BSD 2000 system is an array of microwave antennas operating in the 60-120 MHz range. It is a four-quadrant regional hyperthermia device with phase control permitting the operator to choose the point of constructive interference. A computer preplanning system is provided.

Evaluation of the Sigma 60 applicator for regional hyperthermia in terms of scattering parameters.

Scattering parameters adequately describe the interference between ports of a multiportal electromagnetic device when the device dimensions are comparable with the wavelength of the electromagnetic waves within the device. Since the Sigma 60 applicator is a four-port electromagnetic device, the interference between ports (quadrants) is described by a 4 x 4 scattering matrix. The load and frequency dependence of the scattering parameters were studied.

Dual-antenna applicator for hyperthermia of tumours at intermediate depth.

A dual-antenna applicator with 21 x 26 cm2 aperture, that is fully loaded and operates at 74 MHz, was developed at the Mallinckrodt Institute of Radiology. By placing two antennas into an applicator capable of propagating TE10 mode, a significant enlargement of heating pattern was achieved without an increase in applicator dimensions. When antennas are placed symmetrically about a parallel to the antenna axis of symmetry, the sensitivity of the applicator input impedance to variations of load impedance reduces.