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. Catheters that facilitate the placement of antennas into tumor frequently protrude beyond the tissue. This provides the opportunity of exposing part of the antenna tips (with low radiation output) beyond the tissue. Decoupling of this part of the antennas from the tissue reduced the dead space and improved microwave power transfer to the tissue. This concept was investigated using a muscle equivalent phantom consisting of five segments with thicknesses varying from 3 cm to 8 cm. The transfer of microwave power to the phantom and SAR distributions along the central axis of a rectangular array of four antennas were evaluated by measuring rates of temperature rise. The protrusion lengths that improved the array performance were found for each segment of the phantom.

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. This may yield cold spots in the tissues above the implant. Measurements in a phantom with varying thickness of the superficial layer above the implant demonstrated inadequate Specific Absorption Rates of energy distribution in this layer. A method that improves these distributions in the superficial layers was developed and tested in this work. This method requires placing a deionized water bolus on the phantom (patient) surface. Additional microwave antennas are placed on top of the bolus above and parallel to the implanted antennas. The Specific Absorption Rates distributions were evaluated for the thicknesses of superficial layer ranging from 1.5 mm to 16 mm and two bolus thicknesses (5 and 10 mm). RESULTS: The adequate Specific Absorption Rates distributions were achieved for all tested thicknesses of the superficial layer (1.5, 4, 8, 12, and 16 mm). The use of the 5 mm bolus versus 10 mm bolus is discussed. The use of additional antennas did not significantly increase stray radiation. CONCLUSION: This method has the potential to optimize heating of superficial tissues located above a microwave antenna implant.

Effects of finite angular steps and extent of profile data on the calculation of rotational x-ray dose distributions.

Computer algorithms for rotational therapy beams, in most cases, perform dose calculations by summing stored fixed beam data at finite angular steps. Such an algorithm, based on the Bentley beam model, was evaluated by comparing calculations with measured data for an 18-MV x-ray beam. Measurements were made in a specially constructed cylindrical water phantom of 15-cm radius using a 0.1-cm3 ionization chamber for an arc of 180 degrees and for a field size of 7.2 X 7.2 cm2 at 100-cm source-axis distance. This study revealed that the Bentley beam model, with fixed beams summed every 10 degrees, predicts the dose in the treatment volume, centered about the isocenter, with an accuracy of approximately 2%. However, dose at depths between the phantom surface and the treatment volume could be underestimated by as much as 10% (3% of isocenter). This was shown to be partially due to the truncated tails of the off-axis profiles in the Bentley model, which extend only 8 mm outside the edge of the radiation field, and the large angular increment of integration (10 degrees). Using beam profiles extending to 4 cm outside the edge of the radiation field and angular steps of 5 degrees or less for summation of fixed beams reduced errors to less than 5%. Therefore, extended beam profiles and smaller angular steps for summing fixed beams are recommended for photon rotation calculation when increased accuracy is required.

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 1.5-2 cm along the central axis of a 2 x 2 cm array of regular dipole antennas. Many attempts to improve the performance of dipole antenna arrays have been made by designing antennas with increased current at the tips. While some dipole antennas of new design show negligible dead lengths at close proximity in the single-antenna configuration, phantom experiments have demonstrated that these antennas exhibit at least 1.1 cm dead space along the central axis of the four-antenna array. Therefore, there seems to be a limit to which the array dead length can be reduced by the improvements in the dipole-type antenna design. The goal of this work is to find the theoretical minimum of the array dead length. This was done by assuming a uniform current distribution along the entire antenna. The specific absorption rate (SAR) patterns were calculated for an array with an insertion depth of 7 cm (resonant length for 915 MHz) and a variable spacing between antennas (1-3 cm). It was found that there is a dead length of 6 mm along the central axis of the 2 x 2-cm array with the uniform current distribution, which can be considered as the theoretical limit of the dead length for this array.(ABSTRACT TRUNCATED AT 250 WORDS)

Dosimetric evaluation in heterogeneous tissue of anterior electron beam irradiation for treatment of retinoblastoma.

A dosimetric study of anterior electron beam irradiation for treatment of retinoblastoma was performed to evaluate the influence of tissue heterogeneities on the dose distribution within the eye and the accuracy of the dose calculated by a pencil beam algorithm. Film measurements were made in a variety of polystyrene phantoms and in a removable polystyrene eye incorporated into a tissue substitute phantom constructed from a human skull. Measurements in polystyrene phantoms were used to demonstrate the algorithm's ability to predict the effect of a lens block placed in the beam, as well as the eye's irregular surface shape. The eye phantom was used to measure dose distributions within the eye in both the sagittal and transverse planes in order to test the algorithm's ability to predict the dose distribution when bony heterogeneities are present. Results show (1) that previous treatment planning conclusions based on flat, uniform phantoms for central-axis depth dose are adequate; (2) that a three-dimensional heterogeneity correction is required for accurate dose calculations; and (3) that if only a two-dimensional heterogeneity correction is used in calculating the dose, it is more accurate for the sagittal than the transverse plane.

Field matching of electron beams using plastic wedge penumbra generators.

We describe the use of polystyrene wedges to match adjacent electron beams with improved dose uniformity. These wedges were designed to increase the penumbra width at the field junction from about 1.5 to about 3.5 cm, to achieve dose uniformity. Measurements using thermoluminescent dosimeters (TLD) and therapy localization film showed that the use of polystyrene wedges (penumbra generators) produced only a small increase (less than 3%) in the surface dose and a small increase (less than 1%) in the x-ray contamination. Without wedges at the field junction, lateral mismatching of beam edges by 2 or 3 mm may introduce high dose variations (120% or more or 50% or less). Similar 2-3 mm set-up errors did not cause more than +/- 5% dose variations when plastic wedges were used to match the fields. These wedges are particularly useful when matching fields of different beam energies or matching fields on curved surfaces, such as the chest wall.

A two-dimensional pencil-beam algorithm for calculation of arc electron dose distributions.

A two-dimensional pencil-beam algorithm is presented for the calculation of arc electron dose distributions in any plane that is perpendicular to the axis of rotation. The dose distributions are calculated by modelling the arced beam as a single broad beam defined by the irradiated surface of the patient. The algorithm is two-dimensional in that the anatomical cross section of the patient and the skin collimators are assumed identical in parallel planes outside the plane of calculation. The broad beam is modelled as a collection of strip beams, each strip beam being characterised by its planar fluence, mean projected angular direction and a root-mean-square spread about the mean direction. Using these parameters, the dose distribution is calculated using pencil-beam theory. Examples of strip-beam parameters and resulting dose distributions for patient geometries are presented. Features of the algorithm, which include (1) incorporation of pencil-beam theory for the calculation of dose in heterogeneous tissue, (2) run times of only about twice that of comparable-sized fixed electron fields and (3) the input requirement of only a single depth dose and four off-axis dose profiles of measured data, make the algorithm practical for clinical use.

Dosimetric evaluation of a two-dimensional, arc electron, pencil-beam algorithm in water and PMMA.

The accuracy of dose calculations from a pencil-beam algorithm developed specifically for arc electron beam therapy was evaluated at 10 and 15 MeV. Mid-arc depth-doses were measured for 0 degrees and 90 degrees arcs using 12 and 15 cm radius cylindrical water phantoms. Calculated depth-doses for the 90 degrees arced beams in the build-up region were as much as 3% less than measured values; the maximum dose was similar in magnitude but at a greater depth; and the therapeutic depth, R80, was 2-4 mm deeper. Calculated values of output (dose per monitor unit) at the depth of the maximum calculated dose were compared with measured values; for arcs ranging from 0-90 degrees, 12 and 15 cm radius water phantoms, and collimator widths of 4, 5 and 6 cm, results showed differences as great as 7%. Isodose countours for a 90 degrees arc were also measured in a 15 cm radius PMMA phantom. At the depth of maximum dose the algorithm predicted doses in the penumbral regions, both with and without collimation, which agreed within a few per cent of measured values. The largest discrepancies were 5%, which occurred in the penumbral portion of the depth-dose fall-off region. Differences between measurement and calculation are not believed to be clinically significant and are believed to be primarily due to the fact that the algorithm models neither large-angle scattering nor the effects of range straggling on the pencil-beam dose distribution.

Dosimetry of a kilovoltage radiotherapy x-ray machine.

We present the percent depth doses, half-value thicknesses, exposure rates in air, dose uniformity, backscatter factors, isodose curves, and penumbra widths for 75-kVp, 100-kVp, 150-kVp, 200-kVp, and 250-kVp beams for rectangular cones (4 cm x 6 cm, 6 cm x 8 cm, and 8 cm x 10 cm) and exposure rates in air and backscatter factors for cylindrical cones (diameters 2 cm, 3 cm, and 3.5 cm) with flat and bevelled ends for the Philips RT-250 x-ray machine. Published dosimetry data specific to this machine are limited to percent depth doses and beam qualities. Rectangular cone percentage depth dose curves and beam quality agreed with published values. While measured backscatter factors for large rectangular fields generally agree with calculated and published values, smaller field backscatter factors were less than those reported by others. Monte Carlo calculations for small field backscatter factors appear more accurate than those measured.

A 100-cm pseudosimulation technique for 80-cm isocentric treatments.

A simulator should mimic the geometry of the treatment machine. If the geometry of the simulator does not match that of the therapy machine, true simulation of treatment could be a problem. Pseudosimulation is a simple, practical solution to this problem. We have successfully implemented this technique for over a year in our clinic for about 70 patients.

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. This empirical method offers a simple, practical solution. The accuracy of the method is about 1% to 2%, depending on the shape and size of the irregularly shaped electron field.

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 0.893 (10-mm cone; a variation of 5%), but they fall to 0.83 (7.5-mm cone) and 0.67 (5-mm cone). The dmax increases from about 12.9 (5-mm cone) to 16.3 mm (40-mm cone). The full width half maximum (FWHMs) of the beam profiles, measured at 5 cm depth, agree with the cone diameters within 1 mm. The 10-90% beam penumbra/FWHM ratio is 0.23 +/- 0.03 (> or = 20-mm cones); for the smaller-diameter cones this ratio increases reaching 0.84 (5-mm cone). New tissue maximum ratios (TMRs) are reported for the 5-, 7.5-, 32.5-, and 37.5-mm-diameter cones. TMRs for the other diameter cones are consistent with published data. The measured doses in two verification studies using the 12 cones with diameters > or = 12.5 mm with a single 360 degrees arc agreed to 2% with the planned doses, and to about 10% for the three smaller cones. In a simulated treatment neglecting tissue heterogeneties (skull bone), the measured doses for two five arc studies (22.5-mm cone) were within 4% of the calculated dose to isocenter.