No abstract available

[en] Purpose/Objective: With the advent of megavoltage radiation, spatially fractionated radiation (SFR) has been abandoned for the last several decades. Yet it has been proved safe and effective in delivering large cumulative doses (> 100 Gy) of radiation. At our institution, SFR has been adapted to megavoltage beams. This study evaluates the toxicity and effectiveness of this approach in treatment of advanced bulky cancers. Material and Methods: From January, 1995 through December, 1996, 48 patients with advanced cancers (tumor sizes > 10 cm) were treated with spatially fractionation high-dose external beam radiation using a GRID technique. 8 patients received GRID treatments to multiple sites and a total of 58 sites were irradiated. A 50:50 GRID (open to closed areas) was utilized and a single fraction of 1000-2000 cGy (median 1500 cGy) to dmax was delivered utilizing 6MV photons. 42 patients received high dose GRID therapy for palliation (main, mass, bleeding and dyspnea) with (48) or without (10) fractionated external beam irradiation. In 6 patients, GRID therapy was given as part of a definitive treatment combined with conventionally fractionated external beam irradiation (dose range 4000-7000 cGy) with or without subsequent surgery. 24 treatments were delivered to the abdomen and pelvis, 20 to the head and neck region, 9 to the thorax and 5 to the extremities. Follow-up in these patients ranged from 2 months to 24 months. Results: For palliative treatment, a 76.9% response rate was observed for pain including complete response (CR) of 26.9% and partial response (PR) of 50% in these large bulky tumors. A 64.5% response rate was observed for mass effect (CR 22.6%, PR 41.9%). The response rate observed for bleeding and dyspnea was 100% (66.6% CR, 33.3% PR) and 50% (50% PR) respectively. A relatively higher response rate (CR 30%, PR 55%) was observed in patients who received GRID treatment in the head and neck area. No Grade 3 late skin, subcutaneous, mucosal, or GI complications were observed in any patient in spite of these high doses. In the 6 patients who received GRID treatment for the purpose of definitive treatment, a CR was observed clinically in 5 patients (83.3%) and a complete pathological response was confirmed in the operative specimen in 3 patients (50%). Conclusion: The efficacy and safety of using a large single fraction of SFR was confirmed by this study and substantiates our earlier results. In selected patients with bulky tumors (>10cm), SFR can be combined with fractionated external beam irradiation to yield improved local control and/or palliation of disease, especially where conventional treatment alone has limited chance of success

[en] Daily, monthly, and annual quality control (QC) of linear accelerators are part of the major tasks of the medical physicist to verify that patients are receiving proper radiation treatment. The control tests consist of the measurement of beam output, verification of the beam energy, and determination of the beam flatness and symmetry in a linear accelerator. A new device, referred to as QC phantom, was designed and fabricated for the QC of linear accelerators. This device is accompanied by software generating the reports of all measured data, keeping track of day-to-day data, and plotting the results. The accuracy, reproducibility, and linearity of the QC phantom were evaluated in this project. Also, the user friendliness of this device for morning warmup of linear accelerators was tested

[en] Purpose: Wedge-shaped isodoses are desired in a number of clinical situations. Physical wedge filters have provided nominal angled isodoses with dosimetric consequences of beam hardening, increased peripheral dosing, nonidealized gradients at deep depths, along with the practical consequences of filter handling and placement problems. Dynamic wedging uses a combination of a moving jaw and changing dose rate to achieve angled isodoses. The clinical implementation of dynamic wedge and an accompanying quality assurance program are discussed in detail. Methods and Materials: The accelerator at our facility has two photon energies (6 MV and 18 MV), currently with dynamic wedge angles of 15 deg. , 30 deg. , 45 deg. , and 60 deg. . The segmented treatment tables (STT) that drive the jaw in concert with a changing dose rate are unique for field sizes ranging from 4.0 cm to 20.0 cm in 05 cm steps, resulting in 256 STTs. Transmission wedge factors were measured for each STT with an ion chamber. Isodose profiles were accumulated with film after dose conversion. For treatment-planning purposes, d_max orthogonal dose profiles were measured for open and dynamic fields. Physical filters were assigned empirically via the ratio of open and wedge profiles. Results: A nonlinear relationship with wedge factor and field size was found. The factors were found to be independent of the stationary field setting or second order blocking. Dynamic wedging provided more consistent gradients across the field compared with physical filters. Percent depth doses were found to be closer to open field. The created physical filters provided planned isodoses that closely resembled measured isodoses. Comparative isodose plans show improvement with dynamic wedging. Conclusions: Dynamic weding has practical and dosimetric advantages over physical filters. Table collisions with physical filters are alleviated. Treatment planning has been solved with an empirical solution. Dynamic wedge is a positive replacement for physical filters, and a first step for commercial introduction of dynamic conformal therapy

[en] Purpose: Dose distributions around low energy (< 60 keV) brachytherapy sources, such as ¹²⁵I, are known to be very sensitive to changes in tissue composition. Available ¹²⁵I dosimetry data describe the effects of replacing the entire water medium by heterogeneous material. This work extends our knowledge of tissue heterogeneity effects to the domain of bounded tissue heterogeneities, simulating clinical situations. Our goals are three-fold: (a) to experimentally characterized the variation of dose rate as a function of location and dimensions of the heterogeneity, (b) to confirm the accuracy of Monte Carlo dose calculation methods in the presence of bounded tissue heterogeneities, and (c) to use the Monte Carlo method to characterize the dependence of heterogeneity correction factors (HCF) on the irradiation geometry. Methods and Materials: Thermoluminescent dosimeters (TLD) were used to measure the deviations from the homogeneous dose distribution of an ¹²⁵I seed due to cylindrical tissue heterogeneities. A solid water phantom was machined accurately to accommodate the long axis of the heterogeneous cylinder in the transverse plane of a ¹²⁵I source. Profiles were obtained perpendicular to and along the cylinder axis, in the region downstream of the heterogeneity. Measurements were repeated at the corresponding points in homogeneous solid water. The measured heterogeneity correction factor (HCF) was defined as the ratio of the detector reading in the heterogeneous medium to that in the homogeneous medium at that point. The same ratio was simulated by a Monte Carlo photon transport (MCPT) code, using accurate modeling of the source, phantom, and detector geometry. In addition, Monte Carlo-based parametric studies were performed to identify the dependence of HCF on heterogeneity dimensions and distance from the source. Results: Measured and calculated HCFs reveal excellent agreement (≤ 5% average) over a wide range of materials and geometries. HCFs downstream of 20 mm diameter by 10 mm thick hard bone cylinders vary from 0.12 to 0.30 with respect to distance, while for an inner bone cylinder of the same dimension, it varies from 0.72 to 0.83. For 6 mm diameter by 10 mm thick hard bone and inner bone cylinders, HCF varies 0.27-0.58 and 0.77-0.88, respectively. For lucite, fat, and air, the dependence of HCF on the 3D irradiation geometry was much less pronounced. Conclusion: Monte Carlo simulation is a powerful, convenient, and accurate tool for investigating the long neglected area of tissue composition heterogeneity corrections. Simple one dimensional dose calculation models that depend only on the heterogeneity thickness cannot accurately characterize ¹²⁵I dose distributions in the presence of bone-like heterogeneities

[en] The TG-43 recommended dosimetric characteristics of a new ¹²⁵I brachytherapy source have been experimentally and theoretically determined. The measurements were performed in Solid Water^TM using LiF TLDs. The calculations were performed using Monte Carlo simulations in Solid Water^Tm and water. The measured data were compared with calculated values as well as the reported data in literature for other source designs. The dose rate constant this source in water was 1.01±3% cGy h^-1 U^-1 and the anisotropy constant was 0.956

[en] Purpose: An applicator is described for endocavitary treatment of rectal cancers using a high dose rate (HDR) remote afterloading system with a single high-intensity ¹⁹²Ir source as an alternative to the 50 kVp x-ray therapy contact unit most frequently used in this application. Methods and Materials: The applicator consists of a tungsten-alloy collimator with a 45 deg. beveled end, placed in a protoscope with an elliptical cross-section. The resultant 3 cm diameter circular treatment aperture, located in the beveled face of the proctoscope, is irradiated by circular array of dwell positions located about 6.5 mm from the applicator surface. This beveled end allows patients with posterior wall tumors to be treated in the dorsal lithotomy position. The dose-rate distributions about the applicator were determined using a combination of thermoluminescent dosimetry (TLD-100 detectors) and radiochromic film dose measurement techniques along with Monte Carlo dosimetry calculations. TLD-100 (3 x 3 x 0.9 mm³ chips) measurements were used to measure the distribution of dose over the proctoscope surface as well as the central axis dose-rate distribution. Relative radiochromic film measurements were used to measure off-axis ratios (flatness and penumbra width) within the treatment aperture. These data were combined with Monte Carlo simulation results to obtain the final dose distribution. Results: The tungsten collimator successfully limits the dose to the tissue in contact with the proctoscope walls to less than 12% of the prescribed dose. These results indicate that the HDR applicator system has slightly more penetrating depth-dose characteristics than the most widely used contact therapy x-ray machine. Flatness characteristics of the two treatment delivery systems are comparable, although the HDR endocavitary applicator has a significantly wider penumbra. Finally, the HDR applicator has a lower surface dose rate (1.5-4 Gy/min of dwell time) compared to 9-10 Gy/min for the x-ray unit. Conclusions: An applicator system has been developed for endocavitary treatment of early stage rectal carcinoma that uses a single-stepping source HDR remote afterloading system as a radiation source. The advantages of the HDR-based system over x-ray therapy contact units currently used in this clinical application are (a) enhanced flexibility in applicator design and (b) widespread availability of single-stepping source HDR remote afterloading systems

[en] Dosimetric characteristics of brachytherapy sources are normally determined in water using a Monte Carlo simulation technique and in water equivalent phantom material using both experimental and Monte Carlo simulation techniques. The consensuses of these results are then calculated for clinical applications by converting experimental data obtained in water equivalent material to water using a conversion factor. These conversion factors are normally determined as a ratio of the Monte Carlo-simulated dose rate constant in liquid water to the dose rate constant in a water-equivalent phantom material. However, it has been noted that conversion factors utilized by some investigators have been derived using incorrect phantom material composition and incorrect cross-sectional data information. The impact of errors associated with the cross-sectional data and chemical composition of the phantom material used in dosimetric evaluation of brachytherapy sources has been investigated in this project. Results of these investigations have shown that the use of Solid Water trade mark sign with 1.7% calcium content, as compared to the 2.3% value stated by the manufacturer, may lead to 5% and 9% differences in conversion factors for ¹²⁵I and ¹⁰³Pd, respectively

[en] ¹²⁵I brachytherapy sources are being used for interstitial implants in tumor sites such as the prostate. Recently, the ADVANTAGE^TM125I, Model IAI-125, source became commercially available for interstitial brachytherapy treatment. Dosimetric characteristics (dose rate constant, radial dose function, and anisotropy function) of this source were experimentally and theoretically determined, following the AAPM Task Group 43 recommendations. Derivation of the dose rate constant was based on recent NIST WAFAC calibration performed in accordance with their 1999 standard. Measurements were performed in Solid Water^TM phantom using LiF thermoluminescent dosimeters. The theoretical calculations were performed in both Solid Water^TM and water using the PTRAN Monte Carlo code. The results indicated that a dose rate constant of the new source in water was 0.98±0.03 cGy h^-1 U^-1. The radial dose function of the new source was measured in Solid Water^TM and calculated both in water and Solid Water^TM at distances up to 10.0 cm. The anisotropy function, F(r,θ), of the new source was measured and calculated in Solid Water^TM at distances of 2 cm, 3 cm, 5 cm, and 7 cm and also was calculated in water at distances ranging from 1 cm to 7 cm from the source. From the anisotropy function, the anisotropy factors and anisotropy constant were derived. The anisotropy constant of the ADVANTAGE^TM125I source in water was found to be 0.97±0.03. The dosimetric characteristics of this new source compared favorably with those from the Amersham Health Model 6711 source. Complete dosimetric parameters of the new source are presented in this paper

[en] Recently, elongated brachytherapy sources (active length >1 cm) have become commercially available for interstitial prostate implants. These sources were introduced to improve the quality of brachytherapy procedures by eliminating the migration and seed bunching associated with loose seed-type implants. However, the inability to calibrate elongated brachytherapy sources with the Wide-Angle Free-Air Chamber (WAFAC) used by the National Institute of Standards and Technology (NIST) hinders the experimental determination of dosimetric parameters of these source types. In order to resolve this shortcoming, an interim solution has been introduced for calibration of elongated brachytherapy sources using a commercially available well-type ionization chamber. The feasibility of this procedure was examined by calibrating RadioCoil^Tm103Pd sources with active lengths ranging from 1 to 7 cm