Biomedical subjects

C B Saw

Publications and source records attributed to C B Saw.

At least 37 records · Page 2Linked to original sources

PubMed1998

Review of AAPM Task Group No. 43 recommendations on interstitial brachytherapy sources dosimetry. American Association of Physicists in Medicine.

In 1995, the American Association of Physicists in Medicine (AAPM) Task Group No. 43 (TG-43) published its recommendations on the dosimetry of interstitial brachytherapy sources. The report recommended the use of a new dose calculation formalism based on measured quantities. The formalism in modular form permits the computation of doses in two dimensions for 103Pd, 125I, and 192Ir sources. The TG-43 dose calculation formalism introduced new and updated quantities such as air kerma strength, dose rate constant, radial dose function, anisotropy function and anisotropy factor. The dose rate obtained using the TG-43 dose calculation formalism and updated source dosimetry data can be expected to be different from some of the currently used systems by as much as 17%. For the same treatment and implementing the TG-43 dosimetry with point source approximation, the widely prescribed dose of 160 Gy for 125I permanent implants using model 6711 sources changes to 144 Gy. In addition to the dose calculation formalism, TG-43 report also stated that the air kerma strength provided by NIST is estimated to be approximately 7-10% higher than it should be, due to low energy photon contamination for 125I. This difference has not been accounted for in the TG-43 report.

Brachytherapy↗

PubMed1998

Dosimetric considerations of water-based bolus for irradiation of extremities.

The dosimetry of high-energy photon beams in the treatment of superficial lesions occurring in extremities was examined. Large parallel-opposed fields with different photon beam energies were used. The extremity was immersed in water contained in a commercially available plastic wastebasket. The water bolus serves to even out the surface irregularities of the extremities and to remove the skin sparing effect. A polystyrene block was placed at the floor of the wastebasket to ensure that the extremity was encompassed in the radiation fields. The photon beam energies considered were 4 MV, 6 MV, 10 MV, and 24 MV. The results show that the dose distributions are more homogeneous with higher photon beam energies. The isodose lines are more constricted at mid-plane for low energy photon beams. Higher energy photon beams, 10 MV and up would be preferable for the treatment of superficial lesions of the extremities immersed in water bolus contained in a typical wastebasket size.

Extremities↗

PubMed1997

Local temperature and pressure variations and its effects on daily ionometric measurements of radiation beam outputs from linear accelerators.

As part of the quality assurance protocol for linear accelerators at West Virginia University Hospitals, the local temperature and pressure are recorded on a daily basis by the radiation therapists. These temperature and pressure values are not used for temperature and pressure correction unless the ionometric measurements of radiation beam outputs from linear accelerators are outside the established tolerance levels. This paper presents a study on the variation of temperature and pressure over the past two years. Analysis of the data showed that the variation of pressure is within +/- 1.5% of the average value. Likewise, the variation of temperature is within +/- 1.5% of the average value. the worst possible effects of temperature and pressure on ionometric measurements would be within +/- 3%, which is at the tolerance levels recommended by the American Association of Physicists in Medicine (AAPM) Task Group No. 13. By maintaining machine output fluctuation to within +/- 2%, the maximum deviation of uncorrected ionometric measurements would be within +/- 5%. This study collaborates with the tolerance levels suggested by the AAPM Task Group No. 40.

Humans↗

PubMed1996

Relationship of wedge isodose angle to percent depth-dose curves.

PURPOSE: An expression that relates the wedge isodose angle to the gradient of the percent depth-dose curves for photon beams has been formulated and verified experimentally. METHODS AND MATERIALS: The expression that relates a wedge angle to another via the gradient of their corresponding percent depth-dose curves was derived. Because the field size affects the percent depth-dose curves, measurement of wedged isodose distributions were carried out for different field sizes to validate the expression. The 6 MV and 23 MV photon beams and a range of field sizes from 5 x 5 cm to 20 x 20 cm were used. For each field size, the wedge angle and percent depth-dose curve slope were measured at the depth of 10 cm. Measured and calculated wedge angles were compared. RESULTS: The variation of the wedge angle caused by the change in field size is about 6 degrees. For field sizes larger than 10 x 10 cm, the variation of wedge angle is less than 3 degrees. The calculated wedge angles agree with the measured wedge angles to within 1 degree. The knowledge of the extent of wedge angle variation due to the change in field size and the availability of an expression to determine an accurate wedge angle would be helpful in the use of the universal wedge equation.

Filtration↗

PubMed1996

Dose volume assessment of high dose rate 192Ir endobronchial implants.

PURPOSE: To study the dose distributions of high dose rate (HDR) endobronchial implants using the dose nonuniformity ratio (DNR) and three volumetric irradiation indices. METHODS AND MATERIALS: Multiple implants were configured by allowing a single HDR 192Ir source to step through a length of 6 cm along an endobronchial catheter. Dwell times were computed to deliver a dose of 5 Gy to points 1 cm away from the catheter axis. Five sets of source configurations, each with different dwell position spacings from 0.5 to 3.0 cm, were evaluated. Three-dimensional (3D) dose distributions were then generated for each source configuration. Differential and cumulative dose-volume curves were generated to quantify the degree of target volume coverage, dose nonuniformity within the target volume, and irradiation of tissues outside the target volume. Evaluation of the implants were made using the DNR and three volumetric irradiation indices. RESULTS: The observed isodose distributions were not able to satisfy all the dose constraints. The ability to optimally satisfy the dose constraints depended on the choice of dwell position spacing and the specification of the dose constraint points. The DNR and irradiation indices suggest that small dwell position spacing does not result in a more homogeneous dose distribution for the implant. This study supports the existence of a relationship between the dwell position spacing and the distance from the catheter axis to the reference dose or dose constraint points. Better dose homogeneity for an implant can be obtained if the spacing of the dwell positions are about twice the distance from the catheter axis to the reference dose or dose constraint points.

Brachytherapy↗

PubMed1996

Dose distributions through the supraclavicular-axillary region of patients undergoing breast cancer treatment.

The supraclavicular-axillary region is often irradiated if multiple lymph nodes are found to be positive in the staging of breast cancer. The doses delivered to supraclavicular-axillary region are typically computed using IRREG program. As such, the dose distributions through these regions are rarely examined. The intent of this study is to examine dose distributions through the supraclavicular-axillary region. The results of this study indicate that the anterior region of the axilla receives about 20% more dose than the supraclavicular region or the mid-axillary point. This excess dose results from the posterior axillary field used to supplement the dose from the supraclavicular-axillary field.

Axilla↗

PubMed1995

Dose distribution considerations of medium energy electron beams at extended source-to-surface distance.

PURPOSE: To determine the effects of extended source-to-surface distance (SSD) on dose distributions for a range of medium energy electron beams and cone sizes. METHODS AND MATERIALS: The depth-dose curves and isodose distributions of 6 MeV, 10 MeV, and 14 MeV electron beams from a dual photon and multielectron energies linear accelerator were studied. To examine the influence of cone size, the smallest and the largest cone sizes available were used. Measurements were carried out in a water phantom with the water surface set at three different SSDs from 101 to 116 cm. RESULTS: In the region between the phantom surface and the depth of maximum dose, the depth-dose decreases as the SSD increases for all electron beam energies. The effects of extended SSD in the region beyond the depth of maximum dose are unobservable and, hence, considered minimal. Extended SSD effects are apparent for higher electron beam energy with small cone size causing the depth of maximum dose and the rapid dose fall-off region to shift deeper into the phantom. However, the change in the depth-dose curve is small. On the other hand, the rapid dose fall-off region is essentially unaltered when the large cone is used. The penumbra enlarges and electron beam flatness deteriorates with increasing SSD.

Electrons↗

PubMed1994

Validity of universal wedge equation over the range of 60Cobalt to 25 MV photon beam energies.

PURPOSE: Evaluating the validity of the universal wedge equation over the range of 60Cobalt (60CO) to 25 MV photon beam energies. METHODS AND MATERIALS: The universal wedge equation relates the beam weight of the wedged field to the effective wedge angle produced by combining the wedged field to the nonwedged field and is expressed as tan (theta) = B tan (theta w), where B is the normalized weight imposed on the wedged field, theta w is the maximum wedge angle of the wedge filter, and theta is the effective wedge angle. The isodose distributions from 60Co to 25 MV photon beam energies were used. For each photon energy, the isodose distributions of wedged and nonwedged fields were combined in different proportions. The effective wedge angle was determined from each resultant isodose distribution. RESULTS: The relationship between the weight of wedged field and the effective wedge angle was found to be nonlinear for wedge filters with maximum wedge angle greater than 30 degrees. The universal wedge equation predicts the effective wedge angle to within 2 degrees compared to the measured value.

Cobalt Radioisotopes↗

PubMed1994

Dosimetric considerations of commercially fabricated esophageal applicators.

Two commercially fabricated esophageal applicators for low dose rate intraluminal brachytherapy were compared using the dose non-uniformity ratio (DNR). One of the applicators has a single afterloading catheter while the other applicator has six afterloading catheters to accommodate low dose rate 192Ir ribbon or wire sources. Five sets of source configurations, each with a different ribbon length in the range of 5-17 cm were analysed using the DNR. The DNR shows the delivery of relatively higher total doses at higher dose rates to tissues closer to the applicators. The difference in the inhomogeneity of dose rates and the treatment volume exposed to higher dose rates than the prescribed dose rate between the two applicators are minimal. The dose homogeneity is better for the single-source applicator while the worst dose homogeneity for the six-source applicator is caused by the placement of each source closer to the surface of the applicator. The implications of higher dose rates were discussed using the alpha/beta model.

Brachytherapy↗

PubMed1994

Effects of extended SSD on electron-beam depth-dose curves.

The effects of extended source-to-surface distance (SSD) on the electron-beam depth dose curves were examined for five electron beam energies ranging from 6 MeV to 20 MeV. The depth dose curves were measured with the water phantom surface set at 100 cm, 110 cm, and 120 cm from the source. Except for the dose in the build-up region, negligible depth dose curve changes due to extended SSD were observed for electron energies less than or equal to 12 MeV. For electron beams greater than 12 MeV, the percent surface dose decreases and, additionally, the depth of maximum dose shifts deeper into the phantom as the SSD increases. Changes in the rapid falloff region of the depth dose curves were apparent only for the higher energy electron beams, in particular 20 MeV. Extended SSD has no observable effect on the tail portion of the depth dose curves. As such, the energies of the electron beams are not changed significantly by increasing the SSD.

Electrons↗

PubMed1993

Concept of dose nonuniformity in interstitial brachytherapy.

PURPOSE: Evaluation of the 3-dimensional dose distributions of interstitial implants using the dose uniformity ratio. METHODS AND MATERIALS: Single source, two sources, three and four sources arranged both linearly and in the form of a triangle or a square, ribbons with different seed spacings, a single-plane and double-plane implants were evaluated. The evaluations involved the use of differential dose volume histograms and the dose nonuniformity ratio defined as the ratio of the high dose volume to the reference volume. RESULTS: For a single source, the dose nonuniformity is the same regardless which dose rate is selected as the treatment dose rate. For any multi-source implant, the dose nonuniformity is altered depending on the selection of the reference dose rate. In addition, the dose nonuniformity curve exhibited three characteristics zones. CONCLUSION: The dose nonuniformity ratio can be a useful tool in assessing and optimizing interstitial implants.

Brachytherapy↗

PubMed1993

Evaluation of the substitution of Ir-192 seed ribbons for wires in Paris system using dose nonuniformity ratio.

The substitution of Ir-192 seed ribbons for wires in the Paris system of interstitial implants was re-evaluated using the dose nonuniformity ratio. The dose nonuniformity ratio, which is based on volumetric data, measures the dose nonuniformity of the implant quantitatively. The lower the dose nonuniformity ratio value, the smaller the dose nonuniformity, and the better is the dose homogeneity for the implant. Implants configured in a single-plane and double-planes in the form of squares or triangles using Ir-192 wires or seed ribbons were considered. The difference between the particular reference dose rate that yielded a minimum in the dose nonuniformity ratio curve for wire implants and seed implants is about 4 cGy/hr. The difference between the minimum dose nonuniformity ratio value representing the optimal dose homogeneity of the implants is about 5%. The dose homogeneity may be considered better for implants configured using seed ribbons.

Brachytherapy↗

PubMed1993

Seed strength determination for eye plaque therapy.

Standardized radioactive plaques have been used in the irradiation of intraocular tumors. These plaques have defined slots to accommodate 125I seeds and hence produce predictable isodose distributions. The seed strength has to be adjusted to deliver 100 Gy to the prescription point, which varies with the tumor size. The purpose of this work is to develop lookup tables that relate the seed strength to different prescription distances. Dose rates were determined for a set of standardized eye plaques. Using these dose rates and the source strength decaying expression, the seed strengths were determined as a function of distance along a line through the plaque. Two seed-strength tables were created based on four-day and five-day treatment times delivering a 100 Gy to the prescription distance.

Brachytherapy↗

PubMed1992

High energy photon beam percent depth dose curves.

The percent depth dose of 8 MV and 18 MV photon beams were measured for various field sizes. The percent depth dose curves exhibited crossed over at selective depth in phantom. Hence, the percent depth dose increases at shallower depth, is independent at crossed over depth, and decreases at deeper depth with decreasing field size. This peculiar phenomenon, which cannot be explained using the primary and scattered radiation concept, should be noted.

Humans↗

PubMed1992

Determination of basal dose rates in Paris system of interstitial implants.

The isodose distributions are related to the source configurations in the Paris system of interstitial implants. In this system, the basal dose rates are determined at those points identified relative to the implant. Once these points have been located, the basal dose rates are merely the summation of the dose rate contributions from all the sources. By applying the symmetry properties of the implant, the number of basal dose rate computations and the number of dose rate summations are reduced. The reduction in both the number of computations and the number of summations has facilitated the determination of the average basal dose rate. After the average basal dose rate has been determined, the reference dose rate that is used clinically is calculated. Examples of determining the basal dose rates for a few interstitial implants are presented.

Brachytherapy↗

PubMed1992

Various wedge isodose angles for treatment planning.

Various wedge isodose angles or simply wedge angles smaller than the nominal wedge angle were created by combining the isodose distributions generated from a single physical wedge with the isodose distributions of the open field for the 8-MV photon beam. The particular wedge angle generated depends on the weights imposed on these isodose distributions. The relationship between these weights and the wedge angle were examined and found to be nonlinear. The difference between the wedge angles defined at 10 cm depth and those defined using the 50% isodose curve is less 6 degrees. The present data was fitted using two proposed empirical equations.

Humans↗

PubMed1992

Wedge filter effects on dosimetric parameters of a linear accelerator.

The open-field and wedged-field output factors as a function of field size for two linear accelerators were measured. Wedge factors were determined by taking the ratio of the outputs with and without the wedge filter. For one linear accelerator, the difference in the output factors between the wedged field and open field can be as large as 5%. The wedge factor for this linear accelerator also varies with the field size. On the other hand, the other linear accelerator shows smaller variation of output factors between wedge field and open field. The variation of wedge factor is less than 1% for a 60 degree wedge. In addition to modifying the isodose distributions, the wedge filter also changes the percent depth dose curves, the output factor, and the wedge factor. The degree of wedge effects on these dosimetric parameters is different for different linear accelerator.

Filtration↗

PubMed1991

Quantitative assessment of interstitial implants.

Quantitative assessment of interstitial implants is proposed using volume versus dose curves and four well-defined dosimetric parameters. The volume versus dose curves, both differential and cumulative, provide quantitative data on the volumes of tissues irradiated to different doses. They also offer a qualitative assessment of the variations in dose delivery. The dose nonuniformity ratio (DNR) quantitatively determines the degree of dose nonuniformity specific to the implant configuration. The dose rate at which the DNR shows a minimum value, if selected as the treatment dose rate, gives an optimized dose distribution. The three volumetric irradiation indices are formulated with respect to a well-defined target volume. They offer quantitative data on the extent to which the implant delivers the prescribed dose to the target volume. These dosimetric parameters determine the degree of coverage of the target volume, dose homogeneity within the target volume, and irradiation of tissues outside the target volume. The method of quantitative assessment is demonstrated using, as examples, an ideal Ir-192 double-plane implant and an actual clinical Ir-192 double-plane breast implant.

Brachytherapy↗