Ali S. Meigooni

Photon energy dependence of the sensitivity of radiochromic film and comparison with silver halide film and LiF TLDs used for brachytherapy dosimetry

There is a new radiochromic film, a highly uniform, thin (100-microns) detector whose sensitive layer (6 microns thick) changes from colorless to blue by dye polymerization without processing, upon exposure to ionizing radiation. Because the dose gradients around brachytherapy sources are steep, the high spatial resolution offered by film dosimetry is an advantage over other detectors such as thermoluminescent dosimeters (TLDs). This compares the photon energy dependence of the sensitivities of GafChromic film, silver halide verification film (Kodak X-Omat V Film), and lithium fluoride TLDs (Harshaw), over the photon energy range 28 keV to 1.7 MeV, which is of interest in brachytherapy. Sensitivity of the radiochromic film is observed to decrease by about 30% as effective photon energy decreases from 1710 keV (4-MV x rays) to 28 keV (60-kV x rays, 2-mm A1 filter). In contrast, the sensitivity of verification film increases by 980% and that of LiF TLDs increases by 41%. The variation of the sensitivity of radiochromic film with photon energy is considerably less than that for silver halide film and similar to that for LiF TLDs, but in the opposite direction. Radiochromic film, like LIF TLDs, does not exhibit the drastic sensitivity changes below 127 keV that silver halide film exhibits. Dose distribution in the immediate vicinity of a high activity (370 GBq) brachytherapy 192Ir source has been mapped using radiochromic film and is presented to illustrate the applicability of this new technology to brachytherapy dosimetry.

A comparison of solid phantoms with water for dosimetry of 125I brachytherapy sources

Dosimetry of brachytherapy sources is critically dependent on precise measurement of the source-detector distance. A solid phantom can be precisely machined and hence distances can be accurately determined. In this work LiF thermoluminescent chips are used for absolute dose rate measurements in solid water, polymethylmethacrylate (PMMA), and polystyrene. These media are examined for their suitability in the dosimetry of 125I by comparing depth doses in each phantom. Measurements and Monte Carlo calculations show that solid water is equivalent to water for the dosimetry of 125I seeds, however, polystyrene and PMMA are not equivalent to water. Also, photon energy spectra for several depths in each phantom material have been calculated and are used to determine average photon energy and mass energy absorption coefficients as a function of depth.

Dosimetry on transverse axes of 125I and 192Ir interstitial brachytherapy sources

Dose rates along the transverse axes of 125I model 6702, 125I model 6711 and 192Ir 0.2-mm steel sources for interstitial brachytherapy have been measured in a solid-water phantom for distances up to 10 cm using LiF thermoluminescent dosimeters (TLDs). Specific dose rate constants, the dose rates in water per unit source strength 1 cm along the perpendicular bisector of the source, are determined to be 0.90 +/- 0.03, 0.85 +/- 0.03, and 1.09 +/- 0.03 cGy h-1 U-1 for 125I model 6702, 125I model 6711 and 192Ir 0.2-mm steel sources, respectively (1 U = unit of air kerma strength = 1 microGy m2 h-1 = 1 cGy cm2 h-1). In older and obsolete units of source strength (i.e., mCi apparent), these are 1.14 +/- 0.03, 1.08 +/- 0.03, and 4.59 +/- 0.15 cGy h-1 mCi-1 (apparent). Currently accepted values of specific dose rate constant for 125I sources are up to 20% higher than our measured values which are in good agreement with the results of our Monte Carlo simulations. But for 192Ir there is good agreement between our measured value of the specific dose rate constant and currently accepted values. The radial dose function for 125I model 6702 is found to be consistently larger than that for 125I model 6711, with an increasing difference as the distance from the source increases. Our measured values for the radial dose function for 125I sources are in good agreement with the results of our Monte Carlo simulation as well as the measured values of Schell et al. [Int. J. Radiat. Oncol. Biol. Phys. 13, 795-799 (1987)] for model 6702 and Ling et al.(ABSTRACT TRUNCATED AT 250 WORDS)

Anisotropy functions for 103Pd, 125I, and 192Ir interstitial brachytherapy sources

Anisotropy of dose distributions around 103Pd, 125I, and 192Ir sources for interstitial brachytherapy was examined. Dose rates around 125I models 6702 and 6711 and 192Ir sources were measured using lithium fluoride thermoluminescence dosimeters (LiF TLDs) in a water-equivalent, solid phantom. From these measured data for 125I and 192Ir and the previously published measured data for 103Pd, isodose rate contours were determined using a bivariate interpolation and smooth surface fitting algorithm. The anisotropy functions, F(r,theta), as defined by the Interstitial Collaborative Working Group (ICWG) for each source, were determined. Also, 4 pi-averaged anisotropy factors, phi an(r), for use in point source approximation, have been calculated at radial distances varying from 1-10 cm for 103Pd, 125I, and 192Ir sources. The anisotropy factors had average values of 0.90, 0.93, 0.95, and 0.98 for 103Pd, 125I model 6711, 125I model 6702, and 192Ir, respectively. The anisotropy factors determined from dose measurements in phantom are observed to be closer to unity than from those determined previously from in-air measurements. This can be attributed to the smoothing of two-dimensional dose distributions due to the presence of more scattered photons in the phantom measurements compared to in-air measurements. Because in-phantom measurements simulate more closely the brachytherapy patient, data from these experiments are recommended for a more accurate determination of dose distributions around clinical brachytherapy implants. In this work, we present a complete set of source data for two-dimensional dosimetry following the ICWG formalism.

Influence of the variation of energy spectra with depth in the dosimetry of 192Ir using LiF TLD

Absolute dose measurements around low activity seeds used in brachytherapy are difficult to perform with ionisation chambers. The physical size of the chamber does not allow good resolution close to the seeds and the ionisation current generated is very low. Small thermoluminescent dosimeters (TLDs) overcome these problems but the energy dependence of their response should be considered. In this work, dose in polystyrene was measured at several distances from the high activity 192Ir source (370 GBq) of a remote afterloading device using an ionisation chamber and LiF TLD chips. These data show that over a range of 1-10 cm from the source the sensitivity of LiF varies by up to 8.5%. This is attributed to the higher response of LiF to the lower photon energies, and to the shift of the photon spectrum to lower energies with increasing depth. The sensitivity of LiF to 192Ir was also calculated by weighting the energy-dependent response of LiF by the Monte Carlo calculated photon spectra. The calculations give a similar change in sensitivity with distance from the source.

On the choice of phantom material for the dosimetry of 192Ir sources

Dosimetric characteristics of polystyrene, solid water, and polymethylmethacrylate were examined and compared to water to determine the suitability of these solid materials for the dosimetry of 192Ir. Ionization charge measured in each of the four media as a function of depth and depth-dose curves calculated by Monte Carlo simulation show that the three solids are equivalent to each other and to water under full scattering conditions. Photon energy spectra generated from the Monte Carlo simulation show little variation for the different media. Mass energy absorption coefficients and exposure-to-dose conversion factors were calculated as a function of depth for these spectra. Measured tissue attenuation factors are in excellent agreement with Meisbergers selected values. The radial dose function, which describes the change in dose with distance in phantom exclusive of the inverse square law, was calculated from the tissue attenuation factor and found to be in significant disagreement with Dales Monte Carlo values. The reason for this discrepancy is discussed.

Interseed effects on dose for 125I brachytherapy implants

Dose calculations in multiseed brachytherapy implants are done by adding the contribution of each individual seed and by assuming that radiation from each seed is unaffected by the presence of the other seeds. To test the validity of this assumption, dose measurements with various configurations of multiseed implants of 125I model 6702 and 125I model 6711 sources were performed. For a linear configuration of three 125I model 6702 seeds at 1-cm separation, with their transverse axes coincident, doses at distances of 3.05 and 5.09 cm from the center along the transverse axis were found to be about 8% lower than the sum of doses from the three individual seeds. However, for three seeds at 1-cm intervals with their longitudinal axes coincident, doses at 3.05 and 5.09 cm distances from the center along the longitudinal axis were found to be about equal to the dose sums from individual seeds. These initial experiments indicated that the magnitude of the interseed effect depends upon the orientation of the seed relative to each other in an implant. To evaluate the importance of this interseed effect for multiseed configurations of 125I model 6702 and 125I model 6711 seeds, dose rates at various distances from a two-plane implant (each plane containing a 3 x 3 array of sources in a 1-cm spacing square grid) were measured in a Solid Water phantom with LiF TLDs. These measurements were carried out in two different planes at different orientations relative to the implant. The average values of the interseed effect at distances ranging from 1 to 7 cm outside the implant were observed to be about the same for 125I model 6702 and model 6711 sources. The mean value of the interseed effect was 6% and the maximum was 12%. On the whole, the interseed effect reduces the dose at the periphery of the iodine implant by 6%.

Tissue inhomogeneity correction for brachytherapy sources in a heterogeneous phantom with cylindrical symmetry

In brachytherapy it is customary to perform dose calculations for an implant assuming that the tumor and surrounding tissues constitute a uniform, homogeneous medium equivalent to water. In this work, the validity of the above assumption is studied quantitatively for points along the transverse axis of 103Pd, 125I, and 241Am brachytherapy sources, using measured and Monte Carlo calculated dose rates in homogeneous and heterogeneous media with cylindrical symmetry. The irradiation geometry chosen was a single source implanted in a Solid Water phantom which had a 1- or 2-cm-thick cylindrical Solid Water shell replaced by a polystyrene shell. The Monte Carlo simulations were performed using the integrated tiger series CYLTRAN Code. Experimental data were obtained for the same geometry to test the validity of the Monte Carlo calculations for a heterogeneous phantom. Measured dose rates just beyond a 2-cm-thick polystyrene heterogeneity were observed to be greater than those in a homogeneous Solid Water phantom by about 130%, 55%, and 10% for 103Pd, 125I, and 241Am, respectively. Thus the effect of a relatively small polystyrene heterogeneity in Solid Water can be substantial for lower energy photons. This perturbation of dose was found to increase steeply with decreasing energy and increasing size (thickness) of inhomogeneity. A simple dose calculation formalism has been developed to predict dose rate in a heterogeneous phantom with cylindrical symmetry, which uses as input the radial dose functions of the uniform media comprising the heterogeneous phantom. Dose rate predictions using this formalism are in reasonable agreement with the experimental data and the Monte Carlo calculated values.(ABSTRACT TRUNCATED AT 250 WORDS)

Superheated Drop Detector for Determination of Neutron Dose Equivalent to Patients Undergoing High Energy X-Ray and Electron Radiotherapy

The superheated drop detector (SDD) consists of thousands of superheated drops dispersed in a small vial of gel, which vaporize upon exposure to high LET radiation, thereby providing a directly observable indication of neutron dose. This detector possesses high sensitivity to neutrons and insensitivity to high-energy photons and electrons, making it suitable for the determination of neutron dose equivalent rates around high-energy photon and electron radiotherapy beams. In the present work, the SDD was used to measure the neutron dose equivalent in and around the radiotherapy beams produced by a 32-MeV linear accelerator. For both x-ray and electron beams, the neutron dose profiles were observed to follow the photon/electron radiotherapy beam profiles. For 25-MV x rays, the neutron dose equivalent per photon dose on the central axis increased by a factor of about 3 as field size increased from 5 x 5 to 30 x 30 cm. However, the neutron dose equivalent rate at 50 cm off-axis in the patient plane was essentially independent of field size. The neutron dose equivalent per electron dose was essentially zero for electron beams with energies below 15 MeV, but increased rapidly above 15 MeV. For 25-MeV electrons, neutron dose equivalent on the central axis was about 1/5 that for 25-MV x rays. Analogous to the data for 25-MV x rays, the neutron dose equivalent rate on the central axis of a 25-MeV electron beam exhibited a similar field size dependence and outside the beam it was essentially independent of field size.(ABSTRACT TRUNCATED AT 250 WORDS)

Some treatment planning considerations for 103Pd and 125I permanent interstitial implants

Sealed sources of palladium-103 (103Pd), which decay with a half life of 17 days and emit on average 21 keV photons, are now in clinical use for permanent implants. For seed implantation of prostatic cancer, 103Pd implants are usually planned to deliver 115 Gy to full decay at an initial dose rate of 19.7 cGy/hr whereas 125I implants are usually planned to deliver 160 Gy at an initial dose rate of 7.72 cGy/hr. Because of the lower energy of photons emitted by 103Pd compared to the 125I sources (27 keV average energy), the tissue attenuation is more severe for 103Pd sources. The radial dose function drops more steeply with distance from the 103Pd sources compared to the 125I sources, raising a concern about the possibility of cold spots in the tumors implanted with 103Pd sources. To investigate this issue, a detailed analysis of the dependence of dose uniformity as a function of seed spacing for 125I and 103Pd sources in various cubic and spherical configurations was carried out. Using the measured single source dosimetry data as input, dose distributions for a variety of cubic and spherical implants were generated on a computerized treatment planning system. This study indicates that relative dose distributions for 125I and 103Pd implants with the same geometric configuration and number of seeds are very similar inside the implanted volume for implants. Dose uniformity within a target volume implanted with 103Pd seeds is also very similar to that for 125I. To expedite clinical implementation of 103Pd, an atlas of dose distributions for 103Pd implants has been produced for various seed configurations, seed spacings, and target volumes. Using 125I implants as a guideline, clinical procedures for planning of 103Pd implants have been developed. It was found that the total source strength implanted divided by the dimension of the implant can be expressed as an exponential function of implant size, resulting in a simple method for estimating the strength of seeds necessary in an implant. Also, the air kerma strength of 103Pd seeds is about 3.3 times that of 125I sources in an implant with the same geometric configuration and number of seeds, provided treatment doses of 115 Gy and 160 Gy are chosen for 103Pd and 125I implants, respectively.