Measurement of dose rate from exposure-calibrated 125I seeds

Monte Carlo and experimental dosimetry of an I125 brachytherapy seed

We have performed a comprehensive dosimetric characterization of the Oncura model 6711 125I seed using both experimental [LiF thermoluminscent dosimetry (TLD)] and theoretical (Monte Carlo photon transport) methods. In addition to determining the dosimetric parameters of the 6711, this report quantified: (1) the angular dependence of LiF TLD energy response functions for both point and volume detectors in water, poly(methylmethacrylate), and solid water media; and (2) the contribution of underlying geometric uncertainties to the overall uncertainty of Monte Carlo derived dosimetric parameters according to the National Institute of Standards and Technology Report 1297 methodology. The theoretical value for the dose rate constant in water was 0.942 cGy U(-1) h(-1)+/-1.76% [combined standard uncertainty (CSU) with coverage factor k=1] and the experimental value was 0.971 cGy U(-1) h(-1)+/-6.1%. Agreement between experimental and theoretical radial dose function values was well within the k= 1 CSU, while agreement between experimental and theoretical anisotropy function values was within the k= 1 CSU only after incorporating the use of polar angle-dependent energy response functions. The angular dependence of the relative energy response was found to have a complex and significant dependence on measurement medium and internal geometry of the source.

Thermoluminescent dosimetry of the Symmetra 125I model I25.S06 interstitial brachytherapy seed

As the efficacy of brachytherapy prostate treatment is becoming realized, new models of 125I seeds are being introduced. In this article we present thermoluminescent dosimetry (TLD) in a solid water phantom for a new design of 125I seed (UroMed/Bebig Symmetra, Model I25.S06). TLD cubes, LiF TLD-100, from Bicron (Solon, OH) with dimension 1 x 1 x 1 mm3 were irradiated at various distances from the seed at angles ranging from 0 degrees to 90 degrees in 10 degrees increments. The TLD detectors were calibrated by irradiation in a 60Co teletherapy beam. Monte Carlo simulation was used to account for TLD energy dependence and the deviation of solid water composition (as determined by chemical analysis of a sample) from liquid water. Dose rates per unit air kerma strength were determined based on calibrations traceable to the 1999 NIST standard (corrected for NIST measurement errors made in 1999) for the Symmetra seed. Dose data is presented in TG-43 format as a function of distance and angle. Values for lambda, F(r, theta), g(r), and the anisotropy constant are obtained for use in radiation treatment planning (RTP) software. The dose rate constant was determined to be 1.033+/-6.4% cGy h(-1) U(-1), which is comparable to model 6702 and higher than model 6711. We find the relative dose distributions of the Symmetra seed are similar to model 6702, and less anisotropic than model 6711. After accounting for deviation of measured solid water composition from the manufacturer's specification, good agreement between TLD results and Monte-Carlo-aided values was found.

Monte Carlo-aided dosimetry of the Symmetra model I25.S06 125I, interstitial brachytherapy seed

A dosimetric study of a new 125I seed for permanent prostate implant, the Symmetra 1251 Seed model I25.S06, has been undertaken utilizing Monte Carlo photon transport calculations. All dosimetric quantities recommended by the AAPM Task Group 43 (TG-43) report have been calculated. Quantities determined are dose rate constant, radial dose function, anisotropy function, anisotropy factor, and anisotropy constant. The recently (January 1999) revised NIST (National Institute of Standards and Technology) 1251 standard for air kerma strength calibration was taken into account as well as updated interaction cross-section data. Calculations were done for the competing model 6702 source for the purpose of comparison. The calculated dose-rate constants for the two seeds are 1.010 and 1.016 cGyh(-1)U(-1) for the Symmetra and model 6702 seeds, respectively. The latter value deviates from the value, 1.039 cGyh(-1)U(-1), recommended in the TG-43 report. The calculated radial dose function for the Symmetra new seed is more penetrating than that of the model 6711 seed (by 20% at 5 cm distance) but agrees closely (within statistical errors) with that of the model 6702 seed up to distances of 10 cm. The anisotropy function for the seed is also close to that for the 6702 seed with a tendency of somewhat more pronounced anisotropy (lower values at small angles from the longitudinal axis). Compared to the model 6711 seed, the Symmetra new seed is more isotropic. The anisotropy constants (the anisotropy function averaged with respect to angle and distance) for the three seed models are within 2%.

Radial dose functions for 103Pd, 125I, 169Yb and 192Ir brachytherapy sources: an EGS4 Monte Carlo study

Radial dose functions g(r) in water around 103Pd, 125I, 169Yb and 192Ir brachytherapy sources were estimated by means of the EGS4 simulation system and extensively compared with experimental as well as with theoretical results. The DLC-136/PHOTX cross section library, water molecular form factors, bound Compton scattering and Doppler broadening of the Compton-scattered photon energy were considered in the calculations. Use of the point source approach produces reasonably accurate values of the radial dose function only at distances beyond 0.5 cm for 103Pd sources. It is shown that binding corrections for Compton scattering have a negligible effect on radial dose function for 169Yb and 192Ir seeds and for 103Pd seeds under 5.0 cm from the source centre and for the 125I seed model 6702 under 8.0 cm. Beyond those limits there is an increasing influence of binding corrections on radial dose function for 103Pd and 125I sources. Results in solid water medium underestimate radial dose function for low-energy sources by as much as 6% for 103Pd and 2.5% for 125I already at 2 cm from source centre resulting in a direct underestimation of absolute dose rate values. It was found necessary to consider medium boundaries when comparing results for the radial dose function of 169Yb and 192Ir sources to avoid discrepancies due to the backscattering contribution in the phantom medium. Values of g(r) for all source types studied are presented. Uncertainties lie under 1% within one standard deviation.

Validation of K-edge 125I brachytherapy enhancement with silver compounds

Brachytherapy with radioactive seeds implanted within the tumour volume has demonstrated good success rates in treating certain cancers. In an effort to improve the curative rates in cancer patients, ongoing research is being conducted to enhance the amount of radiation dose that is absorbed within the tumour volume while minimizing the dose absorbed by the surrounding normal tissue. One method for enhancing tumour dose absorption with 125I brachytherapy seeds is to increase the number of photoelectric atomic interactions within the tumour volume by introducing small quantities of a silver compound, taking advantage of the K-edge effect. Because low-energy electrons and Auger electrons are the primary sources of brachytherapy dose enhancement, acquiring accurate experimental measurements of absorbed dose increases is a major challenge. To circumvent this problem, an x ray fluorescence excitation spectroscopy dosimetry technique supplemented with clinically accepted dosimetry calculations was developed to estimate relative absorbed dose increases in a water phantom containing up to 7.5 mM of silver. Excellent agreement was observed between theoretically derived Monte Carlo dosimetric predictions and experimental measurements. These results successfully demonstrated that K-edge enhanced 125I brachytherapy is indeed possible with future development of a non-toxic silver chelate.

TLD dose measurement: a simplified accurate technique for the dose range from 0.5 cGy to 1000 cGy

A simplified TLD technique characterized by high precision and reproducibility of dose measurement is presented. One hundred eighty LiF TLD rods 1 mm diam x 3 mm length as obtained from the manufacturer were annealed for 1 h at 400 degrees C followed immediately by 2 h at 105 degrees C. After exposure to a dose of 1 Gy of 4 MV x rays, TLDs were annealed for 15 min at 105 degrees C, then read out. TLDs were then sorted into five groups, ranging from 26 to 50 rods each with approximately equal sensitivity after correcting for the drift in the sensitivity of the TLD reader during the readout session. Maintaining group identity, the TLDs were again annealed, irradiated and read out. Fewer than 10% of the TLDs were removed from each group because the corrected readings differed from the respective group mean by more than 3.5%. The standard deviation of the readout was approximately 1.5% within each group. The planchet heater was not flushed with nitrogen gas. Various tests were performed to assess the stability of the group sorting technique and the linearity of TLD dose response. After reannealing, five TLDs were randomly drawn from one of the presorted groups, and subjected to various dose of 4 MV radiation over the range from 0.5 to 1000 cGy. This resulted in an average readout standard deviation of 1.2%. Response per unit dose was almost flat over the range from 0.5 cGy to 100 cGy, and increased by 15% over the range from 100 cGy to 1000 cGy. TLD sensitivity was affected by the duration of the anneal, but was virtually independent of the various time delays between irradiation, prereadout anneal, and readout. The group annealing and sorting (GAS) procedure provides a simple, reliable, precise, convenient, and accurate method for TLD measurements.

Dose rate constants for 125I, 103Pd, 192Ir and 169Yb brachytherapy sources: an EGS4 Monte Carlo study

An exhaustive revision of dosimetry data for 192Ir, 125I, 103Pd and 169Yb brachytherapy sources has been performed by means of the EGS4 simulation system. The DLC-136/PHOTX cross section library, water molecular form factors, bound Compton scattering and Doppler broadening of the Compton-scattered photon energy were considered in the calculations. The absorbed dose rate per unit contained activity in a medium at 1 cm in water and air-kerma strength per unit contained activity for each seed model were calculated, allowing the dose rate constant (DRC) A to be estimated. The influence of the calibration procedure on source strength for low-energy brachytherapy seeds is discussed. Conversion factors for 125I and 103Pd seeds to obtain the dose rate in liquid water from the dose rate measured in a solid water phantom with a detector calibrated for dose to water were calculated. A theoretical estimate of the DRC for a 103Pd model 200 seed equal to 0.669 +/- 0.002 cGy h(-1) U(-1) is obtained. Comparison of obtained DRCs with measured and calculated published results shows agreement within 1.5% for 192Ir, 169Yb and 125I sources.

Conformal episcleral plaque therapy

PURPOSE

Episcleral plaque therapy (EPT) with sealed 125I sources is widely used in the treatment of choroidal melanoma. In EPT, as elsewhere in radiotherapy, concern for normal tissue tolerance has frequently been a dose-limiting factor. The concept of conformal therapy, which seeks to improve dose homogeneity within the tumor and greatly reduce the dose to uninvolved structures may provide a solution to this problem. Radioactive sources are typically distributed uniformly over the surface of an episcleral plaque and are sometimes offset slightly from the scleral surface to reduce the dose to the sclera relative to the apex and prescribed therapeutic margin at the tumor base. Nevertheless, it is not uncommon for scleral dose to exceed the dose to the apex of intermediate to tall tumors by a factor of 4 or more. The availability of low-energy sealed sources such as 125I prompted the development of gold-backed plaques to shield noninvolved periocular tissues. The concept of shielding can be extended to include collimation of individual sources. The potential advantages of individual source collimation include reduced scleral dose, more homogeneous tumor dose, and superior shielding of adjacent normal structures such as the fovea as compared to previous plaque designs.

METHODS AND MATERIALS

A three-dimensional treatment-planning system has been extended to design a plaque that incorporates individually collimated 125I sources. Thermoluminescent dosimetry (TLD) and radiochromic film were used to compare calculated dose-rate distributions with measured dose rates in an acrylic phantom.

RESULTS

Calculations predict that source collimation in the form of a "slotted" gold plaque will achieve the purposes of the study. The collimating effect of the slots is demonstrated qualitatively using radiochromic film, and the accuracy of the calculation is demonstrated quantitatively with TLD.

CONCLUSION

The episcleral plaque described in this report is simpler to assemble than previous plaque designs. It produces a more homogeneous dose distribution in the tumor, reduces scleral dose by up to 50% as compared to conventional designs, and significantly reduces radiation dose to uninvolved structures adjacent to the plaque.

Rapid two-dimensional dose measurement in brachytherapy using plastic scintillator sheet: linearity, signal-to-noise ratio, and energy response characteristics

Because of the large dose gradients encountered near brachytherapy sources, an efficient, accurate, low-atomic number areal detector, which can record dose at many points simultaneously, is highly desirable. We have developed a prototype of such a system using thin plates of plastic scintillator as detectors. A micro-channel plate (MCP) image intensifier was used to amplify the optical scintillation images produced by radioactive 125I and 137Cs sources in water placed 0.5-5.7 cm distance from the detector. A charge-coupled device (CCD) digital camera was used to acquire 2-D light-intensity distributions from the image intensifier output window. For both isotopes, a small area (2 x 3 mm2) PVT detector yields a CCD net count rate that is linear with respect to absorbed dose rate within +/- 3% out to 5.7 cm distance. Acquisition times range from 1.5-400 sec with a reproducibility of 0.5-5.5%. If a large-area (6 x 20 cm2) PVT detector is used, a four-fold increase in count rate and large deviations from linearity are observed, indicating that neighboring pixels contribute light to the signal through diffusion and scattering in PVT and water. A detailed noise analysis demonstrates that the image intensifier reduces acquisition time 10000-fold, reduces noise relative to signal 200-fold, and reduces amplifier gain noise as well.

Episcleral plaque thermoradiotherapy in patients with choroidal melanoma

From 1988 to 1991, 21 patients with uveal melanoma were treated in a Phase I study with episcleral plaque radiotherapy (EPRT). This irradiation was combined with localized current field episcleral hyperthermia (LCFHT). Tumor stage was: T3 = 15 (71%) and T2 = 6 (29%). Follow-up ranged from 2 to 42 months (mean 9.2 months). EPRT was given using custom built I-125 gold plaques. Radiation doses to the tumor apex ranged from 13 to 123 Gy (mean dose 70.0 Gy) given at a mean dose rate of 55 cGy/hr. LCFHT was given with 500 KHz frequency for 45 min immediately before EPRT. The temperature was controlled on the scleral surface using four thermocouples. T mean ranged from 42.5 degrees C to 45 degrees C +/- 0.5 degrees C (mean 43.4 degrees C). The study patients showed rapid tumor necrosis. A 25% mean decrease of apical tumor dimension was noted, p = 0.0007. At least ambulatory vision (greater than 5/200) was maintained by 17/21 (81%) patients. Visual acuity was seen to improve greater than 6 months post-plaque therapy in 10 (48%) study patients. This was following an intermediate decrease in visual acuity. Severe complications, including large hemorrhagic retinal detachment and large vitreous hemorrhage, were seen in two (9.5%) of the early study patients. A mean scleral temperature reduction to less than or equal to 44 degrees C +/- 0.5 degrees C resulted in good treatment tolerance and a lack of serious complications in subsequently treated patients. A Phase II prospective randomized trial comparing LCFHT with 60 versus 80 Gy EPRT dose to the tumor apex is currently being activated for patients with choroidal melanoma.

Episcleral plaque radiotherapy in the treatment of uveal melanomas

During an 8-year period, 85 patients with uveal melanomas were treated with episcleral plaque radiotherapy (EPRT). The T-stage was: T1-3 (4%), T2-29 (34%) and T3-53 (62%). The mean tumor elevation was 6.1 mm. Radiation dose was prescribed at the tumor apex and at D5mm. The mean D5mm dose was 150.1 Gy (range 70.5-430 Gy) and the mean dose at the apex was 102.6 Gy (range 29.8-200 Gy). Useful vision (greater than 5/200) was maintained in 73% of patients. The 5-year actuarial survival was 88%. Metastatic disease developed in 9 (11%) patients, 6 of whom died of their disease. Basal tumor dimensions were important factors predicting metastatic disease, p = 0.002. A decrease in tumor evaluation was seen in 82%. There was a much lower incidence of decrease in tumor radial and circumferential dimensions, 47.5 and 46%, respectively, p less than 0.001. Treatment complications were common (56%), particularly in patients with large tumors (72%), p = 0.04. The incidence of complications was higher in patients treated prior to 1988 as compared to those who were treated more recently (67 vs 35%, p = 0.010). There were 13 (15%) patients who had enucleation. This included 12 treated before 1986 and 1 patient treated subsequently (46 vs 2%, p less than 0.001). In a univariate analysis, tumor height and radiation dose at D5mm were important factors predicting enucleation, p = 0.004. In a multivariate analysis, however, the most important factor predicting enucleation was treatment administration prior to 1986, p less than 0.001). A sharp decrease in the incidence of severe complications, including enucleation, as seen after 1985, is likely due to a major effort in treatment optimization.