125I Sources Research Articles

Silver fluorescent x‐ray yield and its influence on the dose rate constant for nine low‐energy brachytherapy source models

The physical characteristics of the photons emitted by a low-energy brachytherapy source are strongly dependent on the source's construction. Aside from absorption and scattering caused by the internal structures and the source encapsulation, the photoelectric interactions occurred in certain type of source-construction materials can generate additional energetic characteristic x rays with energies different from those emitted by the bare radionuclide. As a result, the same radionuclide encapsulated in different source models can result in dose rate constants and other dosimetric parameters that are strikingly different from each other. The aim of this work was to perform a systematic study on the yield of silver fluorescent x rays produced in nine 125I sources that are known to contain silver and its impact on the dose-rate constant. Using a high-resolution germanium spectrometer, the relative 125I spectra emitted by the nine sources on its bisector were measured and found to be similar to each other (the maximum variation in the 125I-Kbeta relative intensity was less than 4%). On the other hand, the measured silver fluorescent x-ray spectra exhibited much greater variations from model to model; the maximum change in the measured Ag-Kalpha relative intensity was over 95%. This larger variation in the measured silver fluorescent x-ray yield was caused by (1) the different amount of silver that was directly exposed to the 125I radionuclide in different source models and (2) the stronger influence of the source's internal geometry on the silver fluorescent x rays. Because the addition of silver fluorescent x rays can significantly alter the photon characteristics emitted by the radioactive sources, a precise knowledge on the silver fluorescent x-ray yield is needed in theoretical calculations of the sources' intrinsic dosimetric properties. This study concludes that the differences in silver fluorescent yield are the primary causes of the variable dose rate constant observed among these source models.

On the physical, spectral, and dosimetric characteristics of a new brachytherapy source

A new 125I source under the name Braquibac has been developed in Argentina for interstitial brachytherapy applications. The aim of this work is to study the new seed's design and to calculate its dosimetric parameters. Radiographic and destructive tests were carried out on inactive seeds to determine the physical characteristics of the source. Values of g(r), Lambda, F(r, Theta), and phi(an)(r), were obtained in water and air by simulation using the MCNP5 Monte Carlo code according to the methodology recommended in TG-43 and updated in TG-43U1. The dose rate constant was determined to be 0.937+/-0.004 cGy h(-1) U(-1) (overall statistical uncertainty +/-2.7%). Sk per unity activity was calculated to be 0.671+/-0.003 cGy cm2 h(-1) mCi(-1) by simulation of the seed in dry air using point detectors. Spectroscopic studies for both the new and the Amersham model 6711 seed were performed using an HPGe planar detector. The emission spectra of both seeds proved to be very similar. The anisotropy of the total photon intensity in air was measured in planes containing the seed's short and long axes using the HPGe detector. The minimum photon intensity for the new seed was 31.14+/-3.10% of the transverse intensity.

SU-FF-T-386: Statistical Distribution of Strengths for Various Models of Low Energy Permanent Brachytherapy Sources

Purpose. Because there is limited data on the distribution of seed strengths across many source models available, this study determines the standard deviation of source strengths in actual brachytherapy seed orders. Additionally, we test a recent analysis indicating that the number of assayed seeds necessary to determine an accurate sample mean depends only on the deviation of source strengths within the order. Methods. Thirty consecutive orders for each of five I-125 source models and three Pd-103 models were analyzed by 100% assay of single seeds in well chambers with appropriate ADCL calibration factors. Results. The aggregate distribution of source strengths as percent deviation from the mean for each model was normal. The strength distribution of individual orders was usually normal, but some were skewed left or right and some were rectangular. The average order consisted of 96 seeds and 109 seeds for I-125 and Pd-103, respectively. For the I-125 models, the average standard deviation from the mean ranged from 2.3% to 3.8%, and for Pd-103 models from 1.1% to 1.8%. An iterative assay was tested in a 30 patient Pd-103 model series where the average number of seeds was 119. The standard deviation of strengths was calculated with each additional seed and stopped when the mean was predicted to be within 3% of the population mean at the 95% CI. The mean number of seeds assayed was 2.8, and only one order required 5 seeds. The mean seed strength agreed within 0.5% of the 100% assay and a maximum absolute difference of 2.2%. Conclusion. The seed strength variation found for the models studied is acceptable for an iterative assay approach that determines the number of seeds to be assayed by the standard deviation of the seed order. Such an approach requires considerably fewer seeds than the 10% assay approach.

Monte Carlo dosimetric characterization of the IsoAid ADVANTAGE brachytherapy source

For roughly 25 years, I125 and Pd103 sources have been used in the treatment of various malignant diseases such as prostate cancer. Various new sources have been marketed and produced to meet the demand for new sources to use in treatment. Recently, IsoAID LLC created the ADVANTAGE Pd103 source. Various dosimetric parameters must be determined to facilitate treatment planning using this source. Theoretical determination of dosimetric characteristics, dose rate constant, radial dose function, and anisotropy function for this new source followed the American Association of Physicists in Medicine (AAPM) Task Group 43U1 recommendations. Theoretical calculations were performed in liquid water using the PTRAN Monte Carlo code version 7.44. The radial dose function of the new source was calculated in liquid water at distances up to 10.0 cm, and the anisotropy function, at distances ranging from 0.5 cm to 7.0 cm. The anisotropy factors and anisotropy constant were derived from the anisotropy function. The results in water indicate that the dose rate constant is 0.709±0.014 cGy•h−1•U−1 and that the anisotropy constant is 0.880±0.040. The dosimetric characteristics of this new source compare favorably with those of other commercially available Pd103 sources.PACS: 87.53.Jw

Dosimetric consideration of individual 125I source strength measurement and a large-scale comparison of that measured with a nominal value in permanent prostate implant brachytherapy

We investigated the difference between measured and manufacturer's nominal source strength in a large sample of a single model of (125)I seeds. Physical characteristics of single seed measurement by the well-type ionization chamber were also investigated to provide dosimetric data. A well-type ionization chamber with a single seed holder was used to measure source strength of all 1935 (125)I seeds implanted in the initial 28 patients in our hospital. Physical characteristics including linearity of readings for different integral time intervals, reproducibility, isotropy, and axial positional sensitivity were assessed. To calculate the source strength, the integral charge during 30 s was measured and converted to air kerma strength. The nominal activity stated by the manufacturer was compared with the measured value. Linearity, reproducibility, and isotropy of the well-type ionization chamber were within 0.2%. Measured source strength was on average 2.1% (range -7.6% to +7.2%), lower than the nominal value. Standard deviation of all measured seeds was 2.0%. The maximum difference between the measured and the manufacturer's nominal source strength in each patient was -3.7%. The standard deviation averaged 1.6%. The nominal source strength of the (125)I seeds agreed well with the measured value. Our study can be helpful as guidance for individual (125)I seed source strength measurement.

Health-Related Quality of Life after Permanent Interstitial Brachytherapy for Prostate Cancer

To determine dosimetric risk factors for increased toxicity after permanent interstitial brachytherapy for prostate cancer. Quality of life questionnaires (Expanded Prostate Cancer Index Composite) of 60 and 56 patients were analyzed after a median posttreatment time of 6 weeks (A—acute) and 16 months (L—late). The corresponding CT scans were performed 30 days after the implant. The prostate, rectal wall, and base of seminal vesicles were contoured. Prostate volume, number of seeds and needles as well as dosimetric parameters were correlated with the morbidity scores. For a prostate volume of 38 ± 12 cm3 (mean ± standard deviation), 54 ± 7 125I sources (Rapid Strands®, activity of 22.6 ± 3.0 MBq [0.61 ± 0.08 mCi]) were implanted using 20 ± 6 needles. Improved late urinary function scores resulted from a higher number of sources per cm3 (≥ 1.35). A prostate D90 < 170 Gy (A)/< 185 Gy (L) and base of seminal vesicle D10 < 190 Gy (A and L) were associated with higher urinary function scores. Late rectal function scores were significantly higher for patients with a prostate V200 < 50% and V150 < 75%. Patients with a prostate volume < 40 cm3 reached better sexual function scores (A and L). A higher number of needles per cm3 (≥ 0.5) resulted in improved late urinary, bowel and sexual function scores. Quality of life after a permanent implant can be improved by using an adequate amount of sources and needles. With an increasing number of seeds per cm3, dose homogeneity is improving. A prostate D90 < 170 Gy and a base of seminal vesicle D10 < 190 Gy (as an indicator of the dose to the bladder neck and urethral sphincter) can be recommended to maintain a satisfactory urinary function.

Evaluation of a EMCCD Detector for Emission-Transmission Computed Tomography

A prototype electron multiplying charged coupled device (EMCCD) based gamma camera is evaluated for emission-transmission computed tomography. The detector has an 8.2times8.2 mm2 active sensor area with a 512times512 pixel array, and can be operated with a 3:1 fiber optic taper to expand the effective imaging area to approximately 25times25 mm2 for small animal studies. We demonstrate the possibility of operating the camera in either charge integrating or single photon counting mode for pinhole SPECT with 125I and 99mTc sources. In photon counting mode, we show that energy discrimination can be used to distinguish between these two sources for dual isotope imaging. X-ray CT images of phantoms are taken without a collimator to evaluate the quality of the camera for transmission imaging

SU‐FF‐T‐441: Updated Solid Water™ to Water Conversion Factors for 125I and 103Pd Brachytherapy Sources

Purpose: The updated TG‐43U1 recommends determination of the consensus of low energy brachytherapy source parameters using experimental and Monte Carlo Simulated data. As per this recommendation, the measured parameters in Solid Water™ are converted into water data for clinical application. However, the published conversion factors may have been obtained using Monte Carlo simulations with a cross section files that may have errors for low energy photons, and also for inaccurate chemical composition of the phantom material. In this project impact of different calcium composition in Solid Water™ phantom on dose rate constant, radial dose function of 103Pd and 125I brachytherapy sources have been investigated. Method and Materials: Conversion factor of Solid water™ to water has been determined using MCNP5 Monte Carlo code. In these simulations two different calcium compositions in Solid Water™ of 2.3% (SWa) and 1.7% (SWb) were utilized. Monte Carlo simulations were performed for 125I sources, with and without silver marker, in order to investigate the impact of characteristic x‐rays (emitted by silver marker) on dosimetric parameters. However, the simulation for 103Pd was performed only on one type of the source. Each simulation was run for a total of 20M histories to keep statistical fluctuation less than 0.3%. Results: For 2.3% calcium content phantom material the calculated conversion factor was found to be 1.027 and 1.045 for 125I and 103Pd sources respectively. However, for 1.7% calcium these values were found to be 0.989 and 0.963, respectively. Conclusion: The Solid water‐to‐water conversion factors for 125I and 103Pd sources are found to be 0.989 and 0.963 respectively as compared to the published value of 1.05 and 1.048. In addition, presence of silver marker had no significant impact on conversion factor for I‐125 sources. Moreover, conversion for g(r) was also introduced for clinical application of measured data.

SU‐FF‐T‐356: Permanent Prostate Implants: Dose to Critical Structures Inferior to the Prostate

The basic goal of any radiation therapy is to maximize the dose to the tumor while minimizing dose to normal structure. With permanent prostate implants, treatment plans are developed to spare the urethra and the rectum. Post‐implant evaluation shows that the actual implant does not mimic the plan primarily due to prostate swelling and source position shift either at the time of deposition or post implant. Dose indices have been developed to evaluate dose coverage to the implant. 90% prescription to the 100% of the prostate (V100) is considered an excellent implant. A good execution of the implant plan will still maintain a cooler urethra and low dose to the rectum. It is common to see under‐dose in the base region of the prostate and seeds below the apex of the prostate. Sources below the apex of the prostate result in dose to the external sphincter, corpus cavernosum and internal pudendal artery. At our institution, we routinely obtain an MR for anatomic delineation and a CT for source localization 2 weeks after the implant. The datasets are registered based on mutual information techniques and dose to the normal tissue are reported. Fifteen patient entered in this study received a permanent implant to 145 Gy using I125 sources. The average volume of the MR prostate was 50.7cc. The average V100 was 90.5%. The average max dose to the right and left corpus cavernosum was 26.1(10.6) Gy and 28.1(12.4) Gy, respectively. The average max dose to the right and left pudendal arteries was 39.7(25.2) and 40.9(18.5) Gy, respectively. The average dose to 90% of the external sphincter was 132.1(44.9) Gy. With sufficient follow‐up it is possible to correlate the dose to the vascular structures and their effect on erectile function, and dose to the external sphincter and the effect on urinary function.

MO‐B‐230A‐01: AAPM TG‐43 Update for 2004 and Beyond

Since publication of the 2004 update to the American Association of Physicists in Medicine (AAPM) Task Group No. 43 Report (AAPM TG‐ 43U1), several new low‐energy photon‐emitting brachytherapy sources have become available. Many of these sources have satisfied the AAPM prerequisites for routine clinical use as of January 10th, 2005, and are posted on the Joint AAPM/RPC Brachytherapy Seed Registry. Consequently, the AAPM has prepared this supplement to the 2004 AAPM TG‐43 update. This paper presents the AAPM‐approved consensus datasets for these sources, and includes the following 125I sources: Amersham model 6733, Draximage model LS‐1, Implant Sciences model 3500, IBt model 1251L, IsoAid model IAI‐125A, Mentor model SL‐125/SH‐125, and SourceTech Medical model STM1251. The Best Medical model 2335 103Pd source is also included. While the methodology used to determine these datasets is identical to that published in the AAPM TG‐ 43U1 report, additional information and discussion are presented here on some questions that arose since the publication of the TG‐43U1 report. Specifically details of interpolation and extrapolation methods are described further. Despite these small changes, additions, and clarifications, the overall methodology, the procedures for developing consensus datasets and the dose calculation formalism remain the same as in the TG‐43U1 report. Thus, the AAPM recommends that the consensus datasets and resultant source‐specific dose‐rate distributions included in this supplement be adopted by all end users for clinical treatment planning of low‐energy photon‐emitting brachytherapy sources. Adoption of these recommendations may result in changes to patient dose calculations, and these changes should be carefully evaluated and reviewed with the radiation oncologist prior to implementation of the current protocol.

TU-EE-A1-05: The Use of Directional Interstitial Sources to Reduce Skin Dose in Breast Brachytherapy

Purpose: To investigate the feasibility of reducing the skindose with temporary LDR multicatheter breast implants with the use of directional 125I interstitial sources in comparison to conventional HDR interstitial breast brachytherapy.Method and Materials: The treatment plan for a patient treated with HDR interstitial brachytherapy with 192Ir was compared to a directional 125I treatment plan in the same dataset. Directional sources contain an internal radiation shield that greatly reduces the intensity of radiation in the shielded direction. They have a similar dose distribution to non‐directional sources on the unshielded side. Several dosimetric parameters are compared including target volume coverage, dose homogeneity index, and the skin surface areas receiving 30%, 50% and 80% of the prescription dose (S30, S50 and S80, respectively). The HDR prescription dose was 34 Gy in 10 fractions. Results: Similar excellent target coverage was achieved by both directional LDR and HDR (99.2% and 97.5%, respectively). Moreover, for a 170‐cc target volume, the dose homogeneity index was 0.82 for both LDR and HDR (V100 was 211.4 cc or 225.7 cc, and V150 was 39.1 cc or 40.4 cc, respectively). However, with directional LDR, the following reductions in skindose may be achieved: S30 is reduced from 100.6 cm2 to 62.6 cm2, S50 from 50.6 cm2 to 16.1 cm2, and S80 at 2 cm2 to null. The reduction in V50 for the whole breast is more than 100 cm3 (386.1 cc vs. 489.2 cc). Conclusion: As compared to HDR, directional interstitial 125I sources allow similar dose coverage to the subcutaneous target, while significantly lowering the skindose due to a quicker fall‐off beyond the target. Directional LDR sources can produce a similar dose homogeneity index, but the biological characteristics are more tolerable to the patient and can potentially reduce the risk of late skin and subcutaneous toxicity.

Studies on the production and quality assurance of miniature 125I radioactive sources suitable for treatment of ocular and prostate cancers

125I sources were prepared by adsorption of 125I on palladium-coated silver wires. The effect of reducing agent on percentage adsorption of 125I was studied and the amount of adsorbed activity on source core was studied by repeated adsorption cycles. The activity per source in the sources produced from the same batch varied with coefficient of variation (i.e. the ratio of standard deviation to mean multiplied by 100) less than 10%. The unencapsulated source exhibited low leachability (< 0.01%). The laser parameters were optimized to obtain quality welds with negligible leak rate. The sources were laser-encapsulated in titanium capsules of 0.8 mm (OD)×4.5 mm (l). The release of radioactivity from encapsulated sources in an immersion test at 50 °C for 5 h was <5 nCi (185 Bq). The surface contamination on the sealed capsules was found to be<0.05 nCi or 1.85 Bq per source. The sources were used in the treatment of a child having retinoblastoma.

Polymer gel dosimetry close to an 125I interstitial brachytherapy seed

Despite its advantages, the polymer gel–magnetic resonance imaging (MRI) method has not, as yet, been successfully employed in dosimetry of low energy/low dose rate photon-emitting brachytherapy sources such as 125I or 103Pd interstitial seeds. In the present work, two commercially available 125I seed sources, each of approximately 0.5 U, were positioned at two different locations of a polymer gel filled vial. The gel vial was MR scanned with the sources in place 19 and 36 days after seed implantation. Calibration curves were acquired from the coupling of MRI measurements with accurate Monte Carlo dose calculations obtained simulating the exact experimental setup geometry and materials. The obtained gel response data imply that while linearity of response is sustained, sensitivity (calibration curve slope) is significantly increased (approximately 60%) compared to its typical value for the 192Ir (or 60Co and 6 MV LINAC) photon energies. Water equivalence and relative energy response corrections of the gel cannot account for more than 3–4% of this increase, which, therefore, has to be mainly attributed to physicochemical processes related to the low dose rate of the sources and the associated prolonged irradiation time. The calibration data obtained from one 125I source were used to provide absolute dosimetry results for the other 125I source, which were found to agree with corresponding Monte Carlo calculations within experimental uncertainties. It is therefore suggested that, regardless of the underlying factors accounting for the gel dose response to 125I irradiations, polymer gel dosimetry of new 125I or 103Pd sources should be carried out as originally proposed by Heard and Ibbot (2004 J. Phys.: Conf. Ser. 3 221–3), i.e., by irradiating the same gel sample with the new low dose rate source, as well as with a well-characterized low dose rate source which will provide the dose calibration curve for the same irradiation conditions.

Relative biologic effectiveness in terms of tumor response of 125I implants compared with 60Co gamma rays

To measure the relative biologic effectiveness (RBE) for 125I seeds compared with external beam radiotherapy using a clinically relevant in vivo system. Photon emission from a detailed source model was simulated using the Monte Carlo code MCNP4C, sampling from a 125I spectrum. The mouse RIF-1 tumor was treated with either temporary implant of an 125I seed or with 60Co gamma rays. The tumors were always the same size at the initiation of treatment, and the endpoint was growth inhibition. The dose-response curve for both modalities was close to linear and was independent of the initial 125I activity (dose rate) for the range investigated. Calculation of the RBE for tumor response requires assigning a unique value for the tumor dose that is not homogenous but depends on the distance from the 125I source. Because tumor regrowth will depend on the subpopulation of cells that have the greatest probability of survival (i.e., those at the greatest distance from the 125I source), one approach is to use the dose to this population. On this basis, the RBE for 125I compared with 60Co gamma rays is 1.5. If the 125I dose is computed as the average dose to the tumor, corrected for the dose that is wasted as overkill in the cell population closest to the center of the 125I seed, the RBE is 1.4. The result, an RBE of 1.4-1.5 is similar to findings obtained by other methods, supporting the validity of this approach to derive an RBE with validity in a clinical context.

MO-E-T-618-02: Dosimetry of a Thyroid Uptake Detected in Seed Migration Survey Following a Patient's Iodine-125 Prostate Implant and Measurements of Seed Leakages

Purpose: To improve the quality of prostate brachytherapy. A case of thyroid uptake following an I-125 prostate implant was previously reported. The abstract presents the methods to estimate the dose to thyroid and data of source leakages measured for different iodine-125 seeds. Method and Materials: Seed migration survey is provided to the prostate brachytherapy patients as a quality control procedure. Different implant techniques were evaluated to minimize the seed migration. To estimate the thyroid uptake dose from a seed ruptured during implantation, I-125 source intentional leakage is measured in vitro. A seed is cut open and placed in saline solution. The concentration of I-125 in the solution is measured over a period of 2 months. A leakage curve is obtained. Using MIRD model, the dose to thyroid is calculated. The thyroid counts at different times are used to verify the thyroid uptake dose. Results: Seven of 11 vendors distributing I-125 seeds in North America provided I-125 seeds in our seed leakage investigation. The measured data shows that the leakage half-life of a seed varies from 12 days to infinite for different manufactured seeds. Based on the leakage data and patient measurements, we conclude that one seed was leaking in our thyroid uptake case. The estimated dose to thyroid is 263 cGy. Conclusion: The elimination of seed migration has been achieved and so the improvement of the implant quality. Since more and more prostate cancer patients will be treated by brachytherapy, increased incidences of thyroid uptake are expected. The measured seed leakage data can be used in the estimation of dose to thyroid in case a seed leak is encountered. Conflict of Interest: The project was partially supported by a fund donated by Mr. Malvin Bank. There is no conflict of interest.

Dose verification with Monte Carlo technique for prostate brachytherapy implants with 125I sources

The Monte Carlo technique has been implemented to generate the dose distributions in a model prostate patient, implanted with iodine-125 ( 125I) brachytherapy sources. The results of these calculations are also compared with the dose distributions calculated by a commercially available treatment planning system. The comparison shows that with the source strength suggested by the Monte Carlo technique, the current clinical planning system is found to provide 100% coverage of the prostate with the prescription dose for the same implant pattern. However, the dose-volume histogram of this investigation shows that the VariSeed™ treatment planning system has a 29% and 136% larger dose coverage for the 150% and 200% isodose lines, respectively, than the Monte Carlo simulation. These differences are attributed to the oversimplification of the current planning system using the point source approximation, and also to the interseed effects from multisources that are neglected in the conventional planning systems. The results of this study provide evidence that supports the use of the Monte Carlo technique in treatment planning systems to provide accurate dose calculations in brachytherapy implants.

Relative Biological Effectiveness of103Pd and125I Photons for Continuous Low-Dose-Rate Irradiation of Chinese Hamster Cells

Monolayers of Chinese hamster lung cells (CCL-16) in a polystyrene phantom were irradiated in vitro by 103Pd and 125I sources at dose rates of 6 to 72 cGy/h. Cell survival curves for acute high-dose-rate irradiation (over 30 Gy/h) were also measured using nearly monoenergetic X-ray beams which were designed to simulate the mean energies of photons emitted by 125I and 103Pd and also using a clinical 250 kVp X-ray beam. A profound dose-rate effect is observed over the dose-rate range of 6 to 20 cGy/h. An inverse dose-rate effect was observed for both radionuclides, with its onset occurring at a dose rate of about 20-30 cGy/h. The average RBE of 103Pd relative to 125I was determined to be 1.45 +/- 0.07, 1.41 +/- 0.07, 0.70 +/- 0.07 and 1.49 +/- 0.07 at dose rates of 6.9, 12.6, 19.0 and 26.7 cGy/h, respectively. Because 103Pd implants are generally prescribed at a higher initial dose rate (21 cGy/h) than the corresponding 125I implants (7 cGy/h), the effects of both dose rate and photon energy on biological response must be considered together. For the CCL-16 cells, the RBE of 103Pd at 19.0 cGy/h relative to that of 125I at 6.9 cGy/h was estimated to be 2.3 +/- 0.5.

Extraprostatic Seed Placement and Its Effect on Seed Loss

The purpose of this study was to examine the relationship between extraprostatic seed placement and seed loss in a cohort of patients who had underwent both computed tomographic (CT) and magnetic resonance (MR) scans at day 0 and day 30 following brachytherapy. Twenty-two patients with 1997 AJC clinical stage T1-T2 prostatic carcinoma were implanted with nonstranded I 125. Patients were selected solely by having a prostate volume between 15 and 60 cc and a willingness to return for 30-day follow-up CT and MR scans. The total number of I-125 sources implanted on day 0 ranged from 50 to 104 (median: 70). Preplan treatment planning methods have been previously described in detail: a modified peripheral loading pattern and treatment margins of 5-10 mm were used. Noncontrast postimplantation CT and MR scans were obtained 1-4 hours after implantation on day 0. The total seed count on days 0 and 30 was verified by plain radiograph. Pelvic MR (T1) images were registered with the CT images in the Varian planning system, using bony landmarks. The number of extracapsular seeds in each quadrant of the circumference was then totaled for each patient. A second set of plain radiographs (for seed counting), as well as CT and MR scans, were obtained 30 days after implantation (day 30) and were similarly analyzed. The number of extraprostatic seeds at day 0 ranged from 13 to 35, making up 17%-48% (median: 34%) of the total number implanted. Of the 22 patients, 10 lost one or more seeds between the implantation day and the 1-month follow-up. The mean number of seeds lost was 1.1 (+/- 1.7). There was no apparent relationship between the percent of extraprostatic seeds and the number of seeds lost. There was no apparent relationship between seed loss and number of seeds less than 3 mm or greater than 3 mm from the prostatic capsule. We have shown here that with CT and MR seed localization, extraprostatic seed placement does not appear to substantially increase the likelihood of seed loss after the procedure.

High-dose regions versus likelihood of cure after prostate brachytherapy

To analyze the effect of high-dose regions on biochemical cancer control rates after prostate brachytherapy. Patients with 1997 American Joint Committee on Cancer clinical Stage T1c-T2a prostate carcinoma (Gleason grade 5-6, prostate-specific antigen level 4-10 ng/mL) were randomized to implantation with 125I (144 Gy) vs. 103Pd (125 Gy, National Institute of Standards and Technology 1999). Isotope implantation was performed by standard techniques, using a modified peripheral loading pattern. Of the 313 patients entered in the protocol, 270 were included in this analysis. The 125I source strength ranged from 0.4 to 0.89 mCi (median, 0.55 mCi), and the 103Pd source strength ranged from 1.3 to 1.6 mCi (median, 1.5 mCi). CT was performed within 4 h after implantation. The dosimetric parameters analyzed included the percentage of the postimplant prostate volume covered by the 100%, 150%, 200%, and 300% prescription dose (V100, V150, V200, and V300, respectively). The median time to the last follow-up for patients without failure was 2.7 years. Freedom from biochemical failure was defined as a serum prostate-specific antigen level of < or =0.5 ng/mL at last follow-up. Patients were censored at last follow-up if their serum prostate-specific antigen level was still decreasing. The mean V100, V150, V200, and V300 value was 90% (+/-8%), 63% (+/-14), 35% (+/-13%), and 14% (+/-7%), respectively. Patients with a V100 of > or =90% had a 3-year freedom from biochemical failure rate of 96% vs. 87% for those with a V100 of <90% (p=0.0029). Overall, patients with more high-dose regions had a greater chance of biochemical control. However, when only patients with a V100 of > or =90% were analyzed, no relationship was found between higher dose regions and the likelihood of cancer control. This lack of effect on biochemical control was apparent for both isotopes. High-dose regions do not appear to affect cancer control rates, as long as >90% of the prostate volume is covered by the prescription dose.

Estimation of a method detection limit for an in vivo XRF arsenic detection system

An x-ray fluorescence measurement system has been developed with an 125I source to detect arsenic in superficial layers of phantoms and tissue. Based on in vivo measurements, in conjunction with Monte Carlo simulations, the detection limit for arsenic in skin ranges between 2.6 ± 0.5 and 5.7 ± 1.1 µg g−1, depending on skin thickness and assuming that arsenic is uniformly distributed in the skin. The effect of skin arsenic distribution was also examined.