Biomedical subjects

B R Thomadsen

Publications and source records attributed to B R Thomadsen.

At least 19 recordsLinked to original sources

PubMed2006

A new internal pair production branching ratio of 90Y: the development of a non-destructive assay for 90Y and 90Sr.

(90)Y is utilized as a therapeutic radioisotope in radiolabeled monoclonal antibodies and in microspheres for targeted radiation therapy of the liver. Currently, the widely used dose calibrator assay of (90)Y can have uncertainties exceeding +/-10%. A non-destructive assay using spectroscopy is possible by reducing the currently published uncertainty (+/-12%) in the internal pair production branching ratio for the 0(+)-0(+) transition of (90)Zr. A high-purity germanium detector was used to determine the branching ratio to be (31.86+/-0.47) x 10(-6).

Calibration↗

PubMed2001

An iterative sequential mixed-integer approach to automated prostate brachytherapy treatment plan optimization.

Conventional treatment planning for interstitial prostate brachytherapy is generally a 'trial and error' process in which improved treatment plans are generated by iteratively changing, via expert judgement, the configuration of sources within the target volume in order to achieve a satisfactory dose distribution. We have utilized linear mixed-integer programming (MIP) and the branch-and-bound method, a deterministic search algorithm, to generate treatment plans. The rapidity of dose falloff from an interstitial radioactive source requires fine sampling of the space in which dose is calculated. This leads to a large and complex model that is difficult to solve as a single 3D problem. We have therefore implemented an iterative sequential approach that optimizes pseudo-independent 2D slices to achieve a fine-grid 3D solution. Using our approach, treatment plans can be generated in 20-45 min on a 200 MHz processor. A comparison of our approach with the manual 'trial and error' approach shows that the optimized plans are generally superior. The dose to the urethra and rectum is usually maintained below harmful levels without sacrificing target coverage. In the event that the dose to the urethra is undesirably high, we present a refined optimization approach that lowers urethra dose without significant loss in target coverage. An analysis of the sensitivity of the optimized plans to seed misplacement during the implantation process is also presented that indicates remarkable stability of the dose distribution in comparison with manual treatment plans.

Algorithms↗

PubMed2001

Tissue mimicking materials for a multi-imaging modality prostate phantom.

Materials that simultaneously mimic soft tissue in vivo for magnetic resonance imaging (MRI), ultrasound (US), and computed tomography (CT) for use in a prostate phantom have been developed. Prostate and muscle mimicking materials contain water, agarose, lipid particles, protein, Cu++, EDTA, glass beads, and thimerosal (preservative). Fat was mimicked with safflower oil suffusing a random mesh (network) of polyurethane. Phantom material properties were measured at 22 degrees C. (22 degrees C is a typical room temperature at which phantoms are used.) The values of material properties should match, as well as possible, the values for tissues at body temperature, 37 degrees C. For MRI, the primary properties of interest are T1 and T2 relaxations times, for US they are the attenuation coefficient, propagation speed, and backscatter, and for CT, the x-ray attenuation. Considering the large number of parameters to be mimicked, rather good agreement was found with actual tissue values obtained from the literature. Using published values for prostate parenchyma, T1 and T2 at 37 degrees C and 40 MHz are estimated to be about 1,100 and 98 ms, respectively. The CT number for in vivo prostate is estimated to be 45 HU (Hounsfield units). The prostate mimicking material has a T1 of 937 ms and a T2 of 88 ms at 22 degrees C and 40 MHz; the propagation speed and attenuation coefficient slope are 1,540 m/s and 0.36 dB/cm/MHz, respectively, and the CT number of tissue mimicking prostate is 43 HU. Tissue mimicking (TM) muscle differs from TM prostate in the amount of dry weight agarose, Cu++, EDTA, and the quality and quantity of glass beads. The 18 microm glass beads used in TM muscle increase US backscatter and US attenuation; the presence of the beads also has some effect on T1 but no effect on T2. The composition of tissue-mimicking materials developed is such that different versions can be placed in direct contact with one another in a phantom with no long term change in US, MRI, or CT properties. Thus, anthropomorphic phantoms can be constructed.

Adipose Tissue↗

PubMed2001

The water-equivalence of phantom materials for 90Sr-90Y beta particles.

Intravascular brachytherapy requires that the dose be specified within millimeters of the source. High dose gradients near brachytherapy sources require that the source-detector distance be accurately known for dosimetry purposes. Solid phantoms can be designed to accommodate these stringent requirements. This study reports dosimeter readings from 90Sr-90Y sources measured in water, A150, polystyrene and in an epoxy-based water-equivalent plastic. Measurements showed that while A150 and the epoxy-based plastic agreed well with water when the surface of the source contacted the detector housing, the relative response in the phantoms decreased with increasing depth in phantom, falling to approximately 0.55 those of water at a depth of 5 mm. Readings in polystyrene were within 4% of those in water between 1 and 2 mm depth. However, while polystyrene followed water more closely than the other two materials, at greater depths the relative response in polystyrene to water varied from 0.65 to 1.34. When the density of the materials is accounted for, the relative response in A150 is nearly constant with increasing areal density. Furthermore, the response in A150 shows the closest agreement with that in water of any of the solid materials for higher areal densities. For values below 0.3 g/cm2, polystyrene shows the closest agreement with water.

Angioplasty↗

PubMed2001

Comparison of two planning systems for HDR brachytherapy gynecological application.

PURPOSE: This report compares the Nucletron NPS and PLATO planning system for patients treated for cervix cancer. MATERIALS AND METHODS: This study compares calculations generated using the older NPS (version 11.43) planning system and the more recent PLATO (version 14.1) system for two cases: 1) a single dwell position and 2) an actual patient application using a tandem and ovoid. RESULTS: For one dwell position: for NPS planning the dose for points along the source axis forward of the cable was 9.85% more than for symmetrically placed points in the cable direction. For PLATO, the same test gave rise to a difference of 10.2%. Comparing the two systems, NPS calculated doses for points in the forward direction 14% greater than those calculated by PLATO. The entry of points using the digitizer accounted for less than 1% of any difference. For the patient case: the dose difference between NPS and PLATO planning for all patient reference points entered from films ranged from 1 to 4%. The difference in dose between optimized and nonoptimized planning was approximately 0.5% for prescription points (points A), while for the bladder and rectum the differences were 6% and 20%, respectively with NPS, and with PLATO, 8% and 22%, respectively. CONCLUSION: This study highlighted the effects of the differences in the calculational algorithm between the older and newer planning systems from Nucletron. While the differences were minimal on the perpendicular bisector of the source, along the axis they become considerable. In a practical gynecological case, these differences mostly affect the dose to the rectum, since that organ receives the greatest proportion of its dose from rays near the same axis. Overall, the PLATO system plan required about 2.5% less integrated reference air kerma than the NPS plan for the same dose to point A. For either planning system, optimization is crucial in decreasing dose to bladder and rectal points.

Algorithms↗

PubMed2000

Radiation injury from x-ray exposure during brachytherapy localization.

Two patients developed skin ulcers secondary to high doses of diagnostic-energy x rays received during localization procedures as part of brachytherapy treatments. Both were morbidly obese and diabetic. The obesity led to the delivery of estimated skin doses of 83 Gy in one case and 29 Gy in the other in attempts to produce readable images on localization radiographs. This report discusses the factors leading to the injuries, the progression of the injuries over time, and the variables involved in the localization procedures with the aim of preventing future mishaps. The greatest contribution to the large skin dose was the need, with the equipment available, to use multiple exposures to produce a single film, because of the effect of the resultant reciprocity failure.

Brachytherapy↗

PubMed1999

Measurements of head-scatter factors with cylindrical build-up caps and columnar miniphantoms.

Head-scatter factors, Sh, also referred to as output factors, are measured in-air with an ion chamber and a semiconductor diode fitted with cylindrical build-up caps and columnar miniphantoms fabricated from materials of different atomic number. Sh increases with field size less rapidly when cylindrical build-up caps are constructed from high atomic number materials. This is a consequence of a net scatter of contamination electrons away from the detector. Ion chambers and diodes give identical results when the same type of build-up caps are used. Contamination electrons can be avoided by the use of columnar miniphantoms that have sufficient wall thickness in the radial direction. This radial wall thickness is characterized in this work for 6, 10, and 18 MV x-ray beams. Sh increases with field size less rapidly when columnar miniphantoms are constructed from high atomic number materials. This is due to the decrease in the average energy of photons at large field sizes. It is concluded that to obtain Sh for dosimetry in water, cylindrical build-up caps and columnar miniphantoms should be constructed from material with an atomic number close to that of water.

Calibration↗

PubMed1998

A solid water pelvic and prostate phantom for imaging, volume rendering, treatment planning, and dosimetry for an RTOG multi-institutional, 3-D dose escalation study. Radiation Therapy Oncology Group.

PURPOSE: With increased interest in 3-D conformal radiation therapy and dose escalation, it is necessary to provide advanced techniques to assure quality in treatment delivery. Multi-institutional trials for these newer treatment techniques require methods of verifying the consistency of treatments between the participating institutions. For this reason, a phantom was designed to address the quality and consistency of Radiation Therapy Oncology Group (RTOG) 3-D prostate treatment protocol. METHODS AND MATERIALS: A solid water pelvic and prostate phantom for imaging, volume rendering, treatment planning, and dosimetry applications for performing comprehensive quality assurance has been designed and fabricated. Its configuration was based upon CT slices obtained from a patient study. Individual slices were machined with corresponding contours of the prostate, bladder, rectum, and the left and right femurs. Most of the phantom is made of solid water (Gammex/RMI, Middleton, WI), while the femurs are made of bone-equivalent material. The CT numbers from patient images were used to adjust the solid water composition within the organ volumes, providing image contrast from the remainder of the phantom. Cylindrical insertion grooves are machined in the phantom to allow placement of ionization chambers and thermal luminal dosimeters (TLDs) for dosimetry applications. During imaging, the cavities are filled with rods fabricated from solid water material. RESULTS: The phantom is being used to evaluate the consistency of a range of processes in radiation therapy simulation, planning, and delivery of 3-D-based treatments for prostate cancer. CONCLUSION: The ultimate study objective is to use the phantom to evaluate the accuracy and consistency of treatments delivered by institutions participating in national collaborative clinical trials involving 3-D conformal dose escalation.

Humans↗

PubMed1998

High dose-rate brachytherapy treatment delivery: report of the AAPM Radiation Therapy Committee Task Group No. 59.

The goals of this task group are to examine the current high dose-rate (HDR) treatment delivery practices and to prepare a document to assure safe delivery of HDR treatments. The document consists of detailed HDR procedures for design of an HDR brachytherapy program, staffing and training, treatment specific quality assurance, and emergency procedures. The document provides an extensive quality assurance (QA) check list. It reviews all aspects of HDR treatment delivery safety, including prescription, treatment plan, treatment delivery, and radiation safety.

Brachytherapy↗

PubMed1997

An investigation of tomotherapy beam delivery.

Experimental simulations for tomotherapy beam delivery were performed using a computer-controlled phantom positioner, a cylindrical phantom, and a 6 MV x-ray slit beam. Both continuous helical beam and sequential segmented tomotherapy (SST) beam deliveries were evaluated. Beam junctioning problem due to couch indexing error or field width errors presented severe dose uniformity perturbations for SST, while the problem was minimized for helical beam delivery. Longitudinal breathing motions were experimentally simulated for helical and SST beam delivery. While motions reduced the dose uniformity perturbations for SST, small artifacts in dose uniformity can be introduced for helical beam delivery. With typical breath frequency and magnitude, for a slit beam of 2.0 cm width at 4 rpm, the dose uniformity perturbation was not significant. A running start/stop technique was implemented with helical beam delivery to sharpen the 20%-80% longitudinal dose fall-off from 1.5 to 0.5 cm. The latter was comparable to the corresponding dose penumbra of a conventional 6 MV 10 x 10 cm2 field. All together, helical beam delivery showed advantages over SST for tomotherapy beam delivery under similar delivery conditions.

Biophysical Phenomena↗

PubMed1997

Innovations and advances in brachytherapy.

The treatment of cancer with radioactive sources using topical molds and intracavitary radium showed the efficacy of brachytherapy long before external radiation therapy became common practice. Brachytherapy is progressing in several clinical directions that reflect technological innovation as well as changes in medical care which combine to provide significant advances in the practice of brachytherapy today. A major change in brachytherapy, the development of high intensity sources, promoted clinical and technologic innovations in high dose-rate brachytherapy. Pulse dose-rate brachytherapy is the use of these sources to treat a target by using pulses of radiation for several minutes each hour with the same overall time and total dose as low dose-rate treatment. Recent novel approaches of using radiotherapy for prevention of restenosis in coronary and peripheral vessels has interested laboratory and clinical researchers. Most of the work toward new source material for brachytherapy centers around intravascular applications. Technology to support brachytherapy continues to advance rapidly. The interaction between images from diverse modalities and the various phases of the treatment planning is the basis of enhanced treatment planning procedures. Optimization in brachytherapy entails calculating the source strengths or source dwell times to satisfy a set of dose criteria to achieve the best dose distribution for a brachytherapy implant. Progress in computerization and calculation modeling has promoted this technical advance. Education and training in brachytherapy indications and techniques is mandatory to maintain high standards of brachytherapy practice.

Animals↗

PubMed1995

The adverse effect of treatment prolongation in cervical carcinoma.

PURPOSE: Proliferation of surviving tumor clonogens during a course of protracted radiation therapy may be a cause of local failure in cervical carcinoma. The effect of total treatment time was analyzed retrospectively in relation to pelvic control and overall survival for squamous cell carcinomas of the uterine cervix. METHODS AND MATERIALS: Two hundred and nine patients (Stage IB-IIIB) treated with a combination of external beam and low dose rate intracavitary irradiation were evaluable for study. Multivariate analysis and Kaplan-Meier statistical methods were used to determine the effect of treatment time on pelvic control and survival at 5 years. RESULTS: The median treatment duration was 55 days. For all stages combined, the 5-year survival and pelvic control rates were significantly different with treatment times < 55 days vs. > or = 55 days: 65 and 54% (p = 0.03), 87 and 72% (p = 0.006), respectively. By stage, a shorter treatment duration (i.e., < 55 days vs. > or = 55 days) was significant for 5-year overall survival and pelvic control for Stages IB/IIA and III, but not for Stage IIB: Stage IB/IIA (81 and 67%, 96 and 84%), Stage III disease (52 and 42%, 76 and 55%) and Stage IIB (43 and 50%, 74 and 80%, respectively). Survival decreased 0.6%/day and pelvic control decreased 0.7%/day for each additional day of treatment beyond 55 days for all stages of disease. Additionally, significant late complications were not influenced by treatment time. CONCLUSION: These results suggest that prolongation of treatment time is associated with decreased local control and survival in patients with cervical carcinoma. This is consistent with emerging data from other institutions. Therapeutic implications include avoidance of unnecessary treatment breaks, the design of fractionation schemes that decrease treatment duration, and possibly the use of tumor cytostatic drugs during conventional radiation.

Brachytherapy↗

PubMed1995

Postoperative vaginal cuff irradiation using high dose rate remote afterloading: a phase II clinical protocol.

PURPOSE: In September 1989, a postoperative Phase II high dose rate (HDR) brachytherapy protocol was started for International Federation of Gynecology and Obstetrics (FIGO) Stage I endometrial adenocarcinoma. This review reports the overall survival, local control, and complication rates for the initial 63 patients treated in this Phase II study. METHODS AND MATERIALS: High dose rate brachytherapy was delivered using an Iridium-192 HDR remote afterloader. Sixty-three patients were entered into the Phase II protocol, each receiving two vaginal cuff treatments 1 week apart (range 4-12 days) with vaginal ovoids (diameter 2.0-3.0 cm). No patient received adjuvant external beam radiation. A dose of 32.4 Gy in two fractions was prescribed to the ovoid surface in 63 patients. The first three patients treated at our institution received 15, 16.2, and 29 Gy, respectively, to determine acute effects. RESULTS: At a median follow-up of 1.6 years (range 0.75-4.3 years) no patient has developed a vaginal cuff recurrence. One regional recurrence (1.6%) occurred at 1.2 years at the pelvic side wall. This patient is alive and without evidence of disease 7 months after completion of salvage irradiation, which resulted in the only vaginal stenosis (1.6%). Fourteen patients (22%) experienced vaginal apex fibrosis by physical exam, which was clinically symptomatic in four patients. Two patients reported stress incontinence; however, these symptoms were noted prior to their HDR therapy. One patient died 2.4 years after HDR therapy due to cardiovascular disease without evidence of cancer at autopsy. CONCLUSION: Preliminary results of our phase II HDR vaginal cuff protocol for postoperative FIGO Stage IA, Grade 3 or Stage IB, Grade 1-2 patients demonstrate that 32.4 Gy in two fractions is well tolerated by the vaginal cuff mucosa. Local control appears comparable to our prior experience and others with low dose rate (LDR) brachytherapy. Additional patient accrual and further follow-up will better determine the late morbidity, local control, and overall survival of these patients.

Adult↗

PubMed1995

Impact of "optimized" treatment planning for tandem and ring, and tandem and ovoids, using high dose rate brachytherapy for cervical cancer.

PURPOSE: Different treatment techniques are used in high dose rate (HDR) remote afterloading intracavitary brachytherapy for uterine cervical cancer. We have investigated the differences between "optimized" and "nonoptimized" therapy using both a tandem and ring (T/R) applicator, and a tandem and ovoids (T/O), applicator. METHODS AND MATERIALS: HDR afterloading brachytherapy using the Madison System for Stage IB cervical cancer was simulated for 10 different patients using both a T/R applicator and a T/O applicator. A treatment course consists of external beam irradiation and five insertions of HDR afterloading brachytherapy. Full dosimetry calculations were performed at the initial insertion for both applicators and used as a reference for the following four insertions of the appropriate applicator. Forty dosimetry calculations were performed to determine the dose delivered to Point M (similar to Point A), Point E (obturator lymph nodes), vaginal surface, bladder, and rectum. "Optimized" doses were specified to Point M and to the vaginal surface. "Nonoptimized" doses were specified to Point M only. Using the linear-quadratic equation, calculations have been performed to convert the delivered dose using HDR to the biologically equivalent doses at the conventional low dose rate (LDR) at 0.60 Gy/h. RESULTS: Major differences between "optimized" and "nonoptimized" LDR equivalent doses were found at the vaginal surface, bladder, and rectum. Overdoses at the vaginal surface, bladder, and rectum were calculated to be 208%, nil, and 42%, respectively, for the T/R applicator with "nonoptimization." However, for the T/O applicator, the overdoses were smaller, being nil, 32%, and 27%, respectively, with "nonoptimization." CONCLUSION: Doses given in high dose rate intracavitary brachytherapy border on tissue tolerance. "Optimization" of either applicator decreases the risk of a dose that may have potential for complications. Optimization of a tandem and ovoids best ensures that the doses are not diminished at the treatment sites, and that the potential for overdose is reduced.

Brachytherapy↗

PubMed1995

Quality assurance tool for high dose rate brachytherapy.

An insert for a well ionization chamber is described for quality assurance tests for high dose rate remote afterloaders. The use of this insert over time has shown its applicability for efficient QA tests for the high dose rate afterloader. The measurement results are six times greater than the uncertainties involved for the measurements. Checks on position can be made better than 1 mm and dwell times better than 1 s. The overall measurements can be done to better than 1.5%. QA checks can be accomplished in a shorter time period with as great or better accuracy than prior film techniques.

Brachytherapy↗

PubMed1995

Intercomparison of normalized head-scatter factor measurement techniques.

Normalized head-scatter factors were measured with cylindrical beam coaxial miniphantoms and high purity graphite buildup caps for 4-, 6-, 10-, and 24-MV photon beams at field sizes from 4 x 4 to 40 x 40 cm2. The normalized head-scatter factors determined by the two methods matched well for 4- and 6-MV photon beams. The miniphantom technique produced normalized head-scatter factors 1.5% and 4.8% lower than the buildup caps for the 10- and 24-MV beams for large field sizes, respectively. At small field sizes, the miniphantom technique produced larger normalized head-scatter factors than the buildup caps. Measurements made with an electromagnet indicate that a significant portion of the ionization measured in the buildup cap at 24 MV arises from contamination electrons. Measurements made with the miniphantom and magnet found no contamination electron contribution. The miniphantom technique may exclude such contamination electrons, potentially leading to inaccuracies in tissue-maximum ratios and phantom scatter factors, as well as inaccuracies in monitor unit calculations.

Humans↗

PubMed1994

A comparison of the efficacy and complication rates of low dose-rate versus high dose-rate brachytherapy in the treatment of uterine cervical carcinoma.

PURPOSE: To compare the outcome and complication rates for treatment of uterine cervical carcinoma with low dose-rate (LDR) vs. high dose-rate (HDR) brachytherapy at the University of Wisconsin Comprehensive Cancer Center (UWCCC). METHODS AND MATERIALS: One-hundred ninety-eight evaluable patients with cervical carcinoma, Stages IB to IIIB, treated with curative intent with a combination of megavoltage teletherapy and LDR brachytherapy from 1977 to 1988 were the subject of an initial review. In 1989, a HDR treatment program was initiated where all patients with cervical carcinoma were subsequently treated with a combination of HDR brachytherapy and teletherapy. Using the linear-quadratic model (LQ), the dose and schedule of HDR brachytherapy and teletherapy were designed to give similar tumor control and late effects as LDR therapy. Technically, the HDR schedule required meticulous attention to treatment geometry to limit severe late effects. Forty patients treated with the HDR program with 2-4 year follow-up were reviewed and compared to the previous LDR patient group. The LDR and HDR treatment groups were comparable with regards to age, weight, stage distribution, bulk of disease, and histology. RESULTS: No significant difference in survival was found between the LDR and HDR groups with 3-year actuarial overall survival being 66% and 77%, respectively. Three-year actuarial pelvic control rates were similar at 80% and 77% for the LDR and HDR groups, respectively. No significant difference in late treatment complications requiring hospitalization or surgery was found between the two treatment groups with a complication rate of 10% (20/198) for the LDR patients and 2.5% (1/40) for the HDR patients. CONCLUSION: As predicted by our LQ calculations, treatment results for LDR and HDR brachytherapy were similar with respect to survival, pelvic control and late complications in the treatment of cervical carcinoma. The HDR brachytherapy program at the UWCCC appears to be a safe and effective alternative to LDR therapy in the treatment of cervical carcinoma.

Adolescent↗

PubMed1994

A practical alternative to conventional five-field irradiation postmastectomy for locally advanced breast cancer.

A combination of electron and photon beams has been used as an alternative for the conventional five-field method to irradiate patients postmastectomy for locally advanced breast cancer. Anterior and posterior opposed photon beams treat in continuity the lateral chest wall, axilla, and supraclavicular lymph nodes. An adjacent anterior electron beam is used at an energy matched to the depth of the internal mammary nodes. It includes the anterior chest wall, but bolus is used in the lateral aspect to spare underlying lung. This electron beam eliminates the diverging junction between the internal mammary and medial tangential fields used in the conventional five-field technique. Overlaps along the junction between the photon and electron beams are minimized by placing the center of the photon field along its medial border. Measurements with an Alderson-Rando phantom show dose-distribution advantages for this technique over the conventional five-field approach. There is less chance of underdosing tumor cells or of overdosing normal tissue along beam junctions. Clinical studies on 29 patients treated by this technique between July 1985 and December 1989 show increased rates of acute skin reactions, but otherwise similar side effects compared with 57 breast cancer patients treated with the five-field technique over the same time period. Local recurrence rates and patient survival rates were similar for the two groups. Given the dose-distribution advantages of this technique and its simple adaptation to accommodate unusual surgical scars or cancer recurrences, its use should be considered for postmastectomy patients with locally advanced breast cancer in well-equipped cancer treatment centers.

Breast Neoplasms↗