PubMed Health⌕ Search

Biomedical subjects

Timothy D Solberg

Publications and source records attributed to Timothy D Solberg.

29 records · Page 2Linked to original sources
PubMed2003

Segmental and dynamic intensity-modulated radiotherapy delivery techniques for micro-multileaf collimator.

A leaf sequencing algorithm has been implemented to deliver segmental and dynamic multileaf collimated intensity-modulated radiotherapy (SMLC-IMRT and DMLC-IMRT, respectively) using a linear accelerator equipped with a micro-multileaf collimator (mMLC). The implementation extends a previously published algorithm for the SMLC-IMRT to include the dynamic MLC-IMRT method and several dosimetric considerations. The algorithm has been extended to account for the transmitted radiation and minimize the leakage between opposing and neighboring leaves. The underdosage problem associated with the tongue-and-groove design of the MLC is significantly reduced by synchronizing the MLC leaf movements. The workings of the leaf sequencing parameters have been investigated and the results of the planar dosimetric investigations show that the sequencing parameters affect the measured dose distributions as intended. Investigations of clinical cases suggest that SMLC and DMLC delivery methods produce comparable results with leaf sequences obtained by root-mean-square (RMS) errors specification of 1.5% and lower, approximately corresponding to 20 or more segments. For SMLC-IMRT, there is little to be gained by using an RMS error specification smaller than 2%, approximately corresponding to 15 segments; however, more segments directly translate to longer treatment time and more strain on the MLC. The implemented leaf synchronization method does not increase the required monitor units while it reduces the measured TG underdoses from a maximum of 12% to a maximum of 3% observed with single field measurements of representative clinical cases studied.

Algorithms↗
PubMed2003

Patient specific quality assurance for the delivery of intensity modulated radiotherapy.

A patient specific quality assurance program has been developed to facilitate the clinical implementation of intensity modulated radiotherapy (IMRT) delivered using a micro-multileaf collimator. The methodology includes several dosimetric tasks that are performed prior to the treatment of each patient. Film dosimetry is performed for each individual field and for the multifield composite plan. Individual field measurements are performed at a depth of 5 cm in a water equivalent slab phantom; export of dose calculations from the treatment planning system is similarly specified. For the composite distribution, parameters from the patient plan are applied to an IMRT phantom, and film is exposed in an axial orientation. Distributions are compared with the aid of software developed for the specific tasks. The measured and calculated dose distributions can be superimposed and positioned graphically using move, rotate, and mirror tools, as well as by specifying isocenter coordinates and using fiducial marks. Horizontal and vertical profiles are available for analysis. Dose difference, distance-to-agreement, and gamma index, the minimum scaled multidimensional distance between a measurement and a calculation point determined in combined dose and physical distance space, are calculated along a specified isodose line and displayed. gamma provides an excellent measure of disagreement between measurement and calculation for complex intensity distributions. We specify 3% dose difference and 3 mm distance as our scaling acceptability criteria. Absolute dosimetry for each composite plan is performed using an ionization chamber. To date, excellent agreement between measurements and calculations has been observed.

Humans↗
PubMed2003

A seed specific dose kernel method for low-energy brachytherapy dosimetry.

We describe a method for independently verifying the dose distributions from pre- and post-implant brachytherapy source distributions. Monte Carlo calculations have been performed to characterize the three-dimensional dose distribution in water phantom from a low-energy brachytherapy source. The calculations are performed in a voxelized, Cartesian coordinate geometry and normalized based upon a separate Monte Carlo calculation for the seed specific air-kerma strength to produce an absolute dose grid with units of cGy hr(-1) x U(-1). The seed-specific, three-dimensional dose grid is stored as a text file for processing using a separate visual basic program. This program requires the coordinate positions of each seed in the pre- or post-plan and sums the kernel file for a three-dimensional composite dose distribution. A kernel matrix size of 81x81x81 with a voxel size of 1.0x1.0x1.0 mm3 was chosen as a compromise between calculation time, kernel size, and truncation of the stored dose distribution as a function of radial distance from the midpoint of the seed. Good agreement is achieved for a representative pre- and post-plan comparison versus a commercial implementation of the TG-43 brachytherapy dosimetry protocol.

Algorithms↗
PubMed2003

Stereotactic radiotherapy for the treatment of lymphocytic hypophysitis. Report of two cases.

Lymphocytic hypophysitis is a rare inflammatory disorder of the pituitary gland. Standard therapy consists of transsphenoidal resection or oral administration of corticosteroid medications. Two patients with symptomatic lymphocytic hypophysitis, which recurred after standard therapy, were treated with low-dose stereotactic radiotherapy. On imaging studies both lesions demonstrated a response to radiation and each patient experienced relief of symptoms. There has been no adverse sequela of the radiation treatment. The authors conclude that stereotactic radiotherapy represents an effective, noninvasive treatment option for patients with lymphocytic hypophysitis, particularly if the disease is recurrent after surgery or resistant to corticosteroid medications.

Aged↗
PubMed2002

Investigations of a minimally invasive method for treatment of spinal malignancies with LINAC stereotactic radiation therapy: accuracy and animal studies.

PURPOSE: A new method for stereotactic irradiation of spinal malignancies is presented, with evaluations of the theoretic and practical limitations of localization accuracy and the implementation of the method in swine. MATERIALS AND METHODS: In a percutaneous procedure, a minimum of three small (1.7-mm-diameter) titanium markers are permanently affixed to a vertebra. Markers are localized on biplanar radiographs while isocenter positions are determined on CT. An external fiducial frame defines a three-dimensional coordinate system through the patient. Radiographs coupled with a rigid body rotation algorithm account for daily differences in patient position. Phantom studies were used to verify theoretic uncertainty calculations from a simulation program. A swine model was used to evaluate the difficulty and duration of the implant technique, the suitability of the vertebral process as an implant site, vertebral motion due to normal respiration, and the ability to target one vertebra with markers in an adjacent vertebra. RESULTS: Theoretic accuracy studies confirmed that localization accuracy is a function of marker separation. Phantom studies involving 296 measurements showed that individual implants could be localized within +/-0.25 mm. The largest targeting error observed in 3,600 measurements of 100 implant configurations was 1.17 mm. The implant procedure took 5-10 minutes per site. No significant migration of implants was observed up to 35 days postimplantation, and respiratory motion had no detectable influence on vertebral position. Adjacent vertebrae may be useful for targeting one another with a small sacrifice in localization accuracy. CONCLUSIONS: The use of implanted markers for localization of spinal malignancies has potential for applications in stereotactic radiotherapy. Phantom measurements suggest that localization accuracy similar to intracranial stereotactic radiotherapy techniques is achievable. Swine studies suggest that the implant technique is expedient and feasible for tumor targeting purposes.

Animals↗
PubMed2002

Dosimetric parameters for three low-energy brachytherapy sources using the Monte Carlo N-particle code.

Monte Carlo calculations have been performed for the purpose of characterizing the dosimetric properties of three brachytherapy sources. The three sources are manufactured by Syncor Pharmaceuticals Inc. and consist of two 125I seeds (BT-125-1 and BT-125-2) and one 103Pd seed (BT-103-3). The BT-125-1 and BT-125-2 seed consists of a solid palladium and silver core, respectively. A thin layer (0.5 microm) of 125I is adsorbed onto the solid core for each seed and encased within a titanium housing. The BT-103-3 seed consists of a central gold marker and four resin balls encased within a titanium housing. A thin layer of 103Pd is bonded onto each resin ball. The dosimetric properties, including the dose rate constant, radial dose function, and anisotropy were calculated in water according to the TG-43 protocol using the Monte Carlo N-Particle code. The dose rate constant was calculated to be 0.955+/-0.005 and 0.967+/-0.005 cGyh(-1) U(-1) for the BT-125-1 and BT- 125-2 seeds, respectively. A dose rate constant of 0.659+/-0.005 cGy h(-1) U(-1) was calculated for the BT-103-3 seed. Radial dose function, g(r), calculated to a distance of 10 cm, and an isotropy function, F(r, theta), calculated for radii from 0.5 to 7.0 cm, were found to be in close agreement with previously published data.

Anisotropy↗
PubMed2002

An evaluation of gating window size, delivery method, and composite field dosimetry of respiratory-gated IMRT.

A respiratory gating system has been developed based on a commercial patient positioning system. The purpose of this study is to investigate the ability of the gating system to reproduce normal, nongated IMRT operation and to quantify the errors produced by delivering a nongated IMRT treatment onto a moving target. A moving phantom capable of simultaneous two-dimensional motion was built, and an analytical liver motion function was used to drive the phantom. Studies were performed to assess the effect of gating window size and choice of delivery method (segmented and dynamic multileaf collimation). Additionally, two multiple field IMRT cases were delivered to quantify the error in gated and nongated IMRT with motion. Dosimetric error between nonmoving and moving deliveries is related to gating window size. By reducing the window size, the error can be reduced. Delivery error can be reduced for both dynamic and segmented delivery with gating. For the implementation of dynamic IMRT delivery in this study, dynamic delivery was found to generate larger delivery errors than segmented delivery in most cases studied. For multiple field IMRT delivery, the largest errors were generated in regions where high field modulation was present parallel to the axis of motion. Gating was found to reduce these large errors to clinically acceptable levels.

Artifacts↗
PubMed2002

A Monte Carlo based phase space model for quality assurance of intensity modulated radiotherapy incorporating leaf specific characteristics.

Dose calculations for intensity modulated radiation therapy (IMRT) require an accurate description of the radiation field defined by the multileaf collimator. A previously developed Monte Carlo phase space model has been modified to provide accurate dose verification for IMRT treatments on a Novalis linear accelerator. We have incorporated into the model the effects of the multileaf collimator geometry, including leaf transmission, interleaf leakage, the rounded leaf tips and the effects of leaf sequencing, as well as the beam divergence and energy variation across the field. The modified source model was benchmarked against standard depth dose and profile measurements, and the agreement between the calculation and measurement is within the AAPM Task Group No. 53 criteria for all benchmark fields used. Film dosimetry was used to evaluate the model for IMRT sequences and plans, and the ability of the model to account for leaf sequencing effects is also demonstrated.

Algorithms↗
PubMed2002

Dosimetric parameters of three new solid core I-125 brachytherapy sources.

Monte Carlo calculations and TLD measurements have been performed for the purpose of characterizing dosimetric properties of new commercially available brachytherapy sources. All sources tested consisted of a solid core, upon which a thin layer of 125I has been adsorbed, encased within a titanium housing. The PharmaSeed BT-125 source manufactured by Syncor is available in silver or palladium core configurations while the ADVANTAGE source from IsoAid has silver only. Dosimetric properties, including the dose rate constant, radial dose function, and anisotropy characteristics were determined according to the TG-43 protocol. Additionally, the geometry function was calculated exactly using Monte Carlo and compared with both the point and line source approximations. The 1999 NIST standard was followed in determining air kerma strength. Dose rate constants were calculated to be 0.955+/-0.005, 0.967+/-0.005, and 0.962+/-0.005 cGy h(-1) x U(-1) for the PharmaSeed BT-125-1, BT-125-2, and ADVANTAGE sources, respectively. TLD measurements were in excellent agreement with Monte Carlo calculations. Radial dose function, g(r), calculated to a distance of 10 cm, and anisotropy function, F(r,theta), calculated for radii from 0.5 to 7.0 cm, were similar among all source configurations. Anisotropy constants, phi(an), were calculated to be 0.941, 0.944, and 0.960 for the three sources, respectively. All dosimetric parameters were found to be in close agreement with previously published data for similar source configurations. The MCNP Monte Carlo code appears to be ideally suited to low energy dosimetry applications.

Brachytherapy↗
PubMed2002

Initial clinical results of stereotactic radiotherapy for the treatment of craniopharyngiomas.

The efficacy and toxicity of stereotactic radiotherapy (SRT) for the treatment of craniopharyngioma has been retrospectively evaluated in 16 patients. The median tumor diameter was 2.8 cm (range 1.5-6.1) and the median tumor volume was 7.7 cc (range 0.7-62.8). SRT was delivered to a single isocenter using a dedicated 6 MV linear accelerator to patients immobilized with a relocatable stereotactic head frame. The three-year actuarial overall survival was 93% and the rate of survival free of any imaging evidence of progressive disease was 75%. The three-year actuarial survival rates free of solid tumor growth or cyst enlargement were 94% and 81% respectively. Our results suggest that SRT is a safe and effective treatment approach for patients with craniopharyngioma. Long-term follow-up is required to determine whether the normal tissue-sparing inherent with SRT results in reduction of the neurocognitive effects of conventional radiotherapy for craniopharyngioma. SRT can be delivered to craniopharyngioma that may be difficult to treat with stereotactic radiosurgery due to proximity of the optic chiasm. Further clinical experience is necessary to determine the clinical utility of beam shaping in the setting of SRT.

Adolescent↗