Biomedical subjects

David Shepard

Publications and source records attributed to David Shepard.

7 recordsLinked to original sources

PubMed2006

Direct aperture optimization of breast IMRT and the dosimetric impact of respiration motion.

We have studied the application of direct aperture optimization (DAO) as an inverse planning tool for breast IMRT. Additionally, we have analysed the impact of respiratory motion on the quality of the delivered dose distribution. From this analysis, we have developed guidelines for balancing the desire for a high-quality optimized plan with the need to create a plan that will not degrade significantly in the presence of respiratory motion. For a DAO optimized breast IMRT plan, the tangential fields incorporate a flash field to cover the range of respiratory motion. The inverse planning algorithm then optimizes the shapes and weights of additional segments that are delivered in combination with the open fields. IMRT plans were generated using DAO with the relative weights of the open segments varied from 0% to 95%. To assess the impact of breathing motion, the dose distribution for the optimized IMRT plan was recalculated with the isocentre sampled from a predefined distribution in a Monte Carlo convolution/superposition dose engine with the breast simulated as a rigid object. The motion amplitudes applied in this study ranged from 0.5 to 2.0 cm. For a range of weighting levels assigned to the open field, comparisons were made between the static plans and the plans recalculated with motion. For the static plans, we found that uniform dose distributions could be generated with relative weights for the open segments equal to and below 80% and unacceptable levels of underdosage were observed with the weights larger than 80%. When simulated breathing motion was incorporated into the dose calculation, we observed a loss in dose uniformity as the weight of the open field was decreased to below 65%. More quantitatively, for each 1% decrease in the weight, the per cent volume of the target covered by at least 95% of the prescribed dose decreased by approximately 0.10% and 0.16% for motion amplitudes equal to 1.5 cm and 2.0 cm, respectively. When taking into account the motion effects, the most uniform and conformal dose distributions were achieved when the open segment weights were in the range of 65-80%. Within this range, high-quality IMRT plans were produced for each case. The study demonstrates that DAO with tangential fields provides a robust and efficient technique for breast IMRT planning and delivery when the open segment weight is selected between 65% and 80%.

Breast Neoplasms↗

PubMed2006

New developments in intensity modulated radiation therapy.

As intensity modulated radiation therapy (IMRT) becomes routine clinical practice, its advantages and limitations are better understood. With these new understandings, some new developments have emerged in an effort to alleviate the limitations of the current IMRT practice. This article describes a few of these efforts made at the University of Maryland, including: i) improving IMRT efficiency with direct aperture optimization; ii) broadening the scope of optimization to include the mode of delivery and beam angles; and iii) new planning methods for intensity modulated arc therapy (IMAT).

Humans↗

PubMed2005

Effect of beamlet step-size on IMRT plan quality.

We have studied the degree to which beamlet step-size impacts the quality of intensity modulated radiation therapy (IMRT) treatment plans. Treatment planning for IMRT begins with the application of a grid that divides each beam's-eye-view of the target into a number of smaller beamlets (pencil beams) of radiation. The total dose is computed as a weighted sum of the dose delivered by the individual beamlets. The width of each beamlet is set to match the width of the corresponding leaf of the multileaf collimator (MLC). The length of each beamlet (beamlet step-size) is parallel to the direction of leaf travel. The beamlet step-size represents the minimum stepping distance of the leaves of the MLC and is typically predetermined by the treatment planning system. This selection imposes an artificial constraint because the leaves of the MLC and the jaws can both move continuously. Removing the constraint can potentially improve the IMRT plan quality. In this study, the optimized results were achieved using an aperture-based inverse planning technique called direct aperture optimization (DAO). We have tested the relationship between pencil beam step-size and plan quality using the American College of Radiology's IMRT test case. For this case, a series of IMRT treatment plans were produced using beamlet step-sizes of 1, 2, 5, and 10 mm. Continuous improvements were seen with each reduction in beamlet step size. The maximum dose to the planning target volume (PTV) was reduced from 134.7% to 121.5% and the mean dose to the organ at risk (OAR) was reduced from 38.5% to 28.2% as the beamlet step-size was reduced from 10 to 1 mm. The smaller pencil beam sizes also led to steeper dose gradients at the junction between the target and the critical structure with gradients of 6.0, 7.6, 8.7, and 9.1 dose%/mm achieved for beamlet step sizes of 10, 5, 2, and 1 mm, respectively.

Algorithms↗

PubMed2005

Racial differences in the use of aspirin: an important tool for preventing heart disease and stroke.

BACKGROUND: Regular aspirin use, particularly as secondary prevention, reduces morbidity from heart disease and stroke. Few studies have examined racial/ethnic differences in aspirin use for the prevention of cardiovascular disease (CVD). METHODS: Data from the 2001 Behavioral Risk Factor Surveillance System (n=2,514 African Americans; n=865 Hispanics; n=28,038 Whites) were used to assess racial/ethnic differences in aspirin use. Multivariable logistic regression was used to examine whether the likelihood of aspirin use differs by race/ethnicity after accounting for sociodemographic and CVD risk factors. RESULTS: Regular aspirin use was associated with increasing age, male gender, lower educational attainment, hypertension, diabetes, overweight, and histories of myocardial infarction, coronary heart disease, and stroke. Aspirin use was lower among African Americans and Hispanics than Whites (28.6% and 28.7% vs 37.1%, respectively). After adjustment for sociodemographic and CVD risk factors, African Americans and Hispanics were 30%-40% less likely than Whites (OR=0.6, 95% C1=0.5, 0.7, African Americans; OR=0.7, 95% CI=0.5, 1.0, Hispanics) to take aspirin. Although aspirin use was higher among those with CVD (73.6% African Americans, 73.6% Hispanics, and 82.7% Whites), after multivariable adjustment, African Americans and Hispanics with CVD remained 30% to 50% less likely to use aspirin than Whites (OR=0.7, 95% CI=0.4, 1.2, African Americans; OR=0.5, 95% CI=0.2, 1.1, Hispanics). CONCLUSIONS: African Americans and Hispanics are less likely to take aspirin than their White counterparts. Differences in sociodemographic characteristics and CVD risk factors do not account for lower aspirin use among racial/ethnic minorities. Additional studies should examine methods to increase aspirin use in these populations.

Adult↗

PubMed2003

Treatment planning for stereotactic radiosurgery with photon beams.

Stereotactic Radiosurgery (SRS) has evolved as a unique discipline that combines aspects of both surgery and radiation oncology. Technological developments in the past few decades have provided a wide array of treatment techniques, including (i) the Gamma Knife; (ii) Linac-based stereotactic techniques using circular collimators or using micro multileaf collimators (mMLCs); (iii) the Cyber Knife, using an x-band linac mounted on a robotic arm; and (iv) serial and spiral tomotherapy. This paper provides a review of the treatment planning methods for stereotactic radiosurgery. Because of the differences in planning strategies used for each SRS technique, this paper will provide both a general review of the pre-requisites and common features of SRS treatment planning and the planning techniques specific to each of the SRS techniques.

Humans↗

PubMed2002

Clinical implementation of intensity-modulated arc therapy.

PURPOSE: Intensity-modulated arc therapy (IMAT) is a method for delivering intensity-modulated radiation therapy (IMRT) using rotational beams. During delivery, the field shape, formed by a multileaf collimator (MLC), changes constantly. The objectives of this study were to (1) clinically implement the IMAT technique, and (2) evaluate the dosimetry in comparison with conventional three-dimensional (3D) conformal techniques. METHODS AND MATERIALS: Forward planning with a commercial system (RenderPlan 3D, Precision Therapy International, Inc., Norcross, GA) was used for IMAT planning. Arcs were approximated as multiple shaped fields spaced every 5-10 degrees around the patient. The number and ranges of the arcs were chosen manually. Multiple coplanar, superimposing arcs or noncoplanar arcs with or without a wedge were allowed. For comparison, conventional 3D conformal treatment plans were generated with the same commercial forward planning system as for IMAT. Intensity-modulated treatment plans were also created with a commercial inverse planning system (CORVUS, Nomos Corporation). A leaf-sequencing program was developed to generate the dynamic MLC prescriptions. IMAT treatment delivery was accomplished by programming the linear accelerator (linac) to deliver an arc and the MLC to step through a sequence of fields. Both gantry rotation and leaf motion were enslaved to the delivered MUs. Dosimetric accuracy of the entire process was verified with phantoms before IMAT was used clinically. For each IMAT treatment, a dry run was performed to assess the geometric and dosimetric accuracy. Both the central axis dose and dose distributions were measured and compared with predictions by the planning system. RESULTS: By the end of May 2001, 50 patients had completed their treatments with the IMAT technique. Two to five arcs were needed to achieve highly conformal dose distributions. The IMAT plans provided better dose uniformity in the target and lower doses to normal structures than 3D conformal plans. The results varied when the comparison was made with fixed gantry IMRT. In general, IMAT plans provided more uniform dose distributions in the target, whereas the inverse-planned fixed gantry treatments had greater flexibility in controlling dose to the critical structures. Because the field sizes and shapes used in the IMAT were similar to those used in conventional treatments, the dosimetric uncertainty was very small. Of the first 32 patients treated, the average difference between the measured and predicted doses was -0.54 +/- 1.72% at isocenter. The 80%-95% isodose contours measured with film dosimetry matched those predicted by the planning system to within 2 mm. The planning time for IMAT was slightly longer than for generating conventional 3D conformal plans. However, because of the need to create phantom plans for the dry run, the overall planning time was doubled. The average time a patient spent on the table for IMAT treatment was similar to conventional treatments. CONCLUSION: Initial results demonstrated the feasibility and accuracy of IMAT for achieving highly conformal dose distributions for different sites. If treatment plans can be optimized for IMAT cone beam delivery, we expect IMAT to achieve dose distributions that rival both slice-based and fixed-field IMRT techniques. The efficient delivery with existing linac and MLC makes IMAT a practical choice.

Brain Neoplasms↗

PubMed2002

Fetal and ovarian radiation dose in patients undergoing gamma knife radiosurgery.

BACKGROUND: It is difficult to estimate the fetal or ovarian radiation dosage for female patients undergoing Gamma Knife radiosurgery. Our goals are to determine the fetal and ovarian radiation dose at various distances from a cranial isocenter, to provide a reference for practitioners to estimate the fetal dose with respect to gestational age, and to identify the components of pelvic extracranial radiation. METHODS: An anthropomorphic phantom and ion chamber were used to measure the dose at 50, 60, and 70 cm from a cranial isocenter and at three points within the transverse plane for the supine position. Each measurement consisted of a 5-minute exposure. Additional measurements were taken for four collimator sizes, the prone position, off-axis, and with one-half of all collimator holes plugged. RESULTS: The values of the fetal and ovarian dose rates ranged from 0.27 cGy/min to 0.05 cGy/min based on distance from the isocenter. The fetal and ovarian dose can be up to 8.1 cGy for a 30-minute Gamma Knife treatment. The dose fell off more rapidly than predicted by the inverse square law. There was no dependence of fetal dose rate on collimator size. No advantage to the prone position could be shown. Leakage and collimator scatter are the main components of extracranial dose 50 to 70 cm from the isocenter. CONCLUSIONS: The fetal and ovarian dose is a function of treatment time and distance from the isocenter. We recommend pregnancy status assessment in women of reproductive age and treatment plan design using large volume shots in order to minimize treatment time.

Brain Neoplasms↗