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P Alaei

Publications and source records attributed to P Alaei.

9 recordsLinked to original sources
PubMed2006

Dose uncertainty due to aperture effects in dynamic fields.

Dosimetry of intensity modulated radiation therapy requires accurate modeling of the beamlets that comprise each treatment segment. Planning systems such as Varian Eclipse and Philips Pinnacle recommend measuring dose distributions and output factors for fields as small as possible, generally down to at least 2 x 2 cm2. Conventionally, we perform these measurements for regular fields, defined by the secondary collimators. In practice, it is the multileaf collimation system (MLC) that defines the intensity map and provides dynamic dose modulation in either a moving window or segmented step-and-shoot mode. For this review we have only considered the latter delivery mode. Using this method, we have studied aperture motion effects on the dynamic collimator scatter (S(c)), total scatter (S(c,p)), and phantom scatter (S(p)) factors for various combinations of collimator settings (4 x 4-14 x 40 cm2) and dynamically stepped leaf gaps (0.1 to 1.0 cm) in comparison with those for static field factors. For two different Varian linear accelerators, we found similar results in a systematic dependence of collimator scatter on gap width and collimator setting. As the gap increases from 0.1 to 1.0 cm the dynamic collimator scatter factors converge from a maximum difference of about 30% toward the static field values. At the same time, there is no measurable difference between dynamic field phantom scatter factors and those conventionally obtained for static fields. Second, we evaluated the two planning systems as to how well they account for collimator scatter by attempting to mimic the dynamic apertures used above by planning and measuring dose distributions to several small, cylindrical targets for a similar range of fixed collimator settings. We found that the ratio of measured-to-planned doses as a function of target size were similar to the measured, dynamic S(c) data for the Varian Eclipse planning system, indicating underestimation of dose for targets smaller than 1 cm diameter, but were close to unity for the Philips Pinnacle system, suggestive of the underlying differences in the dose calculation algorithms. We discuss the measurements and results and potential impact on the dosimetry of small clinical targets.

Air↗
PubMed2004

Radiation treatment decreases bone cancer pain, osteolysis and tumor size.

Radiotherapy is the cornerstone of palliative treatment for primary bone cancer in animals and metastatic bone cancer in humans. However, the mechanism(s) responsible for pain relief after irradiation is unknown. To identify the mechanism through which radiation treatment decreases bone cancer pain, the effect of radiation on mice with painful bone cancer was studied. Analysis of the effects of a 20-Gy treatment on localized sites of painful bone cancers was performed through assessments of animal behavior, radiographs and histological analysis. The findings indicated that radiation treatment reduced bone pain and supported reduced cancer burden and reduced osteolysis as mechanisms through which radiation reduces bone cancer pain.

Animals↗
PubMed2003

In vivo diode dosimetry for routine quality assurance in IMRT.

Due to the complexity of IMRT dosimetry, dose delivery evaluation is generally done using a treatment plan in which the optimized fluence distribution has been transferred to a test phantom for accessibility and simplicity of measurement. The actual patient doses may be reconstructed in vivo through the use of electronic portal imaging devices or films, but the assessment of absolute dose from these measurements is time-consuming and complicated. In our clinic we have instituted the use of routine diode dosimetry for IMRT patients following the same procedure used for standard radiation therapy patients in which each new treatment field is checked at the start of treatment. For standard cases the dose at dmax is calculated as part of the monitor unit calculation. For the IMRT cases, the dose contribution to the dmax depth for each field is taken from the treatment plan. We found that about 90% of the diode measurements agreed to within +/- 10% of the planned doses (45/51 fields) and 63% (32/51 fields) achieved +/- 5% agreement. By using this direct in vivo method to verify the clinical doses delivered, we have been able to make a uniform startup procedure for all patients while simplifying our IMRT QA process.

Humans↗
PubMed2001

Lung dose calculations at kilovoltage x-ray energies using a model-based treatment planning system.

The determination of the dose to organs from diagnostic x rays has become important because of reports of radiation injury to patients from fluoroscopically guided interventional procedures. We have modified a convolution/superposition-based treatment planning system to compute the dose distribution for kilovoltage beams. We computed lung doses using this system and compared them to those calculated using the CDI3 organ dose calculation program. We also computed average lung doses from a simulated radiofrequency ablation procedure and compared our results to published doses for a similar procedure. Doses calculated using this system were an average of 20% lower for AP beams and 7% higher for PA beams than those obtained using CDI3. The ratio of the average dose to the lungs to the skin dose from the simulated ablation procedure ranged from 25% higher to 15% lower than that determined by other authors. Our results show that a treatment planning system designed for use in the megavoltage energy range can be used for calculating organ doses in the diagnostic energy range. Our doses compare well with those previously reported. Differences are partly due to variations in experimental techniques. Using a three-dimensional (3-D) treatment planning system to calculate dose also allows us to generate dose volume histograms (DVH) and compute normal tissue complication probabilities (NTCP) for diagnostic procedures.

Female↗
PubMed2001

Measurement of the dose deposition characteristics of x-ray fluoroscopy beams in water.

The purpose of this study was to characterize the x-ray dose distribution of fluoroscopy beams by measuring their percent depth dose curves and lateral dose profiles in a water phantom. Percent depth dose curves were measured near the surface with an Attix parallel plate chamber and deep within the water phantom with a Farmer-type cylindrical chamber. Percent depth dose curves were compared to published data where applicable. Lateral dose profiles were measured at depths of 2, 5, 10, and 15 cm in phantom with a Farmer chamber. Pulsed, 50 mA x-ray beams with peak tube potentials of 60, 80, 100, and 120 kV and half value layers of 1.89, 2.52, 3.20, and 4.09 mm Al, respectively, were investigated.

Biophysical Phenomena↗
PubMed2000

Evaluation of a model-based treatment planning system for dose computations in the kilovoltage energy range.

The ability to determine dose distribution and calculate organ doses from diagnostic x rays has become increasingly important in recent years because of relatively high doses in interventional radiology and cardiology procedures. In an attempt to determine the dose from both diagnostic and orthovoltage x rays, we have used a commercial treatment planning system (Pinnacle, ADAC Laboratories, Milpitas, CA) to calculate the doses in phantoms from kilovoltage x rays. The planning system's capabilities for dose computation have been extended to lower energies by the addition of energy deposition kernels in the 20-110 keV range and modeling of the 60, 80, 100, and 120 kVp beams using the system. We compared the dose calculated by the system with that measured using thermoluminescent dosimeters (TLDs) placed in various positions within several phantoms. The phantoms consisted of a cubical solid water phantom, the solid water phantom with added lung and bone inhomogeneities, and the Rando anthropomorphic phantom. Using Pinnacle, a treatment plan was generated using CT scans of each of these phantoms and point doses at the positions of TLD chips were calculated. Comparisons of measured and computed values show an average difference of less than 2% within materials of atomic number less than and equal to that of water. The algorithm, however, does not produce accurate results in and around bone inhomogeneities and underestimates attenuation of x rays by bone by an average of 145%. A modification to the CT number-to-density conversion table used by the system resulted in significant improvements in the dose calculated to points beyond bone.

Algorithms↗
PubMed1999

Generation and use of photon energy deposition kernels for diagnostic quality x rays.

Accurately determining the dose from low energy x rays is becoming increasingly important. This is especially so because of high doses in interventional radiology procedures and also because of the desire to model accurately the dose around low energy brachytherapy sources. Various methods to estimate the dose from specific procedures are available but they only give a general idea of the true dose to various organs. The use of sophisticated three-dimensional (3D) dose deposition algorithms designed originally for radiation therapy treatment planning can be extended to lower photon energy regions. The majority of modern 3D treatment planning systems use a variation of the convolution algorithm to calculate dose distributions. This could be extended into the diagnostic energy range with the availability of lower energy deposition kernels ( < 100 keV). We have used version four of the Electron Gamma Shower (EGS4) system of Monte Carlo codes to generate photon energy deposition kernels in the energy range of 20-110 keV and have implemented them in a commercial 3D treatment planning system (Pinnacle, ADAC Laboratories, Milpitas, CA). The kernels were generated using the "SCASPH" EGS4 user code by selecting the appropriate transport parameters suitable for the relative low energy of the incident photons. The planning system was subsequently used to model diagnostic quality beams and to calculate depth dose and cross profile curves. Comparisons of the calculated curves have been made with measurements performed in a homogeneous water phantom.

Algorithms↗
PubMed1996

Inhibition of protein prenylation down-regulates signalling by inflammatory mediators in human keratinocytes.

Several inflammatory mediators have been shown to activate phospholipase C in human keratinocytes via GTP-binding protein-coupled receptors. Since GTP-binding proteins are prenylated proteins, we have examined the role of prenylation in signal transduction in HaCaT keratinocytes. Indirect inhibition of prenylation with the HMG CoA reductase inhibitors fluvastatin or compactin decreased bradykinin-stimulated inositol 1,4,5-triphosphate generation. This effect was abolished by mevalonic acid but not by serum, indicating a requirement for a non-sterol metabolite for signal generation. The BK response was also inhibited by zaragozic acids B and C, known inhibitors of prenyl protein transferases. These results suggest that protein prenylation may be a novel therapeutic target in dermatological conditions where an up-regulation of the inositol lipid pathway has been demonstrated.

Adenosine Triphosphate↗