Biomedical subjects

M Sarfaraz

Publications and source records attributed to M Sarfaraz.

8 recordsLinked to original sources

PubMed2003

A quality assurance method for analyzing and verifying intensity modulated fields.

A quality assurance method is developed for measuring, verifying and analyzing intensity modulated radiation fields. It is applicable for rotational and fixed-beam intensity modulated radiation therapy (IMRT) treatments. A gantry-mount device was constructed to measure the transmission dose of an IMRT field using radiographic films. A double-exposure technique with optimal kernel estimate method was developed to minimize the errors from measurements. A chi2 confidence level test method was developed to detect the discrepancies between measured and prescribed IMRT fluence distributions. Our method was tested for rotational and fixed-beam IMRT treatment verifications. The method was found insensitive to the hardware-related parameters for rotational and fixed-beam IMRT deliveries. The chi2 confidence level test was found to be more sensitive than linear correlation method in detecting relative small errors for cases with a few segments or narrow regions of interest. In conclusion, we demonstrated a quantitative method for verifying and analyzing IMRT treatment deliveries.

Models, Statistical↗

PubMed2001

Optimized intensity-modulated arc therapy for prostate cancer treatment.

We recently implemented intensity-modulated arc therapy (IMAT) at our institution. In this study, we evaluate the dosimetric merits of the application of this technique to the treatment of prostate cancer. Each IMAT treatment plan incorporated bilateral overlapping arcs. The dose from each beam segment was computed using the three-dimensional dose model of a clinical treatment planning system (Render Plan 3.5, Precision Therapy). The weights assigned to the individual arc segments were optimized using a gradient search method. For 12 patients, comparisons were made between the IMAT treatment plans and corresponding plans using fixed cone-beam intensity-modulated radiotherapy (IMRT) from a commercial inverse planning system (CORVUS, NOMOS Corp.). We found that the optimized IMAT treatments produced similar dose distributions to the IMRT deliveries. Compared with the IMRT treatments, the IMAT treatments produced slightly less target dose homogeneity with consistently greater sparing of the rectum in regions of lower dose. The trade-off between target dose conformity and rectum sparing can be adjusted in both optimization procedures. Because the total beam-on time for IMAT delivery is 1 to 2 minutes with approximately 5-6 minutes of patient setup time, the delivery efficiency of the IMAT treatment was significantly better than the multiple-beam IMRT treatment.

Humans↗

PubMed2001

Analysing collimator structure effects in head-scatter calculations for IMRT class fields using scatter raytracing.

The frequent blocking of the irradiated volume in intensity modulated radiation therapy (IMRT) makes the head-scatter fraction of the incident photon fluence more significant than that in conventional therapy with open fields. On the other hand. certain collimator configurations block scatter photons directed to a given observation point while allowing primary photons to be transmitted. The 'anomalous blocking' makes the primary field a poor indicator of the scatter fluence. Since large MU-to-cGy ratios in IMRT can magnify head-scatter uncertainties, it becomes necessary to accurately model both the effective scatter source and the collimator structure that limits the scatter reaching the irradiated volume. First we obtain a dual-source model, using a Taylor series expansion to derive the effective scatter source distribution from the data measured for the Elekta SL20 linac equipped with a multi-leaf collimator (MLC). Then, using a raytracing algorithm, we calculate the transmission of scatter rays from the effective scatter source plane to points in the patient plane. The method can account for the anomalous blocking of scatter by the MLC leaves and the backup diaphragms. For a variety of collimator settings tested, the calculations agree with measurements to an accuracy of 0.002psi10 x 10, where psi10 x 10 is the total (primary + scatter) photon fluence of an open 10 x 10 cm2 field for the same MU delivered. Although the significance of collimator structure in IMRT depends strongly on fields shapes employed for the delivery, potential cumulative errors on the order of a few per cent can be avoided in fluence calculations if the proposed method is used.

Algorithms↗

PubMed2001

A translational couch technique for total body irradiation.

We have constructed a computer controlled translational couch to administer total body irradiation reproducibly and safely. The system has replaced the previous stationary anterior-posterior technique in our institution and 30 plus patients have been treated with it so far. In this technique, patients comfortably lie on a couch in supine and prone positions and are transported slowly through a narrow beam with the gantry in an upright position. Dose to the patient is determined by the couch velocity that is calculated based on physical parameters such as patient's dimensions, beam geometry, and machine dose rate. In our design, the couch velocity is continuously updated to compensate for machine dose rate fluctuations. The translational couch technique provides better dose uniformity within the patient compared to fixed beam techniques, and allows a more precise shielding block placement for organs at risk. At the same time, it presents a special challenge for dosimetry calculations. A dosimetry parameter is introduced that converts the moving beam output to the fixed beam output factor. Based on this factor, a simple dosimetry calculation method has been developed that takes advantage of conventional dosimetry parameters, eliminating extensive dosimetry measurements. Multiple point dose measurements within a phantom confirmed the validity of the calculation method.

Dose Fractionation, Radiation↗

PubMed2000

A dosimetric leaf-setting strategy for shaping radiation fields using a multileaf collimator.

A dosimetric leaf-setting strategy of using multileaf collimators (MLC) for shaping radiation fields has been developed. Existing MLC leaf-setting strategies are all based upon geometric criteria. This new approach, however, matches a prescribed field contour with a MLC using clinically consistent dosimetric criteria. The leaf positions are determined using an iterative optimization algorithm. An empirical dose model was developed to compare the dosimetric-based leaf-setting strategy with the geometric-based leaf-setting strategies. Differences up to half a centimeter in the leaf positions and isodose lines were found between setting the MLC geometrically and setting the MLC dosimetrically. The dosimetric leaf-setting strategy provides the ability to achieve better dose conformation for a clinically desired isodose line. Since the desired isodose line that covers a treatment volume is typically higher than 50% of the maximum dose, the scalloping effects due to the finite leaf width at the leaf edge or 50% isodose lines are much reduced. Another benefit of the dosimetric leaf-setting is that it separates the leaf-setting process from the treatment planning process, and this frees the treatment planning vendors from developing detailed dose models for various existing types and future upgrades of MLC systems.

Algorithms↗

PubMed2000

An investigation of eye lens dose for gamma knife treatments of trigeminal neuralgia.

Stereotactic Gamma Knife radiosurgery has been widely used for treating trigeminal neuralgia (TN). A single large fractional dose of 7000 to 9000 cGy is commonly prescribed as the maximum dose for these treatments. For this reason, if a small percentage of the prescribed dose such as 2-3% scattered to the eye, it could reach or even exceed the tolerance dose of the lens. For several TN cases, we found that the Leksell Gamma Plan system calculates the lens dose about 0.5-2% of the maximum dose independent of the use of eye shielding. These dose values are significantly high and it motivated us to investigate the lens dose for the TN patients treated with stereotactic Gamma Knife radiosurgery. Phantom studies and in vivo dosimetry measurements were carried out for six patients treated at our institution. The average dose to the lens ipsilateral to the treated nerve was measured to be 7.7+/-0.6 cGy. Based on the biological model of Lyman and Emami [Int. J. Radiat. Oncol. Biol. Phys. 21, 109-122 (1991)], the probability of the lens complication (cataract) was determined to be 0.1%. Our findings suggest that few TN patients would develop cataracts after receiving Gamma Knife radiosurgery.

Brain Stem↗

PubMed1999

Validation of an analytical expression for the absorbed dose from a spherical beta source geometry and its application to micrometastatic radionuclide therapy.

The purpose of this study was to validate an analytical expression for the absorbed-dose calculation from the spherical source of beta-emitting radionuclides and to apply it to micrometastases treated with radiolabeled monoclonal antibodies. The self-absorbed fractions from I-131 and P-32 uniform spherical sources were calculated using the analytical expression introduced by P. K. Leichner (J. Nucl. Med., 35: 1721-1729, 1994). The calculated absorbed fractions were compared with previously reported values and were found to be in reasonable agreement, with a maximum difference of 15% for smaller masses and a long-range beta emitter. The expression was subsequently applied to estimate the absorbed dose within spheroid models with nonuniform penetration of radiolabeled antibody. The corresponding absorbed dose for I-131 was compared with reported micro-thermoluminescence dosimeter measurements and found to be in good agreement. This work has independently substantiated the methodology outlined by Leichner and may be reliably incorporated into new software developments for radionuclide dosimetry treatment planning.

Beta Particles↗

PubMed1998

Use of the fast Hartley transform for three-dimensional dose calculation in radionuclide therapy.

Effective radioimmunotherapy may depend on a priori knowledge of the radiation absorbed dose distribution obtained by trace imaging activities administered to a patient before treatment. A new, fast, and effective treatment planning approach is developed to deal with a heterogeneous activity distribution. Calculation of the three-dimensional absorbed dose distribution requires convolution of a cumulated activity distribution matrix with a point-source kernel; both are represented by large matrices (64 x 64 x 64). To reduce the computation time required for these calculations, an implementation of convolution using three-dimensional (3-D) fast Hartley transform (FHT) is realized. Using the 3-D FHT convolution, absorbed dose calculation time was reduced over 1000 times. With this system, fast and accurate absorbed dose calculations are possible in radioimmunotherapy. This approach was validated in simple geometries and then was used to calculate the absorbed dose distribution for a patient's tumor and a bone marrow sample.

Antibodies, Monoclonal↗