摘要:

The dose distribution from radioimmunotherapy is very heterogeneous because of variability in antigen expression, antibody penetration, and tumor architecture. Many models of dose distribution have been constructed but it has been very difficult to confirm these predictions with actual measured doses. The purpose of this study was to determine what degree of resolution could be obtained using mini‐thermoluminescent dosimeter(s) (TLD) in a micrometastasis model. TLDs were inserted into 1‐mm‐diam multicell tumor spheroids that had been treated with131I‐labeled antibody. The spheroids were then sectioned at 30‐μm intervals and the TLD sections (measuring 0.14×0.1×0.03 mm) were removed and read. Calibration of the TLDs was done with whole TLDs using external beam radiation and an131I‐containing gel, and with TLD sections using external beam radiation. Predicted doses were determined by measuring the activity in individual spheroids and the time the TLDs were in the spheroids and incorporating these numbers into a model that assumed either surface binding of131I or some degree of penetration. There was a correlation between the measured TLD dose and the predicted absorbed dose when comparing the low‐ to high‐dose regions. However, there was considerable variation within any particular dose range, probably due to heterogeneity in TLD grain size. TLD sections of this size, originating from standard chips available from the manufacturer, are useful in measuring cumulative absorbed doses from131I‐labeled antibody within 4% of the calculated dose for doses in the range of 150 Gy, within 6% to 20% for the 40‐Gy range, and within a factor of two for doses less than 25 Gy.

ISSN:0269-3127    

DOI:10.1118/1.596753

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A technical evaluation was made of a commercial intraoperative radiation probe. This device utilizes a CsI(Tl) scintillation detector and light pipe arrangement to count gamma radiationinvivo. After determining the optimal window and threshold setting, additional evaluations included linearity, distance response function, detector dead time, counter reproducibility, detector sensitivity, angular resolution, and energy resolution. Detector dead time (21.2 μs) was found to be characteristic of a nonparalysable system. Activity response for each radionuclide was linear (R=0.99) both with and without collimation. Energy resolution, 25% at 210 keV, was not sufficient to separate the two photons (172 and 247 keV) emitted by111In. Detector sensitivity was 1136 and 626 counts per s per microcurie of111In and99mTc, respectively. The mean effective distance from the face of the uncollimated probe to the crystal was determined to be 2.03 cm in air.

ISSN:0269-3127    

DOI:10.1118/1.596754

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.596755

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A formalism for clinical external beam dosimetry based on use of ion chamber absorbed‐dose calibration factors is outlined in the context and notation of the AAPM TG‐21 protocol. It is shown that basing clinical dosimetry on absorbed‐dose calibration factorsNDleads to considerable simplification and reduced uncertainty in dose measurement. In keeping with a protocol which is used in Germany, a quantitykQis defined which relates an absorbed‐dose calibration factor in a beam of qualityQ0to that in a beam of qualityQ. For 38 cylindrical ion chambers, two sets of values are presented forND/NXandNgas/NDand forkQfor photon beams with beam quality specified by theTPR1020ratio. One set is based on TG‐21's protocol to allow the new formalism to be used while maintaining equivalence to the TG‐21 protocol. To demonstrate the magnitude of the overall error in the TG‐21 protocol, the other set uses corrected versions of the TG‐21 equations and the more consistent physical data of the IAEA Code of Practice. Comparisons are made to procedures based on air‐kerma or exposure calibration factors and it is shown that accuracy and simplicity are gained by avoiding the determination ofNgasfromNX. It is also shown that thekQapproach simplifies the use of plastic phantoms in photon beams sincekQvalues change by less than 0.6% compared to those in water although an overall correction factor of 0.973 is needed to go from absorbed dose in water calibration factors to those in PMMA or polystyrene. Values ofkQcalculated using the IAEA Code of Practice are presented but are shown to be anomalous because of the way the effective point of measurement changes for60Co beams. In photon beams the major difference between the IAEA Code of Practice and the corrected AAPM TG‐21 protocol is shown to be thePreplcorrection factor. CalculatedkQcurves and three parameter equations for them are presented for each wall material and are shown to represent accurately thekQcurve for all ion chambers in this study with a wall of that specified material and a thickness less than 0.25 g/cm2. Values ofkQcan be measured using the primary standards for absorbed dose in photon beams.

ISSN:0269-3127    

DOI:10.1118/1.596921

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In this article, the fifth in a series on the calculation of electron dose using multiple‐scattering theory, the predictions of a new model for dealing with localized inhomogeneities will be examined (Med. Phys.18, 123–132, 1991). That model is in the form of a perturbation series, and for a thick half‐slab configuration explicit formulas are worked out for the dose directly deposited by the primary electrons, for three reasonable cutoffs of the series. The predictions of this model with EGS4 Monte Carlo calculations for the half‐slab configuration are compared, and they are found to be quite accurate in the region under the edge of the half‐slab. On the other hand, the “Hogstrom algorithm,” which is currently the most advanced method in routine clinical use, is found to give poor accuracy for this configuration.

ISSN:0269-3127    

DOI:10.1118/1.596756

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In order to initiate the use of a multileaf collimator (MLC) in the clinic, a set of technical procedures needs to be available sufficient to create MLC leaf settings and to deliver an accurate dose of radiation through the MLC‐shaped field. Dosimetry data for clinical use of the MLC were measured. Dosimetric characteristics included central axis percent depth dose, output factors, and penumbra. In this paper, it has been concluded that a dose control monitor unit calculation procedure that has been applied to the use of conventional secondary field‐shaping blocks can be applied to the multileaf collimator dosimetry. The multileaf collimator penumbra (20% to 80%) is only slightly wider (1–3 mm) than the penumbra of the conventional collimator jaws. Beam's‐eye‐view comparisons made between the isodose curves in fields shaped by conventional Cerrobend blocks and isodose curves in fields shaped by the multileaf collimator demonstrated that the 50% isodose line at 10‐cm depth exhibited the discrete steps of the multileaf collimator leaves, but that the 90% and 10% isodose curves of the multileaf were close to those shaped by Cerrobend blocks.

ISSN:0269-3127    

DOI:10.1118/1.596757

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The use of immobilization plastic masks in head and neck radiotherapy can partially eliminate skin benefits derived from the utilization of megavoltage photon beams. Filters and blocks between the patient and the accelerator can further increase the skin dose value. In this study, the increase in surface dose due to 2 and 3.2 mm of plastic material utilized for patient immobilization was measured. Then, the effect of blocking trays, wedges, and blocks on skin dose in typical conditions for head and neck irradiation was evaluated. The measurements were obtained with a NE2534 chamber (Markus type) on a perspex phantom for 6 MeV x‐rays from an accelerator.

ISSN:0269-3127    

DOI:10.1118/1.596758

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A commercial system for producing retracted compensators has been adapted to suit local needs, and is evaluated here. It comprises a magnetic field surface digitizer and computer‐driven milling machine. Improvements in dose distributions, resulting in standard deviations of the mean dose between 2% and 3%, have been achieved for treatment fields in wax phantoms simulating the head and neck regions. Optimization of compensator shape to allow for changes in the amount of scattered radiation has resulted in a further improvement in dose uniformity, particularly near the field borders; for these compensators the standard deviation was as low as 1.6%. The system using the basic algorithm has been in clinical use since July.

ISSN:0269-3127    

DOI:10.1118/1.596759

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In this work, the accuracy of the asymmetric jaws planning feature in a commercial treatment planning (TP) system is assessed. In the latest version of this software, the off‐axis beam quality variation is handled by a functiong(d,r), which is derived from measured horizontal beam profiles at four different depths. The calculated and measured isodoses for a 6‐MV linear accelerator with asymmetric jaws agree to ±0.5% along the central axis and to within 2 mm at the beam edge. Formulas for treatment time calculations using the output data reported by the computer program are described, as well as formulas for manual calculations based on pregenerated data tables. Doses calculated based on these formulas are compared to measurement and the accuracy is ±1% and ±2% for the computer and manual calculations, respectively. It is concluded that this version of the treatment planning system as well as the treatment time calculation formulas can be used adequately for asymmetric jaw computerized and manual treatment planning.

ISSN:0269-3127    

DOI:10.1118/1.596760

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A Monte Carlo computer study of the total dose distribution from neutrons and prompt gamma emissions (but excluding the contribution from conversion and Auger electrons) for gadolinium neutron capture therapy of brain tumors has been carried out in order to test the theoretic feasibility of this modality using commercially available magnetic resonance contrast media. The three‐dimensional dose distribution calculations were performed in a spherical head phantom with a spherical tumor at the center. Potentially achievable gadolinium concentrations of 150 μg/g of tissue in tumor and 3 μg/g in normal tissue were assumed with enrichment to 79.9% gadolinium‐157, as supplied by Oak Ridge National Laboratory. Irradiation was assumed to be with a 2‐keV monoenergetic cylindrical epithermal neutron beam having a radius of 4 cm. The three‐dimensional thermal neutron fluence resulting from the 2‐keV beam propagation through the tissue was modeled. For a single neutron beam, the maximum dose is delivered within the tumor but the dose is very inhomogeneous across the tumor volume due to rapid decrease of thermal neutron fluence with depth. Two parallel opposed neutron beams deliver to the interface of normal and malignant tissue 70%–80% of the maximum dose received at the center of the tumor. To deliver an average tumor dose of 500 cGy in 10 min would require a 2‐keV source neutrons number of 8.0×1011per s within the geometry of the beam.

ISSN:0269-3127    

DOI:10.1118/1.596761

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY