Lawrence H. Lanzl

The application of multiple scattering theory to therapeutic electron dosimetry.

Fermi–Eyges multiple scatteringtheory for electrons is explained, and a general three‐dimensional formalism is developed for its application to problems of therapeutic electron dosimetry. The formalism is illustrated by a number of elementary examples: a rectangular beam, an isotropic point source, and a scanning line source.

Clinical radiotherapy physics

1 Scope of Clinical Radiotherapy Physics.- 2 Atoms, Molecules, and Matter.- 3 Propagation of Energy by Electromagnetic Waves.- 4 Nuclear Transitions and Radioactive Decay.- 5 Radioactive Decay Calculations.- 6 Collision and Radiation Loss in Charged-Particle Interactions.- 7 Photon Interactions.- 8 Conventional X-Ray Machines.- 9 Equipment for Radioisotope Teletherapy.- 10 Particle Accelerators.- 11 Quantification of Radiation Field: Radiation Units and Measurements.- 12 Instruments for Radiation Detection.- 13 Basic Ratios and Factors for the Dosimetry of External Beam.- 14 Beam Dosimetry: Additional Corrections - Special Situations.- 15 Treatment Dose Distribution Planning: Photon Beams.- 16 Physical Aspects of Electron Beam Therapy.- 17 Physics of the Use of Small Sealed Sources in Brachytherapy.- 18 Radiation Safety Standards.- 19 Radiation Safety in External-Beam Therapy.- 20 Radiation Safety in Brachytherapy.- Appendix A and B.

A simple algorithm for planar image registration in radiation therapy

A simple algorithm is presented for planar image registration and the method is applied to the simulator and portal image registration for patient setup verification in radiation therapy. Basically, the algorithm follows the concept proposed by Balter et al. [Med. Phys. 19, 329-334 (1992)], which converts the problem of open curve registration into matching a series of points along the curves. Balters algorithm consists of three steps: (1) to determine a common starting point for each curve pair, (2) acquire two corresponding point sets along each curve, and (3) obtain a global transform matrix by matching two point sets. We integrate all three steps into one simple procedure which fits the sampled points along the intended curve pair by taking the relative path length shift as an independent fitting parameter. After being modified, the algorithm is able to take the different magnification factors of images into account, and it avoids curvature calculations. Numerical simulation as well as clinical and phantom images have been utilized to test the accuracy of the algorithm. The typical errors are less than 1 mm in translation and 1 degree in rotation. We also made a comparison study with the chamfer method. The results of the two methods agree to within 0.5 mm in translation and 0.5 degree in rotation.

Retrospective analysis of hyperthermia for use in the palliative treatment of cancer: a multi-modality evaluation.

Forty-two patients with local or superficial metastatic or recurrent malignant tumors were treated in a non-randomized Phase I/II study to assess the tumoricidal effects of heat combined with radiation and/or chemotherapy. Radiation doses administered averaged 3130 +/- 350 cGy; chemotherapeutic agents employed included bleomycin, mitomycin-C, adriamycin, and cis-platin, heat was induced by radiative or interstitial microwave applicators operating at frequencies ranging from 95 to 900 MHz. Forty-one of the forty-two patients were evaluated for initial therapeutic effects yielding the following response distributions: local hyperthermia with radiation--42% complete response (CR), 44% partial response (PR), and 15% no response (NR); local hyperthermia with chemotherapy--0% CR, 50% PR and 50% NR. Long-term response duration was evaluated for local hyperthermia with radiation, yielding mean time to recurrence of 9.4 months for CRs and mean time to progression of 3.4 months for PRs. In retrospective analysis, we examined the correlations of previously established response-predictor variables of tumor volume and minimum thermal dose with both initial and long-term response rates. Initial complete response rates were correlated directly with non-site-specific minimum thermal dose, varied inversely with tumor volume and exhibited a positive correlation for a limited histologic type/treatment site combination. Surprisingly, long-term response did not correlate either with tumor volume or thermal dose. The frequency of thermally induced complications, which did not correlate with any measured thermal parameters, was found to be 42%, expressed on a per-patient basis.

Electron dose calculation using multiple-scattering theory: thin planar inhomogeneities.

In this article in our series on electron dose calculation using multiple-scattering theory, we apply the Fermi-Eyges theory to the problem of a thin planar inhomogeneity present in an otherwise-layered medium. We derive expressions for the distribution function P and the location distribution L (which multiplied by the restricted mass collision stopping power is the dose directly deposited by the primary electrons) for various types of incident beams: a completely arbitrary distribution, a Gaussian point source, a pencil beam, an isotropic point source, and a broad parallel beam. We show how divergent-beam dose distributions can be determined from parallel-beam calculations, through use of equivalent configurations dependent upon the depth of dose calculation. Also, we indicate how this work can be applied to the design of wedges (or compensators) for beam shaping to provide desired dose distributions or to match juxtaposed radiation fields. Explicit formulas for thin plates are then worked out, and we examine the appearance of hot and cold spots distal to the edge of a localized inhomogeneity, for thin half-slabs and for narrow strips. Finally, considering the case of a thin straight wedge-shaped inhomogeneity, we theoretically discover the phenomenon of a focused hot spot without an accompanying cold spot, and suggest the design of a multiple-scattering lens.

Design optimization of intraoperative radiotherapy cones

PURPOSEnElectron intraoperative cones (EIORCs) commonly used for intraoperative radiation therapy (IORT) often generate high-dose regions at superficial depths. This study was performed to optimize the use of rings in the EIORC design that reduces the high-dose region while minimizing the loss of the treatment volume at the prescribed depth.nnnMETHODS AND MATERIALSnMonte Carlo simulations were performed to study the dosimetry properties of various EIORC designs. Simulations were conducted with EIORCs of various internal radii, lengths, and material compositions irradiated by available electron beam energies. The data were analyzed in terms of volume receiving > 105% and < 90% of the prescription dose, respectively.nnnRESULTSnThe high-dose volume increases with the EIORC size and the electron beam energy. The use of a ring inside the EIORC reduces the 105% dose volume but also increases the sub-90% volume. The degree of change of these volumes depends on the ring thickness and position.nnnCONCLUSIONnThe optimal ring position is about 10 cm from the bottom of the EIORC, regardless of the EIORC material, geometry, or electron energy. The optimal thickness of the ring is dependent on its material composition, the beam energy, and the preferred compromise between a uniform dose profile and a loss of treatment volume.

A method for more efficient source localization of interstitial implants with biplane radiographs

The conventional method for source localization in an interstitial ribbon implant by means of biplane radiographs can be difficult, especially when a large number of seeds are involved. We present a new algorithm for more efficient source localization with the same conventional biplane radiographs. The method does not require a one-to-one source correspondence between two radiographs. The user needs only to digitize several points, following the shape of each ribbon from both films. The points that are digitized do not need to be the location of the seeds, and they do not have to correspond to the same points on both films. The algorithm uses the multidimensional minimization method to reconstruct the three-dimensional locus of the ribbon. The location of each seed is then determined by its pathlength relative to the corresponding starting point. We have used phantom experiments and clinical cases to test the reliability of the algorithm. The results show that the errors in the determination of seed locations are less than 2 mm, and the efficiency in source digitization for data entry can be increased by a factor up to 5.

Collimated electron beams and their associated penumbra widths

The Fermi-Eyges multiple-scattering theory for electrons is applied to calculate profiles of collimated electron beams. The dose profile below the collimator is a convolution of the intensity distribution of the electrons at the level of the collimator and the distribution arising from the propagation of a Gaussian point source from the collimator to the level of the calculation. The electrons at the level of the collimator possess an angular distribution characteristic of the configuration of the electron beam at the vacuum window. Hence, the dose profile and its associated penumbra width can be expressed in terms of the angular moments of the distribution of the electrons at the collimator. The dependence of the penumbra width on the configuration-dependent angular spread of the electrons at the collimator accounts for differences in the size of the penumbra between two broad-beam configurations. These differences are also seen experimentally. We have also studied the dependence of the angular moments of the electrons upon scattering foils present above the collimator and the position of the beam-broadening device in the accelerator head.

An overview of errors in line source dosimetry for gamma-ray brachytherapy

The generally used approach for routine brachytherapy dosimetry depends on a value of source strength provided by the source supplier, the theoretical concept of exposure rate constants for different nuclides, and the applicability of inverse square law to point sources. Corrections for filtration of the radiation within the capsule of the source and attenuation in tissue medium are effected based on attenuation coefficients and data published in the literature. Therefore there is no unique or ideal way of selecting the required data and there could be a spread in the dose values derived by different users. The errors inherent in the current practice of dosimetry of linear sources in brachytherapy are discussed. If the brachytherapy sources are specified in terms of exposure rate at a distance from the source by the source suppliers, the overall uncertainty in the dosimetry at regions of clinical interest around the source could be limited to about +/- 6%.

The radiation therapy dosimetry network in the United States

The United States government and the governments of sixteen other countries have established radiation standards laboratories which house the primary standards dosimeters of the respective countries. In the United States, three regional calibration laboratories were established to disseminate the national radiation standards through the use of secondary standards dosimeters. These, in turn, are used for calibration of radiation field instruments used to measure radiation applied in therapy. These laboratories are periodically accredited by the American Assocation of Physicists in Medicine (AAPM). In 1976, AAPM established a Committee on Radiation Calibration Needs in Therapy to conduct a study of the adequacy of the therapy dosimetry network in the United States. According to this study, 1322 centers were performing radiotherapy in 1978, and 1057 radiation instruments were used to calibrate the therapy machines in these facilities. In more than 99% of the institutions, medical physicists are responsible for carrying out the necessary calibrations. The study showed that there is a need, though not an urgent one, for an additional regional calibration laboratory in the Far Western United States. Establishment of such a laboratory would not be excessively disruptive to the existing laboratories.