Experimental determination of depth-scaling factors and central axis depth dose for clinical electron beams.

Depth-scaling factors rho(eff) for clear polystyrene and polymethylmethacrylate (PMMA) phantoms have been determined experimentally as a function of nominal electron-beam energy in the range 6 to 22 MeV. Values of rho(eff) have been calculated from the ratio rho(eff) = R(wat)(50) / R(med)(50), where R(wat)(50) and R(med)(50) are the measured depths of 50% ionization in electron solid water and plastic (clear polystyrene and PMMA) phantoms, respectively. Measurements were made using an Attix chamber in an electron solid water phantom, a Holt chamber in a clear polystyrene phantom, and a Markus chamber in a PMMA phantom. The average value of measured rho(poly)(eff) was found to be 0.999 +/- 0.009. This is higher than the value of 0.975 recommended by Task Group 25 (TG-25) of the American Association of Physicists in Medicine (AAPM) by 2.5%. Depending on energy, the maximum differences between the AAPM TG-25-recommended and the measured values lie in the range 1% to 3.5%. Similarly, the average value of measured rho(PMMA)(eff) was found to be 1.168 +/- 0.023. This is higher than the AAPM TG-25-recommended value of 1.115, by 5%. Depending on energy, the maximum differences between the AAPM TG-25-recommended and the measured values lie in the range 3% to 8%. Central axis depth dose curves in water were generated for 6, 15, and 20 MeV electron beams from measured depth-ionization data in PMMA and clear polystyrene phantoms following the recommendations of the AAPM TG-25 report and using both TG-25-recommended and experimentally determined values of depth-scaling factors rho(eff). For both phantoms, either the TG-25-recommended value or the experimentally determined values of rho(eff) yielded agreement to within about 2 mm among all depth doses in water at the depths of clinical relevance.

A generalized film technique for the verification of vertex fields used in the treatment of brain tumors.

With the availability of commercial three-dimensional (3D)-treatment planning systems, more and more treatment plans call for the use of noncoplanar conformal beams for the treatment of brain tumors. However, techniques for the verification of many noncoplaner beams, such as vertex fields which involve any combination of gantry, collimator, and table angles, do not exist. The purpose of this work is to report on the results of an algorithm and a technique that have been developed for the verification of noncoplanar vertex fields used in the treatment of brain tumors. This technique is applicable to any geometric orientation of the beam, i.e., a beam orientation that consists of any combination of gantry, table, and collimator rotations. The method consists of superimposing a central plane image of a correctly magnified vertex field on a lateral or oblique field port film. To achieve this, the 3D coordinates of the projection of the isocenter onto the film for lateral (or oblique) as well as the vertex fields are determined and then appropriately matched. Coordinate transformation equations have been developed that enable this matching precisely. A film holder has been designed such that a film cassette can be secured rigidly along the side rails of the treatment table. The technique for taking a patient treatment setup verification film consists of two steps. In the first step, the gantry, table, and collimator angles for the lateral (or oblique) field are set and the usual double exposures are made; the first exposure corresponds to that of the treatment portal with the isocenter clearly identified and the second one a larger radiation field so that the peripheral anatomy is visible on the film. In the next step, the gantry, table, and collimator angles are positioned for the vertex field and the table is moved laterally and vertically and the film longitudinally to a position that will enable precise matching of the isocenter on the film. A third exposure is then taken with the vertex portal. What is seen on the film is a superposition of a central plane image of the vertex field onto the image of the lateral or oblique field. This technique has been used on 60 patients treated with noncoplanar fields for brain tumors. In all of these cases, the coincidence of the projection of the isocenter for the lateral (or oblique) and the vertex fields was found to be within 3 mm.

Experimental determination of fluence correction factors at depths beyond dmax for a Farmer type cylindrical ionization chamber in clinical electron beams.

Recently, it has been recommended that electron beam calibrations be performed at a new reference depth [Burns et al., Med. Phys. 23, 383 (1996)] given by dref = 0.6R50-0.1 cm, where R50 is the depth of 50% depth dose. In order to calibrate electron beams at dref with a Farmer type cylindrical ionization chamber, the values of the perturbation correction factors Pwall and Pfl at dref are required. Using a parallel plate Holt chamber as a reference chamber, the product PwallPfl has been determined for a 6.1-mm-diameter PTW cylindrical ionization chamber at dref as a function of R50 of clinical electron beams (6 < or = nominal energy E < or = 22 MeV). Assuming that Pwall for the PTW chamber is unity in electron beams, the measured Pfl values ranged from 0.96 to 0.98 as the energy is increased. These results are in close agreement with recently reported calculated values. Determination of dref requires the knowledge of R50. A relation between I50 and R50 is given in the IAEA Protocol [TRS No. 277 (IAEA, Vienna, 1987), pp. 1-98] for broad beams at SSD = 100 cm. It has been shown experimentally that the equation R50 = 1.029 x I50-0.063 cm, derived by Ding et al. [Med. Phys. 22, 489 (1995)] from Monte Carlo simulations of realistic clinical electron beams, can be used satisfactorily to obtain R50 from I50, where I50 is the depth of 50% ionization. The largest difference between the measured value of R50 and that calculated by using the above equation has been found to be about 1 mm at 22 MeV.

Anisotropy of an 192iridium high dose rate source measured with a miniature ionization chamber.

The anisotropy of a high dose rate (HDR) 192Ir source was measured in air and in water using a miniature (0.147 cm3) ionization chamber. Measurements were made at a distance of 5 cm from the source center at polar angles from 10 degrees-170 degrees. The anisotropy was found to be less pronounced in water, and the anisotropy is asymmetric about the transverse axis. The results agree with previous ionization chamber and TLD measurements to within +/- 4%. Mean anisotropy factors were determined at each angle from all existing data at 5 cm distance, and compared to published Monte Carlo calculations, and to the values used in the microSelectron HDR brachytherapy planning system (BPS). The Monte Carlo photon transport code appears to systematically underestimate the anisotropy factor by up to 4% in the forward direction and overestimate it by up to 3% in the backward direction. The mean anisotropy factors also indicate that the BPS systematically underestimates the anisotropy factor by up to 3% in the forward direction, and overestimates it by up to 15% in the backward direction. However, the 15% difference occurs at 180 degrees where it is not likely to be clinically significant.

Efficient CT simulation of the four-field technique for conformal radiotherapy of prostate carcinoma.

PURPOSE: Conformal radiotherapy of prostate carcinoma relies on contouring of individual CT slices for target and normal tissue localization. This process can be very time consuming. In the present report, we describe a method to more efficiently localize pelvic anatomy directly from digital reconstructed radiographs (DRRs). MATERIALS AND METHODS: Ten patients with prostate carcinoma underwent CT simulation (the spiral mode at 3 mm separation) for conformal four-field "box" radiotherapy. The bulbous urethra and bladder were opacified with iodinated contrast media. On lateral and anteroposterior DRRs, the volume of interest (VOI) was restricted to 1.0-1.5 cm tissue thickness to optimize digital radiograph reconstruction of the prostate and seminal vesicles. By removing unessential voxel elements, this method provided direct visualization of those structures. For comparison, the targets of each patient were also obtained by contouring CT axial slices. RESULTS: The method was successfully performed if the target structures were readily visualized and geometrically corresponded to those generated by contouring axial images. The targets in 9 of 10 patients were reliable representations of the CT-contoured volumes. One patient had 18 mm variation due to the lack of bladder opacification. Using VOIs to generate thin tissue DRRs, the time required for target and normal tissue localization was on the average less than 5 min. CONCLUSION: In CT simulation of the four-field irradiation technique for prostate carcinoma, thin-tissue DRRs allowed for efficient and accurate target localization without requiring individual axial image contouring. This method may facilitate positioning of the beam isocenter and provide reliable conformal radiotherapy.

Differential dose delivery using a nondocking applicator for intraoperative radiation therapy.

PURPOSE: Although treatment of a field within a field to deliver a boost dose is quite common with external photon beam radiation therapy, the same is not always true with electron beam radiation or in intraoperative radiation therapy (IORT). The purpose of this work is to report the results and details of a new technique developed to treat a field within a field in intraoperative radiation therapy. METHODS AND MATERIALS: This technique makes use of the nondocking IORT system currently used at our institution. Treatment is given in two segments: the large field is first treated by using standard circular lucite cones; the second dose segment is delivered using a new circular brass cone designed to fit concentrically within the large lucite cone. RESULTS: Central axis depth dose, surface dose, output factors, and two-dimensional beam profiles have been measured for a 7 cm inner diameter (i.d.) flat lucite cone and 3.8 and 5 cm i.d. flat brass cones for electron beam energies ranging from 4-22 MeV. For different clinical target volumes, summed dose distributions differentially weighted in both energy and dose are presented. CONCLUSIONS: A simple technique for delivering differential dose in intraoperative radiation therapy is presented. The technique provides a method for escalating dose to higher values for a defined target volume.

A dosimetry system for 192IR interstitial breast implants performed at the time of lumpectomy.

PURPOSE: 192Ir interstitial breast implants performed at the time of lumpectomy present a unique problem because they cannot be preplanned, and yet they are expected to produce a treatment dose rate (TDR) from 0.3 to 0.5 Gy/h using sources already procured. The purpose of this work is to describe a system of dosimetry that works within these constraints and has been used to perform more than 600 such implants. METHODS AND MATERIALS: The underlying principle is to fix the ribbon spacing, the interplaner separation, and the linear activity (1 mCi/cm) so that the TDR will depend only on the area (L x W) implanted. The ribbons are spaced 1.5 cm and 2.0 cm apart in single plane and double implants, respectively. Idealized implants were used to study the TDR as a function of the implant dimensions, and to study the effects of varying the ribbon spacing and interplanar separation. Volume-dose histograms were generated to study the homogeneity of dose. RESULTS: The TDRs of single plane implants range from 0.3 Gy/h for small 4 x 4 cm2 implants to 0.4 Gy/h for large 10 x 10 cm2 implants. The TDRs for double plane implants are similar for the same range of dimensions. CONCLUSIONS: Implants with a TDR between 0.3 and 0.5 Gy/h can be performed for a wide range of geometries without preplanning using fixed ribbons spacings of 1.5 and 2.0 cm for single and double plane implants, respectively, and a linear activity of 1 mCi/cm.

Limitations of the minimum peripheral dose as a parameter for dose specification in permanent 125I prostate implants.

PURPOSE: The objective of this work is to investigate whether the minimum peripheral dose is a practical parameter for dose specification in permanent 125I implants of the prostate. METHODS AND MATERIALS: The investigation was carried out by use of a computer model of ellipsoidal 125I implants in which the average dimension and elongation factor were varied to provide a wide range of geometries. Both ideal and nonideal implants were investigated. The 125I seeds were confined to the target volume except for a portion of the study in which the effect of placing seeds outside the target volume was investigated. RESULTS: The minimum peripheral dose was found to be very sensitive to the seed placement. The irregularities in the seed spacing that inevitably occur in actual implants tend to lower the minimum peripheral dose. As a result, the minimum peripheral dose is generally significantly less than planned by an amount that is unpredictable, and often exceeds 25%. However, the percentage of the target volume that receives a dose less that the prescribed minimum peripheral dose is generally less than 10%. Implanting seeds outside the target volume improves the dose uniformity, but does not appear to offer any advantage in dose coverage, and increases the volume of normal tissue irradiated. CONCLUSION: If a minimum peripheral dose is prescribed for a permanent 125I prostate implant, and the implant is planned using an idealized implant having precisely spaced seeds, the prescribed dose will rarely, if ever, be achieved. Reasonable agreement with the prescribed dose can be achieved only if the requirement for coverage is relaxed from 100 to 90%, or if the total source strength is increased by 20% to compensate for the anticipated imperfections in seed placement.

Dosimetric properties of megavoltage grid therapy.

PURPOSE: Grid therapy is a technique used to deliver a high dose of radiation (15-20 Gy) in a single fraction to many small volumes within a large treatment field. This treatment modality is used for the palliative treatment of large, deeply seated tumors, which have either been treated to tolerance with conventional radiation, or, due to massive tumor bulk, would most likely not benefit from a conventional course of radiation therapy. As the dose distribution from megavoltage grid therapy differs significantly from that of conventional radiation therapy (i.e., many large dose gradients exist within the tumor volume), we have measured various dosimetric properties inherent in this unique treatment modality. METHODS AND MATERIALS: The grid is a 16 x 16 array of 1-cm diameter holes in a 7-cm thick piece of custom blocking material. The ratio of shielded to open surface area is 1:1. Depth dose, valley-to-peak ratios, and output factors for this square array grid were measured in a water phantom for several field sizes, as well as for a 1-cm diameter narrow beam using 6 MV and 25 MV photon beams. RESULTS: The depth dose curves for the grid fields lie between those for an open portal and a narrow beam. For the 6-MV beam at dmax, the ratios of the doses delivered to the center of the shielded regions to that under the center of the holes, expressed as valley-to-peak ratios, range from 15 to 40%. At 10 cm, the ratios increase to between 25 and 45%. At 25 MV at both dmax and 10 cm, the valley-to-peak ratios are between 40 and 60%. The output factors, 0.89 for 6 MV and 0.77 for 25 MV, do not depend on field size. CONCLUSION: Megavoltage grid therapy is a unique treatment modality where the dose is delivered differentially to a large volume in one fraction. Characterization of the dosimetric properties has allowed clinical implementation of the grid.

Dosimetric characteristics of a commercial multileaf collimator.

The dosimetric characteristics of a multileaf collimator (MLC) retrofitted to a SL25 linear accelerator have been investigated. Central-axis depth dose, surface dose, penumbra, beam flatness and symmetry, field size factors, beam transmission through leaves and/or diaphragms, and leakage between the leaves were measured. Quantitative measurements of all beam parameters show good agreement with the design specifications of the manufacturer. No changes were observed in flatness, symmetry, penumbra, and penetration for both 6- and 25-MV photon beams when compared to the values for the standard collimator. No significant differences were observed in the penumbra as a function of leaf position. Transmission measurements in areas shielded by either X diaphragms or leaves plus diaphragms are less than 1% of dose within open field. The average leakage between leaves is about 2.5% for 6-MV and 3.5% for 25-MV photon beams. The peak value of the leakage at any point between leaves is less than 5%. The dosimetric features of shaped fields using the MLC are comparable to those of alloy shaped fields with the standard SL25 collimator.

Concept of dose nonuniformity in interstitial brachytherapy.

PURPOSE: Evaluation of the 3-dimensional dose distributions of interstitial implants using the dose uniformity ratio. METHODS AND MATERIALS: Single source, two sources, three and four sources arranged both linearly and in the form of a triangle or a square, ribbons with different seed spacings, a single-plane and double-plane implants were evaluated. The evaluations involved the use of differential dose volume histograms and the dose nonuniformity ratio defined as the ratio of the high dose volume to the reference volume. RESULTS: For a single source, the dose nonuniformity is the same regardless which dose rate is selected as the treatment dose rate. For any multi-source implant, the dose nonuniformity is altered depending on the selection of the reference dose rate. In addition, the dose nonuniformity curve exhibited three characteristics zones. CONCLUSION: The dose nonuniformity ratio can be a useful tool in assessing and optimizing interstitial implants.

Experimental verification of a three-dimensional dose calculation algorithm using a specially designed heterogeneous phantom.

A solid heterogeneous phantom made up of 25- and 50-mm cubes of materials with different electron densities was used to verify the accuracy of a three-dimensional (3-D) dose calculation algorithm. This algorithm uses 3-D information obtained from contiguous CT (computed tomography) slices, spaced 5 mm apart. Primary and scatter doses at a point are calculated by using information from ray-tracing CT voxels. The algorithm was developed on a Stardent model 1500 Supergraphic workstation. Cubes of materials with different electron densities were stacked up to simulate finite heterogeneities in three dimensions. This design allows verification of the algorithm for surface contour corrections and finite heterogeneities in the treatment field. Thermoluminescent lithium fluoride chips were placed in grooves milled on the cubes for dose measurement at various points. Different experiments were performed to investigate both the accuracy of the dose calculation algorithm and the utility of the versatile test phantom.

Quantitative assessment of interstitial implants.

Quantitative assessment of interstitial implants is proposed using volume versus dose curves and four well-defined dosimetric parameters. The volume versus dose curves, both differential and cumulative, provide quantitative data on the volumes of tissues irradiated to different doses. They also offer a qualitative assessment of the variations in dose delivery. The dose nonuniformity ratio (DNR) quantitatively determines the degree of dose nonuniformity specific to the implant configuration. The dose rate at which the DNR shows a minimum value, if selected as the treatment dose rate, gives an optimized dose distribution. The three volumetric irradiation indices are formulated with respect to a well-defined target volume. They offer quantitative data on the extent to which the implant delivers the prescribed dose to the target volume. These dosimetric parameters determine the degree of coverage of the target volume, dose homogeneity within the target volume, and irradiation of tissues outside the target volume. The method of quantitative assessment is demonstrated using, as examples, an ideal Ir-192 double-plane implant and an actual clinical Ir-192 double-plane breast implant.

Characteristics of photon beams from Philips SL25 linear accelerators.

The Philips SL25 accelerator is a multimodality machine offering asymmetric collimator jaws and a new type of beam bending and transport system. It produces photon beams, nominally at 6 and 25 MV, and a scattered electron beam with nine selectable energies between 4 and 22 MeV. Dosimetric characteristics for the 6- and 25-MV photon beams are presented with respect to field flatness, surface and depth dose characteristics, isodose distribution, field size factors for both open and wedged fields, and narrow beam transmission data in different materials.

Electron beam characteristics on a Philips SL25.

Dosimetry measurements at nominal electron energies of 4, 6, 8, 10, 12, 15, 17, 20, and 22 MeV were made for different sized, open-sided applicators on two Philips SL25 linear accelerators. Measurements include beam flatness, percentage depth dose, surface dose, isodose curves, field size dependence, output at extended distances, virtual source position, and required low melting point alloy thickness for field shaping. These measurements are presented to document the characteristics of electron beams with a new type of applicator design on this series of Philips accelerators.

Optimization of parameters for fitting linear accelerator photon beams using a modified CBEAM model.

Measured beam profiles and central-axis depth-dose data for 6- and 25-MV photon beams are used to generate a dose matrix which represents the full beam. A corresponding dose matrix is also calculated using the modified CBEAM model. The calculational model uses the usual set of three parameters to define the intensity at beam edges and the parameter that accounts for collimator transmission. An additional set of three parameters is used for the primary profile factor, expressed as a function of distance from the central axis. An optimization program has been adapted to automatically adjust these parameters to minimize the chi 2 between the measured and calculated data. The average values of the parameters for small (6 X 6 cm2), medium (10 X 10 cm2), and large (20 X 20 cm2) field sizes are found to represent the beam adequately for all field sizes. The calculated and the measured doses at any point agree to within 2% for any field size in the range 4 X 4 to 40 X 40 cm2.

Asymmetric field arc rotations.

Optimal treatment planning of target volume that surrounds a vital critical structure is often very difficult. Treatment techniques using moving beam therapy with fields asymmetric with respect to rotational axis of the collimator head allow treatment of such target volumes with minimal dose to critical structures. The availability of independent motion of the collimator jaws on new medical accelerators allows easy setting up of asymmetric treatment portals. Therefore, treatment techniques utilizing asymmetric field arc rotations with acceptable dose distributions have been possible.

Dosimetric considerations of stereotactic brain implants.

Dose distributions of stereotactic brain implants performed by four institutions were analyzed. In these implants 192Ir or 125I sources were used. The analyses involved an evaluation of the isodose distributions in two orthogonal planes, the dose gradient outside, and the dose homogeneity within the target volume. Quantitative evaluation of the dose homogeneity was performed using three volumetric irradiation indices. The dose homogeneity was observed to improve as the number of catheters increased. However, the number of catheters used is influenced by neurosurgical considerations. Thus, it is necessary to make a compromise between dose homogeneity and the maximum number of catheters to be used. The dose gradient, a centimeter outside the target volume, was found to depend on the geometry of the implant and at distances beyond, it was found to depend on the type of radioisotopes used.