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The loss in therapeutic ratio accompanying a conversion from low dose-rate (LDR) to high dose-rate (HDR) intracavitary brachytherapy (ICR) requires increased attention to the precision and accuracy of dose distribution calculations and treatment delivery. While the HDR-ICR treatment unit allows better custom-tailored dose distributions compared to LDR, it also requires more attention to detail to achieve the distribution desired. Because the relative biological effectiveness of different isodose levels in a dose distribution varies with the absolute dose (as described in Part 1 of this article), the relative dose distribution used with LDR must be modified for HDR to produce the same expected biological effect. Because of the difference in the radiobiology and physical positioning, simply duplicating applications as performed with LDR misses opportunities for dose distribution improvement as well as opens possibilities for significant complications. Due to differences in positioning the applicator (e.g., retraction of the cervix low in the pelvis instead of packing the applicator high), traditional definitions of points of interest (such as point A) apply poorly with HDR-ICR, compelling new systems of dose specification. With HDR-CR, irreparable mistakes can happen very quickly, and quality assurance for the treatment plan and calculated dwell times prove much more important than with LDR. Key features of the dose distribution and constant relationships involving doses and dwell times help screen planned treatments for mistakes. This paper details the procedural and physical consideration of the Madison system for HDR-ICR brachytherapy for carcinoma of the cervix.

A comprehensive 3D visualization package developed for CT-based 3D radiation treatment planning has been modified to volume-render SAR data. The program accepts data from sequential thermographic thermometry measurements as well as calculated data from thermal models. In this presentation sample data obtained from a capacitive heating system ‘Thermotron-RFS’ is presented. This capability allows the generation of accurate standardized volumetric images of SAR and provides a valuable tool to better preplan hyperthermia treatments.

Purpose: To determine the effect of body weight and anatomical fat on initial setup for prostate patients receiving daily image guidance on helical tomotherapy. Method and Materials: Initial setup corrections from MVCT imaging were retrospectively analyzed for 25 prostate patients. All patients were set up in the head‐first supine position to skin tattoos, and immobilization consisted solely of foam knee pillows and foot rings. Patients were stratified into two groups based on body weight being above and below the median weight of all analyzed patients. Distributions of setup corrections in the lateral, longitudinal, and vertical directions were assessed. Additional preliminary analysis of anatomical fat comparing measurements of transverse areas of the body at the L4‐L5 vertebral inter‐space and setup tattoo levels was performed on planning CTimages of 13 out of the 25 patients. Results: The mean setup corrections in the positive vertical direction were significantly greater for Group A (patients weighing > 213 lbs) than for Group B (⩽ 213 lbs), which were 11.5 mm and 4.3 mm, respectively. This may be attributable to increased couch sag occurring with increased weight and with placing the patient head‐first instead of feet‐first. Variances for Group A were larger than for Group B, which show an increased variation in initial setup for the heavier patient group. The anatomical fat measures corresponded well with each other, indicating that the location of the body used for initial patient setup seems to correlate with methods used to characterize abdominal fat. Conclusion: Our measurements of setup corrections suggest that daily imaging is of greater benefit for heavier patients. Analysis of anatomical fat measures suggests that characteristics of the location of the setup tattoos are affected by abdominal fat. Conflict of Interest: Some authors have a financial interest in TomoTherapy, Inc.

The dosimetry of small photon beams used for stereotactic radiosurgery was investigated using Monte Carlo simulation, convolution calculations, and measurements. A Monte Carlo code was used to simulate radiation transport through a linear accelerator to produce and score energy spectrum and angular distribution of 6 MV bremsstrahlung photons exiting from the accelerator treatment head. These photons were then transported through a stereotactic collimator system and into a water phantom placed at isocenter. The energy spectrum was also used as input for the convolution method of photon dose calculation. Monte Carlo and convolution results were compared with the measured data obtained using an ionization chamber, a diode, and film.

M. W. DEWHIRST, D.V.M., PH.D.,* T. L. PHILLIPS, M.D.,+ T. V. SAMULSKI, PH.D.,+ P. STAUFFER, MSEE,? P. SHRIVASTAVA, PH.D.,+ B. PALIWAL, PH.D.,+ T. PAJAK, PH.D.,+ M. GILLIM, PH.D.,+ M. SAPOZINK, M.D., PH.D.,+ R. MYERSON, M.D., PH.D.,+ F. M. WATERMAN, PH.D.,+ S. A. SAPARETO, PH.D.,+ P. CORRY, PH.D.,+ T. C. CETAS, PH.D.,+ D. B. LEEPER, PH.D.,+ P. FESSENDEN, PH.D.,+ D. KAPP, M.D., PH.D.,+ J. R. OLESON, M.D., PH.D.+ AND B. EMAMI, M.D.*

Purpose: The purpose of this study was to evaluate volumetric kV Cone Beam CT(CBCT)image quality at different scan parameter settings in context to treatment planning tolerances. Method and Materials: Both large and small density phantoms with eight density inserts were scanned by GE LS CT/PET system, as well as the Varian's OBITM system in half fan and full fan scanning modes. Scans for CBCTimages were performed at different tube currents (20‐, 40‐ and 80‐mA) and source‐imager distance (SID) (150cm and 160cm) after prior calibration of each mode. Deviation of the Hounsfield Unit (HU) values at different settings compared to conventional kV CTimages were obtained for further evaluation. We also adjusted the CT number in CTimages to simulate CBCT artifacts that was not produced by our experiments, and to see how much degradation of image would violate dosimetric feasibility of CBCT based treatment planning. Treatment plans for single beam or multiple beams were calculated based on CT,CBCT and modified CTimages for various phantoms geometries and patients. Results and Conclusions: Results show that the HU for different anatomies in the body have different amount of change for different scan parameters settings (including current, SID and fan angle used) for CBCTimage acquisition. Larger variations in HU appeared in lung and dense bone regions, compared to those with HU closer to tissue. Maximum variations in HU were found in the images with data truncation. Dose profiles, dose volume histograms, isodose distributions and Gamma values of CBCT based plan with images scanned at full fan mode agree relatively well with CT based plan. Larger dose discrepancy appears in lung or dense bone region. Results from the CT‐modified images based plans show that the dosimetric error becomes significant as the HU variation goes beyond 50.

Purpose: To assess treatment site‐specific corrections in patient positioning from TomoTherapy Hi‐ART® megavoltage images.Method and Materials: Initial positioning corrections, determined by fusion of daily pre‐treatment MVCT to planning CTimages,wereanalyzed for different anatomical treatment sites.Over 600,1400, and 1200 fractions of head and neck (H&N) and brain,lung, and prostate tomotherapy treatments, respectively, were assessed. Translational and rotational, per‐fraction setup corrections were retrospectively compiled from individual patient archive files. Setup corrections were compared amongst anatomical treatment sites. Setup reproducibility was assessed by analyzing standard deviations and histograms of setup corrections. Setup accuracy was also assessed by analyzing 3D vector lengths and magnitudes of corrections. Results: Large variations in setup corrections were seen in all three disease sites. H&N treatments had a significantly smaller (p

Purpose: The objective of this study is to compare the calculation accuracy of Analytical Anisotropic Algorithm (AAA) to that of Adaptive Collapse Cone Convolution (CCC) algorithm for small beam sizes where the electron disequilibrium becomes significant. Method and Materials: The comparison of CCC and AAA calculations were performed for various field sizes (2×2, 4×4, 6×6 and 10×10) at 6‐ and 10‐MV photon energies on different phantom situations (homogeneous water phantom, cork/water heterogeneous phantoms, step phantom and IMRTlung phantom). Treatment plans for open beams, MLC shaped beams and IMRT plans were created and calculated by each algorithm. Absolute dose comparisons were made based on measurements, calculations and Monte Carlo Simulations. The XV radiographic films, GAFCHROMIC_EBT radiochromic films, PTW Ion Chamber and Sun Nuclear Map checker were used for measurements. Results and Conclusions: Point dose comparison along the central axis of beams shows that in homogeneous phantom AAA predicts dose within 2%, which is compatible to CCC. For the heterogeneous phantom with vertical density gradient, AAA predicts an up to 5% difference and compared to CCC with less than 2.5% difference. Depth Dose curves also showed that AAA overestimates the dose after passing through low density region. But inside the low density region, AAA gives a compatible prediction to CCC for very small fields. For those points far from heterogeneity, AAA results show a relatively good estimation. Profiles at different depths in the phantoms with density gradient along horizontal direction show that AAA does not model lateral scatter adequately which leads to discrepancies of up to ± 7% in the region of ±1cm lateral from the heterogeneous interface, compared to less than 2% for CCC. Planar dose comparison for IMRT plans shows AAA calculated dose fluence matches with film measured data.

Purpose: To evaluate treatment planning based on cone beam CT(CBCT) using latest software on a LINAC 21IX CBCTimaging system. Method and Materials: An anthropomorphic chest phantom having bone, soft tissue, and lung components was used to create and evaluate treatment plans based on conventional CT and CBCTimages. Conventional CTimages of 2.5 mm slice thickness were taken with a GE discovery LS CT/PET system. CBCTimages slices were also reconstructed from flat panel system on a Varian LINAC 21IX. Eclipse treatment planning system was used to compare treatment plans from the conventional CT and CBCTimages. The AAA algorithm was used in the treatment planning system for inhomogeneity correction. Regions of interest around the bone, soft tissue, and lung in both CT and CBCTimages using identical HU threshold values were drawn. Identical targets located in the lung were used in each treatment plan. Analysis of the treatment plans was performed by comparison of geometrical dimensions, total volumes and dose volume histograms of the target and regions of interest. Results: Geometric comparison of actual external spatial dimensions and others in lung and bone were found to be within 1 mm. Volumetric comparison of the regions of interest resulted in a 2.8% difference of the vertebrae, 3.3% of the right lung, and 3.7% of the total external volume. Dosimetric results show similar dose distributions. Dose volume histograms are also comparable. Conclusion: Results demonstrate that treatment planning based on CBCT is feasible. Plans created from CBCTimages are comparable to plans created with conventional CT systems. Conflict of Interest: Partly funded by Prostate Cancer Foundation grant UW 133‐HR30.

Results: 1. The average tumor motion in the entire cohort was 1.54-cm (range 0.7-1.9) in the cephalo-caudad directions, 1.26-cm(range 0.1-3.9) in the antero-posterior directions, and 0.56-cm ( range 0-1.1) in the lateral directions between maximal inspiration and maximal expiration. 2. All plans provided excellent tumor coverage with comparable EUD values for a prescription of 60Gy in 30 fractions of 2Gy; minimum 90% coverage. 3. Cord dose was kept below tolerance in all plans without significant compromise in BTV margins. 4. The dose to the esophagus was similarly not compromised with an NTDmean average of 13.8 Gy (1.0-36.9) 5. MIBH produced superior lung sparing evidenced by lower V20, mean lung dose, and NTDmean values. The average NTDmean for all plans was 20.5 Gy in FB and 14.3 Gy in MIBH. For standard plans (Trial 1, Table 1), the NTDmean values were 21.8 Gy (FB) and 15.4 Gy (MIBH), corresponding to a projected risk of grade 2 pneumonitis of 21% an d8%a t 6months, respectively (1). The NTDmean values for gated treatment plans (Trial 3, Table 1) were 19.3 Gy(FB) and 12.8 Gy (MIBH), corresponding to a 19% and 5% projected risk of pneumonitis at 6 months, respectively. Tomotherapy plans resulted in further improvement in lung DVHs, NTDmean, and a decrease in projected pneumonitis rates. Conclusions: If a 20% risk of Grade 2 pneumonitis is acceptable, the dose to standard expansion volumes can be escalated by a factor of 1.4 using MIBH; using gated delivery techniques dose can be escalated by a factor of 1.7. Tomotherapy enhances this factor still further. Previous studies have demonstrated that dose levels of 85 Gy are required to achieve 50% local control (2). MIBH, using narrow margins with the utilization of IMRT, should allow dose escalation up to and beyond 85Gy.

A technique for accomplishing 41-42 degrees C whole body hyperthermia (WBH) in unrestrained, unanesthetized mice using a simple apparatus is reported. This method combines a radiant heat technology with monitoring of individual rectal temperatures at 10-min intervals. In 66 heating sessions, involving 116 AKR mice and a total of 619 WBH treatments, the mortality rate was less than 1%. Treatment at 41-42 degrees C for periods of up to 120 min were accomplished. Relationships of several variables of mouse temperature-time profiles were studied including (1) initial core temperature, (2) rate of heating, (3) whole body irradiation, and (4) the presence of tumor (transplanted AKR leukemia). Sham treatments produce a consistent temperature-time profile showing about a 1 degree C rise. Between-mouse variability, as well as between-treatment variability in individual animals was estimated. Thermal mapping of the device demonstrates a range from 32-38 degrees C of the air temperature in the zone with the mice to a high of 47 degrees C near to the radiant heating surface at the top of the apparatus.

These Hyperthermia Quality Assurance guidelines are a result of a joint workshop of the Hyperthermia Committee of the American College of Radiology and the Hyperthermia Physics Center, which is the national quality assurance program under Contract No. N01-CM-37512 with the National Cancer Institute. Hyperthermia technology presently lacks the kind of standardization in equipment, treatment procedures, patient monitoring, and treatment documentation available in radiotherapy. Therefore, preventing unacceptable variability in treatment data demands a strong commitment to in-house quality control procedures and to centralized quality assurance reviews in cooperative multi-institutional trials. This paper presents a set of test procedures necessary to ensure proper operation of equipment, suggests a frequency for such tests, and also includes guidelines on quality control procedures to be used during treatment to improve the safety, effectiveness, and reproducibility of hyperthermia treatments. A set of forms are presented to indicate the minimum data, albeit incomplete, that must be collected for acceptable documentation of treatment. These guidelines should be valuable not only to the new entrants in the field but also to those participating in multi-institutional cooperative hyperthermia trials. They have been approved by the Hyperthermia Committees of American College of Radiology, American Society for Therapeutic Radiology and Oncology, Radiation Therapy Oncology Group and the American Association of Physicists in Medicine.

The methods of energy deposition, the power absorbtion properties of biological tissues and the basic components of a typical hyperthermia system are described. In addition, the clinical requirements of hyperthermia treatment are discussed. A perspective on treatment planning and quality control is presented.

Results of the evaluation of thermometry devices used during hyperthermia treatments at 14 different clinics in the USA are presented. Measurements were made by the Hyperthermia Physics Center (HPC, a national hyperthermia quality assurance program under NCI contract No. N01-CM-37512) according to a protocol. Our sample included thermocouples, fiberoptic thermometers, and high lead resistance thermistors. We found that only some but not all of the thermometers of each kind performed within the +/- 0.2 degrees C acceptability criteria of accuracy. The precision, stability, and response times achieved with each type of thermometer are presented. A summary of perturbations and artifacts typical for each system is presented together with suggested precautions to avoid them during clinical usage. We conclude that although the technology used with each thermometer system is capable of producing a temperature accuracy of 0.2 degrees C, this accuracy is clinically achievable only with a concerted effort and a constant alertness on the part of the investigator. Based on the combined experience of this survey, the clinical investigators we visited, and published reports, we present certain guidelines and procedures that can help to reduce the inaccuracies and improve the reliability of temperature data obtained in clinical hyperthermia trials.

Clinac-18 electron-beam energies and spectral energy distributions were measured by a magnetic analysis method to verify results obtained by the standard range method. A comparison of these measurements with nominal energies quoted by the manufacturer indicates that electron-beam measured by both methods are in agreement with the manufacturer's specifications to +/- 0.6 MeV. Spectral energy distribution of the electron beam was measured to be approximately 1 MeV (FWHM) at all energies.

When high energy electrons are used for chest wall irradiation the presence of ribs may contribute excessive inhomogeneity of dose distribution in the volume of interest. We have used a 10 kg (SI unit: IT) bending magnet to reduce these effects. We present measurements performed in polystyrene phantoms that are embedded with simulated ribs.

The formulation, manufacture and testing of an epoxy resin-based solid substitute for water is presented. This "solid water" has radiation characteristics very close volumetrically to those of water. When it is used as a dosimetry phantom for x- and gamma-ray beams in the radiotherapy range, phantom-to-water corrections and density corrections are eliminated. Relative transmission measurements have shown that the transmission through 10 cm of solid water is within 0.2% of that through an equal thickness of water for x and gamma rays. The use of this material for calibration phantoms can help achieve the goal of radiotherapy beam calibrations within +/- 1.0% of the true dose rate, easier to achieve.

Experimental verification of the difference between absorbed dose in tissue and the collision fraction of kerma requires precise knowledge of the absorbed dose curve, particularly in the build-up and build-down regions. A simple method for direct measurement of contamination-free build-up for 60Co, which should also be applicable for most of the photon energies commonly employed for treatment, is presented. It is shown that the contribution from air-scattered electrons to the surface dose may be removed by extrapolating measurements of build-up to zero field size. The remaining contribution to contamination from the collimators and other source-related hardware may be minimised by measuring these build-up curves sufficiently far from the source. These results were tested by measuring the build-up using a magnet to sweep scattered electrons from the primary photon beam and by measuring the surface dose in the limit of an evacuated beam path. The relative dose at zero depth in polystyrene was found to be approximately 8.9 +/- 0.3% of the dose at the depth of maximum build-up.

Cobalt-60 gamma-ray absorbed dose measurements were carried out using LiF thermoluminescent (TL) dosemeters to obtain additional insight into cavity theory as applied to dosemeters of a size intermediate relative to the range of secondary electrons. Stacks of dosemeters (0.4 mm x 3 mm x 3 mm, hot-pressed LiF TLD-100) were surrounded by homogeneous LiF, polystyrene, aluminium, copper or lead. The absorbed dose in the stack (0.1 to 0.7 g cm-2 thick) was derived from TL readings of the individual dosemeters, and was compared with calculations based on the Burlin cavity theory. A relatively strong asymmetry in the experimental absorbed-dose distribution was found between the front, rear and middle dosemeters of the stack, especially when lead was the surrounding medium. Cavity theory shows good agreement with the experimental results obtained in polystyrene, while theory tends to overestimate the dose in the LiF dosemeters enclosed in Al, Cu, and Pb. The general trend predicted by the Burlin theory is somewhat steeper relative to cavity size than that observed experimentally. The theory was modified to make it more applicable to the dose at the centre of the stack. For lead this clearly agreed more closely with experimental results than did the conventional theory.

All radiotherapyphoton beams are accompanied to some extent by secondary electrons which originate in interactions with source hardware, collimator, shadow tray, and/or the air through which the beam passes. Skin sparing, the shape of the dose buildup curve, and the depth of the dose maximum are all influenced by this electron ‘‘contamination.’’ The present study of a 6 0Co source employs a flat ion chamber to measure dose buildup curves in polystyrene at source distances of 72 to 200 cm, with an open beam or a filter of Lucite, Cu, Pb‐loaded acrylic, or Ba‐ or Pb‐loaded nonbrowning glass placed 57 cm from the source, using 5×5, 20×20, and 35×35‐cm2 beams as defined at 80 cm SSD. The effect of electron generation in the air was studied by placing a He‐gas‐filled plastic bag in the beam. A value of about 12% is estimated for the lowest relative dose obtainable with a polystyrene phantom in a ‘‘clear’’ 6 0Co γ‐ray beam of 1‐cm diameter.

An animal model was used to investigate the potential for delivering a regional temperature boost using microwave energy after whole body hyperthermia had been induced in a pig using low density, radiant heat. Thermal profiles and an unexpected toxicity in the region of the microwave energy deposition, which included the heart and liver, are reported.

Several methods have been employed to calculate the relative contribution to skin dose due to scattered electrons in Co-60 gamma-ray beams. Either the Klein-Nishina differential scattering probability is employed to determine the number and initial energy of electrons scattered into the direction of a detector, or a Gaussian approximation is used to specify the surface distribution of initial pencil electron beams created by parallel or diverging photon fields. Results of these calculations are compared with experimental data. In addition, that fraction of relative surface dose resulting from photon interactions in air alone is estimated and compared with data extrapolated from measurements at large source-surface distance (SSD). The contribution to surface dose from electrons generated in air is 50% or more of the total skin dose for SSDs greater than 80 cm.

The characteristics of wedged fields which affect their clinical use have been examined for the 10-MV x-ray beam from the Clinac-18. The methods used for obtaining and analyzing the wedged-field data are discussed. These characteristics have been examined in terms of (a) the wedge angle, (b) the variation of the wedge angle with field size, (c) the variation in the angle through which isodose curves between the approximate depths of 5 and 15 cm are turned relative to the central axis as a function of depth, and (d) the variation of wedge central-axis transmission factor with field size. Analysis of the data pertinent to these categories is presented for the 15-, 30-, 45-, and 60-deg wedges. In addition, the effect of the 60-deg wedge upon the position of maximum dose on the central axis was determined.

Many radiotherapy linear accelerators use electron beam applicators which extended close to the patient's surface when treating at the regular distance. The relatively large size of these applicators often necessitates the use of a larger SSD than that designed by the manufacturer. In such cases, the addition of shielding blocks to the applicator can significantly alter the factors which should be used to calculate the dose rate at the new SSD as compared with the unshielded beam condition. In some typical clinical situations, errors of greater than 60% may result from failure to account for this perturbation. This paper presents the proper correction of the dose vs SSD function for the presence of such shielding blocks for the Clinac-18 linear accelerator.

A simple technique to evaluate styrofoam cutouts used in irregular field shaping is described. The technique consists of rendering visible the edges of the cutout on a radiograph. The edges of a styrofoam cutout are made radiopaque by spraying Renografin-76 onto the side walls of the cutout; the cutout then is radiographed with the patient in treatment simulation position. The radiograph shows dearly the outline of the area to be shielded on the patient's anatomy, with a sharp image of the opaque media.

Solid phantoms designed with nesting, rotatable cylinders, allow increased flexibility in the positioning of dosimeters or test objects, permitting continuous movement as in a water phantom, but with increased ease of handling. Using such phantoms, measurements can be made at almost any point in the cross section of a phantom, an ability not achievable with currently existing solid or water phantoms. A separate phantom design allows easy measurement at points along circles or radii in a phantom.

Often, the radiation exposure levels around brachytherapy patient rooms exceed the limits allowed by Nuclear Regulatory Commission (NRC) regulations and National Council on Radiation Protection (NCRP) recommendations. Since a neighboring patient may be present in the radiation field from a brachytherapy patient, or patients, continuously for 7 days, the dose equivalent rate must not exceed 6 muSv/hr (0.6 mrem/hr). Several boundary conditions constrain solutions to the problem of excessive radiation levels: (1) the cost should not be recurring, (2) the rooms used for brachytherapy patients must be compatible with use for nonbrachytherapy patients, (3) free access through halls must be maintained, (4) all brachytherapy rooms must be compatible with independent use, (5) shielding must not eliminate space required for a resuscitation cart, (6) shielding should not require storage, (7) the weight of shielding must remain within the tolerance of the floor, and (8) the location of beds must match utility outlets in the wall. Possible solutions to the problem include leaving rooms empty around the patient, use of portable shields, or the addition of substantial amounts of shielding material in the walls of the brachytherapy rooms, as well as changing the radionuclides routinely used. The optimum solution for a given institution depends on the frequency of brachytherapy procedures, bed occupancy rate, and available empty space.

The Hyperthermia Physics Center (HPC), under contract with the National Cancer Institute (NCI), has conducted review-type quality assurance (QA) measurements at the five Hyperthermia Equipment Evaluation Centers involved in evaluating the clinical efficacy of a variety of devices for delivering heat treatments to deep-seated human tumours. A summary of the QA protocol, results, testing procedures, standards, criteria, conclusions and recommendations are presented in this paper. The QA review measurements indicate (a) that 81.5 per cent of temperatures surveyed were within the 0.2 degrees C HPC criterion (91 per cent were within 0.4 degrees C), (b) that only 66 per cent of power indications were within the 10 per cent criterion, (c) that the heat patterns in a phantom produced by the BSD Annular Phased Array (AA) had significant variability, (d) that each treatment facility had at least a few potentially occupiable locations where the maximum permissible American National Standards Institute standards of electromagnetic leakage were exceeded, and (e) that these levels of accuracy and safety were achieved only after stringent inhouse QA efforts. From the combined data, it is concluded that the temperature accuracy in this cooperative trial was sufficient to justify a common analysis of clinical data as presented in this series. Also, stringent quality control of every parameter must continue to be stressed in all future hyperthermia trials.

Heating patterns generated by a commercially available 13.5 MHz radiofrequency generator and induction coil hyperthermia system in human size phantoms and a 230 pound pig were studied using a multichannel computermonitored thermometry system that is noninteractive in electromagnetic fields. The phantom studies were composed of synthetic muscle equivalent material and fresh tissue. The pig was heated in the regions of the upper abdomen and the midthorax, both under anesthesia and dead. The temperature was measured along fine penetrating catheters at 1 cm intervals in all experiments. In a homogeneous cylindrical phantom, under our measurement conditions, the temperature profile across the diameter is parabolic with marked superficial heating and essentially no central heating. In nonhomogeneous phantoms and in the pig, the symmetry of this profile was distorted but the basic pattern of marked superficial heating and nearly absent deep central heating remained. Blood flow in the living animal produced some thermal smoothing. It is considered probable that substantial radial temperature gradients will exist within eccentrically located human tumors heated with this device and that certain deep central tumors will be difficult or impossible to heat. Determination of its ultimate value for investigational clinical hyperthermia studies will require accurate temperature mapping of tumors and normal tissues in various anatomic sites in comparison with other approaches to deep heating.

An empirical expression of cavity theory for electron fields is developed in a fashion similar to Burlin's general theory of cavity ionization for photons. It incorporates a term that relates the absorbed dose ratio in the cavity and material medium to the differences in electron scattering. This new expression correlates very well with experimental observations for progressively increasing LiF cavity sizes embedded in different media, the energy response of LiF at high electron energies relative to 60Co and the absorbed dose conversion factors CE in the ionization chamber dosimetry of electron beams.

An auxiliary flatness filter for large field treatment on 4 MV linear accelerator provides an isodose distribution which does not vary more than 6% over the central 80% of the axis of a 32 x 32-cm field at 80 cm in air, and at a 2.5, 6, and 12-cm depth in water. This lead disc filter does not affect the central axis depth dose values, and results in only a 16% reduction in dose rate on the central field axis.

Modern curative radiotherapy requires higher doses to the tumor volume and, necessarily, minimal doses to the surrounding normal tissues. Attempts to use heavy charged particles to achieve such optimization are currently under investigation in many centers. Our data indicate that a static, superimposed magnetic field on a clinical electron therapy beam also offers the capability of some “tailoring” of isodose distributions. Furthermore, a variable, superimposed magnetic field minimizes those tissue generated dose heterogeneities which are inherent with all charged particle beams. We suggest that magnetically modified, clinically available electron beams also offer a practical and less expensive means of achieving tailored, heterogeneity-corrected, isodose distributions.

Summary Complex irregular fields are often used for the treatment of tumors in radiation therapy. The dose distributionof irregularly shaped fields depend strongly on the shape of field irregularities. The effect of field irregularities on the uniformity of the dose distribution of a 4 MV photon beam has been studied. The uniformity index of the blocked field which is a measure of the dose uniformity of the field has been compared to its corresponding unblocked field. The measured and computer calculated dose are compared for points within 1 and 2 cm from the edge of the field irregularities at the depth of 10 cm. The discrepancies between the computer calculated dose and the measured dose are discussed. Some suggestions are made to improve the dose uniformity of the irregularly shaped blocked fields.