Quick, efficient and effective patient-specific intensity-modulated radiation therapy quality assurance using log file and electronic portal imaging device

AIM

The aim of work is to explore a quick, efficient, and effective patient-specific intensity-modulated radiation therapy (IMRT) quality assurance (QA).

MATERIALS AND METHODS

Software tools were developed to extract and analyze the multi-leaf collimator (MLC) leaf positions (LPs) from electronic portal imaging device (EPID) images for Varian C-series machine and TrueBeam, to extract useful data from MLC log file of C-series linear accelerator (LINAC), to extract useful information from the trajectory log binary file of TrueBeam LINAC, to compare LPs derived from EPID images with log file/trajectory log data, and to analyze IMRT treatment files using the MATLAB programming language. The difference in LP determined from the trajectory log and EPID images was proposed for patient-specific QA.

RESULTS

It was found that the differences in LP for regular radiation fields generated using stationary leaves are <0.5 mm for all the field sizes while for regular radiation fields generated using the moving leaves are more but <2 mm. The differences in LPs for IMRT field were also determined and found to be <2 mm.

CONCLUSIONS

The methodology demonstrated can be used for establishing the accuracy of trajectory log data and for independent routine IMRT QA by generating single number like gamma index to indicate pass or fail of an IMRT treatment plan. The QA indices such as numbers of occurrences of ≥2 mm error in LPS are found more than 5% of total number of occurrences; the dosimetric review of planned treatment is advisable.

Acrylonitrile Butadiene Styrene (ABS) plastic based low cost tissue equivalent phantom for verification dosimetry in IMRT

A novel IMRT phantom was designed and fabricated using Acrylonitrile Butadiene Styrene (ABS) plastic. Physical properties of ABS plastic related to radiation interaction and dosimetry were compared with commonly available phantom materials for dose measurements in radiotherapy. The ABS IMRT phantom has provisions to hold various types of detectors such as ion chambers, radiographic/radiochromic films, TLDs, MOSFETs, and gel dosimeters. The measurements related to pretreatment dose verification in IMRT of carcinoma prostate were carried out using ABS and Scanditronix‐Wellhofer RW3 IMRT phantoms for five different cases. Point dose data were acquired using ionization chamber and TLD discs, while Gafchromic EBT and radiographic EDR2 films were used for generating 2D dose distributions. Treatment planning system (TPS) calculated and measured doses in ABS plastic and RW3 IMRT phantom were in agreement within ± 2%. The dose values at a point in a given patient acquired using ABS and RW3 phantoms were found comparable within 1%. Fluence maps and dose distributions of these patients generated by TPS and measured in ABS IMRT phantom were also found comparable both numerically and spatially. This study indicates that ABS plastic IMRT phantom is a tissue‐equivalent phantom and, dosimetrically, it is similar to solid/plastic water IMRT phantoms. Although this material is demonstrated for IMRT dose verification, it can also be used as a tissue‐equivalent phantom material for other dosimetry purposes in radiotherapy.

PACS number: 87.53Kn, 87.55Qr, 87.53Bn and 87.55Km

Operator-free, film-based 3D seed reconstruction in brachytherapy

In brachytherapy implants, the accuracy of dose calculation depends on the ability to localize radioactive sources correctly. If performed manually using planar images, this is a time-consuming and often error-prone process-primarily because each seed must be identified on (at least) two films. In principle, three films should allow automatic seed identification and position reconstruction; however, practical implementation of the numerous algorithms proposed so far appears to have only limited reliability. The motivation behind this work is to create a fast and reliable system for real-time implant evaluation using digital planar images obtained from radiotherapy simulators, or mobile x-ray/fluoroscopy systems. We have developed algorithms and code for 3D seed coordinate reconstruction. The input consists of projections of seed positions in each of three isocentric images taken at arbitrary angles. The method proposed here consists of a set of heuristic rules (in a sense, a learning algorithm) that attempts to minimize seed misclassifications. In the clinic, this means that the system must be impervious to errors resulting from patient motion as well as from finite tolerances accepted in equipment settings. The software program was tested with simulated data, a pelvic phantom and patient data. One hundred and twenty permanent prostate implants were examined (105 125I and 15 103Pd) with the number of seeds ranging from 35 to 138 (average 79). The mean distance between actual and reconstructed seed positions is in the range 0.03-0.11 cm. On a Pentium III computer at 600 MHz the reconstruction process takes 10-30 s. The total number of seeds is independently validated. The process is robust and able to account for errors introduced in the clinic.

An independent dose-to-point calculation program for the verification of high-dose-rate brachytherapy treatment planning

PURPOSE

We describe computer software that performs, quickly and accurately, secondary dose calculations for high-dose-rate (HDR) treatment plans, including those employed for prostate treatments.

METHODS

The program takes as primary input the data file used by the HDR remote afterloader console for treatment. Dosimetric calculations are performed using the Meisberger polynomial and the anisotropy table for the HDR Iridium-192 source. For standard applicators, treatment geometry is automatically reconstructed and the dose is calculated at relevant reference point(s). Template-based treatment plans (e.g., prostate) require additional user input; the dose calculation is then performed at user-selected reference points. A total dwell time calculation for volume and planar implants using the Manchester tables was also implemented.

RESULTS

For fixed-geometry HDR procedures, secondary dose calculations are within 2% of the treatment plan, and results are available for review instantly. For more general applications, the calculated and planned doses are typically within 3% at the prescription isodose line. The Manchester-based dwell time calculation is within 10% of the planned time.

Treatment planning for prostate implants using magnetic-resonance spectroscopy imaging

PURPOSE

Recent studies have demonstrated that magnetic-resonance spectroscopic imaging (MRSI) of the prostate may effectively distinguish between regions of cancer and normal prostatic epithelium. This diagnostic imaging tool takes advantage of the increased choline plus creatine versus citrate ratio found in malignant compared to normal prostate tissue. The purpose of this study is to describe a novel brachytherapy treatment-planning optimization module using an integer programming technique that will utilize biologic-based optimization. A method is described that registers MRSI to intraoperative-obtained ultrasound images and incorporates this information into a treatment-planning system to achieve dose escalation to intraprostatic tumor deposits.

METHODS

MRSI was obtained for a patient with Gleason 7 clinically localized prostate cancer. The ratios of choline plus creatine to citrate for the prostate were analyzed, and regions of high risk for malignant cells were identified. The ratios representing peaks on the MR spectrum were calculated on a spatial grid covering the prostate tissue. A procedure for mapping points of interest from the MRSI to the ultrasound images is described. An integer-programming technique is described as an optimization module to determine optimal seed distribution for permanent interstitial implantation. MRSI data are incorporated into the treatment-planning system to test the feasibility of dose escalation to positive voxels with relative sparing of surrounding normal tissues. The resultant tumor control probability (TCP) is estimated and compared to TCP for standard brachytherapy-planned implantation.

RESULTS

The proposed brachytherapy treatment-planning system is able to achieve a minimum dose of 120% of the 144 Gy prescription to the MRS positive voxels using (125)I seeds. The preset dose bounds of 100-150% to the prostate and 100-120% to the urethra were maintained. When compared to a standard plan without MRS-guided optimization, the estimated TCP for the MRS-optimized plan is superior. The enhanced TCP was more pronounced for smaller volumes of intraprostatic tumor deposits compared to estimated TCP values for larger lesions.

CONCLUSIONS

Using this brachytherapy-optimization system, we could demonstrate the feasibility of MRS-optimized dose distributions for (125)I permanent prostate implants. Based on probability estimates of anticipated improved TCP, this approach may have an impact on the ability to safely escalate dose and potentially improve outcome for patients with organ-confined but aggressive prostatic cancers. The magnitude of the TCP enhancement, and therefore the risks of ignoring the MR data, appear to be more substantial when the tumor is well localized; however, the gain achievable in TCP may depend quite considerably on the MRS tumor-detection efficiency.

A language for generating tomographic images of mathematical phantoms

A computer language is presented that can be used to generate image files, as if the images are created with a CT or a MR scanner. The language defines objects in the "scanner's" coordinate system, as sets of quadratic inequalities. Each of these objects, e.g., an ellipsoid or a half-plane or a cylinder, has its own density. Objects can be superimposed and collections of objects are allowed to translate and rotate. The language allows for a concise way of describing complex objects with precisely defined geometries and densities. An implementation of the language can be used for testing, developing, and analyzing diagnostic software, treatment planning systems, etc. A software module that is based on the language can be made available. The utility of the module for acceptance testing of radiation therapy treatment planning systems is described.

Methods to improve dose uniformity for radioactive stents in endovascular brachytherapy

Intravascular brachytherapy (IVBT) has rapidly gained acceptance as a new treatment modality for reducing restenosis and improving the success rate of percutaneous transluminal coronary angioplasty (PTCA). Recent clinical results on patients treated with beta-emitting 32P stents suggest that radiation reduces in-stent restenosis but may exacerbate neointimal growth at the edges of the stents. This has been referred to as the "candy wrapper effect." It is well known that radioactive stents yield extremely inhomogeneous dose distributions, with low doses delivered to tissues in between stent struts, at the ends of the stent, and also at depth. Some animal model studies suggest that low doses of radiation may stimulate rather than inhibit neointimal growth in an injured vessel, and it is hypothesized that dose inhomogeneity at the ends of a stent may contribute to the candy wrapper effect. We present here a theoretical study comparing dose distributions for beta stents vs. gamma stents; "dumbbell" radioactive loaded stents vs. uniformly loaded stents; and stents with alternate strut design. Calculations demonstrate that dose inhomogenieties between stent struts, at the ends of stents, and at depth can be reduced by better stent design and isotope selection. Prior to the introduction of radioactive stents, criteria for stent design included factors such as trackability, flexibility, strength, etc. We show here that if stent design also includes criteria for strut shape and spacing that improved dose distributions are possible, which in turn could reduce the candy wrapper effect.

Practical considerations in using calculated healthy-tissue complication probabilities for treatment-plan optimization

PURPOSE

Healthy and neoplastic tissues are generally exposed nonuniformly to ionizing radiation. It is thus useful to develop algorithms that predict the probability of tumor control or normal tissue complication probability (NTCP) for any given spatial pattern of dose delivery. The questions addressed here concern: (a) the sensitivity of the NTCP predictions to the actual model used for extrapolation from uniform irradiation (where some clinical data exist) to nonuniform exposures, (b) its dependence on tissue type, and (c) consequences for treatment-plan optimization.

METHODS AND MATERIALS

Two (of several possible) NTCP formulations are used here: the Lyman model and a binomial equation. The effective volume-reduction scheme of Kutcher and Burman is used to obtain the NTCP for an arbitrary distribution of dose. NTCP was calculated for seven organs by postulating a dose distribution of maximum nonuniformity.

RESULTS

Both models fit available NTCP data well, but have very different extrapolations for exposures of small tissue volumes and very low values of NTCP (e.g., < 5%) where no data exist. Organs with pronounced volume effects (lung, kidneys) show substantial NTCP differences between the two models. Even in organs where the volume effect is small (e.g., spinal cord, brain), differences in NTCP due to the model selected may still have serious clinical consequences, as an actual example (for the spinal cord) indicates.

CONCLUSIONS

NTCP calculations based on extrapolations to volume fractions and/or NTCP levels for which reliable data do not exist depend on the model used to fit the data and the degree of dose nonuniformity. If NTCP is to be used in treatment-plan optimization, the prudent approach is to design plans that reproduce the conditions under which available dose-volume data were taken (e. g., uniform dose distributions).

Review of endovascular brachytherapy physics for prevention of restenosis

Intraluminal irradiation of coronary and peripheral arteries has been shown to reduce neointimal hyperplasia following balloon angioplasty, thereby inhibiting restenosis. Several irradiation techniques are being investigated, including temporary intravascular insertion of high activity gamma- or beta-emitting seeds and wires; inflation of dilatation balloon catheter with radioactive liquid or gas; insertion of miniature x-ray tubes via coronary catheters; permanent implantation of radioactive stents; and postangioplasty fractionated external beam irradiation. Unlike conventional brachytherapy, intravascular treatment of restenosis requires accurate knowledge of dose at distances of 0.5-5 mm from the radioactive source. This requirement presents special problems with regard to source calibration and dose specification, because dose gradients at such close distances from a radioactive source are extremely large. This makes it virtually impossible to define the characteristics of an ideal radiation source without some knowledge of the location and radiosensitivity of the target tissues, plus the radiotolerance of normal tissues. Hence, the current debate over whether beta or gamma sources are to be preferred. Imprecise knowledge of dose-volume effects for coronary arteries, plus uncertainties in the biological time sequencing of restenosis fuel a second debate on whether external beam treatments may be efficacious, and whether or not permanent radioactive stents may prove superior to high dose, single fraction brachytherapy. We review here the dosimetric properties of the various irradiation techniques and isotopes that have been proposed, including aspects of radiation safety, dose homogeneity, and practical aspects of source delivery.

Intracoronary radiation for prevention of restenosis: dose perturbations caused by stents

BACKGROUND

Intravascular irradiation with beta-emitters has been proposed for inhibition of restenosis in coronary arteries after balloon angioplasty or stent implantation. Previous studies have shown the effectiveness of gamma-radiation to prevent recurrent restenosis, even in the presence of an implanted stent. The limited range of beta-particles compared with gamma-radiation, however, opens the question of whether absorption and scattering of beta-particles by stent struts will cause significant perturbations in the uniformity and magnitude of the radiation dose, which may in turn compromise treatment.

METHODS AND RESULTS

Nine different stents were deployed with a balloon filled with a beta-emitting radioactive liquid. Dose distributions were measured with Gafchromic film. Stents varied significantly in their absorption of beta-particles. Some stents, constructed of fine meshed wires, produced minimal dose perturbations. Others, with thicker, high-atomic-number struts, induced cold spots in the dose distribution adjacent to the wires of </=35%. Average dose reduction varied from 4% to 14% in the presence of various stents.

CONCLUSIONS

Radiation strategy may have to be tailored to stent design. Stents that minimally perturb the dose distribution may be deployed before irradiation. Those that significantly alter the radiation dose might be better deployed after irradiation. Dose prescriptions may require modification if such perturbations prove clinically significant. Observed dose perturbations, however, decreased rapidly with increasing distance from the stent, which may mitigate the clinical impact of these findings. This, as well as the effects of stents on gamma-dose distributions, requires further investigation.

A little to a lot or a lot to a little: is NTCP always minimized in multiport therapy?

PURPOSE

We address the question of whether or not, for the same average (or integral) dose, a smaller uniform dose to an entire normal tissue structure always results in a lower normal tissue complication probability (NTCP) than does a proportionally larger dose to a partial volume of the same structure.

METHODS AND MATERIALS

A recent compilation of NTCP data and two theoretical formulations of the dependence of NTCP on dose and partial volume irradiated-the Lyman probit equation and the binomial model-are used to examine this question. Both models fit equally well available NTCP data.

RESULTS

Empirical data indicate that for lung, kidney, and possibly liver (but not for esophagus, brain, or heart), given a fixed tumor dose and fixed integral dose, NTCP can be minimized by irradiating a partial volume fraction rather than the entire normal organ. The binomial model supports this interpretation, whereas the probit model predicts that for all organs uniform irradiation of the whole organ always results in the lowest possible NTCP.

CONCLUSIONS

In contrast to what is commonly believed, this study suggests that for at least two normal tissues, namely lung and kidney, there may be situations where "a lot to a little" (i.e., fewer treatment ports) will result in higher tumor control probability and better treatment plan than "a little to a lot" (i.e., multifield treatment). This finding, which is independent of the binomial or probit models used here, depends only on the accuracy of the empirical NTCP data. It is also interesting to note that: a) lung and kidney are commonly classified as parallel tissues, while the others have more of a serial architecture; and b) the choice of the NTCP model can have a profound impact on treatment planning decisions.

Predicting optical densitometer response as a function of light source characteristics for radiochromic film dosimetry

Various forms of GAFChromic (GC) film have been used for several years as radiographic media for measuring dose distributions of brachytherapy sources and small radiation fields. In order to optimize the measurement sensitivity and thus improve precision, we describe a method to calculate the dose response curves (net optical density at a give wavelength or spectrum versus absorbed dose) for different densitometer light sources using measured GC film absorption spectra. Comparison with measurements on the latest version of GC film (model MD-55-2) using four types of densitometers [He-Ne laser, broadband (white light) densitometer, and two LED (red-light) filtered densitometers] confirm the accuracy of this predictive model. The linearity and sensitivity of the dose response curves are found to be highly dependent on the light source spectrum. Initial slope is a function of the average weighted absorbance. Early saturation and decreased linearity of the dose response curves are ascribed to the nonuniform transmission of the light source through the GC film. We found that an LED (red-light) source with a narrow bandpass filter centered at 671 nm near the major absorption peak achieves nearly the maximum possible sensitivity (almost four times more sensitive than He-Ne laser, 632.8 nm) and may be suitable for in vivo dosimetry.

Physician/patient-driven risk assignment in radiation oncology: reality or fancy?

PURPOSE

Treatment plan optimization in radiation oncology entails designing multiple x-ray beams to irradiate a tumor to a dose that will achieve locoregional control while minimizing normal tissue complications. For some anatomical sites, it is possible to estimate tumor control probabilities (TCP) and normal tissue complication probabilities (NTCP) as a function of radiation dose. Thus, treatment plan optimization can be based on biologic end points rather than on dose calculations alone. Given multiple plans with different NTCPs and TCPs, a tradeoff must be made between maximizing TCP and maintaining an acceptable NTCP. How do physicians reach these decisions? Can the process be quantified? Should patients participate in the process?

METHODS AND MATERIALS

Physicians and patients were asked to rank a series of treatment plans having different combinations of TCP and NTCP. Responses were parametrized into a figure of merit (FM) equation which quantifies predilections of TCP and NTCP.

RESULTS

Physician-based FM equations are site- and patient-specific. Variations exist among physicians, but treatment plan selection is often conservative in accordance with the primum non nocere dictum. FM equations generated from the responses of patients suggest that some patients may be willing to accept higher treatment toxicity in exchange for increased TCP.

CONCLUSION

The term "optimized treatment plan" contains inherently subjective criteria which reflect one's willingness to accept treatment morbidity in exchange for probability of cure. These criteria may differ among patients and/or physicians. A quantifiable FM may permit the design of custom-made treatment plans that include physician and patient input.

Dosimetric considerations for catheter-based beta and gamma emitters in the therapy of neointimal hyperplasia in human coronary arteries

PURPOSE

Recent data indicate that intraluminal irradiation of coronary arteries following balloon angioplasty reduces proliferation of smooth muscle cells, neointima formation, and restenosis. We present calculations for various isotopes and geometries in an attempt to identify suitable source designs for such treatments.

METHODS AND MATERIALS

Analytical calculations of dose distributions and dose rates are presented for 192Ir, 125I, 103Pd, 32P, and 90Sr for use in intracoronary irradiation. The effects of source geometry and positioning accuracy are studied.

RESULTS

Accurate source centering, high dose rate, well-defined treatment volume, and radiation safety are all of concern; 15-20 Gy are required to a length of 2-3 cm of vessel wall (2-4 mm diameter). Dose must be confined to the region of the angioplasty, with reduced doses to normal tissues. Beta emitters have radiation safety advantages, but may not have suitable ranges for treating large diameter vessels. Gamma emitters deliver larger doses to normal tissues and to staff. Low energy x-ray emitters such as 125I and 103Pd reduce these risks but are not available at high enough activities. The feasibility of injecting a radioactive liquid directly into the angioplasty balloon is also explored.

CONCLUSIONS

Accurate source centering is found to be of great importance. If this can be accomplished, then high energy beta emitters such as 90Sr would be ideal sources. Otherwise, gamma emitters such as 192Ir may be optimal. A liquid beta source would have optimal geometry and dose distribution, but available sources, such as 32P are unsafe for use with available balloon catheters.

Microdosimetric evaluation of relative biological effectiveness for 103Pd, 125I, 241Am, and 192Ir brachytherapy sources

PURPOSE

To determine the microdosimetric-derived relative biological effectiveness (RBE) of 103Pd, 125I, 241Am, and 192Ir brachytherapy sources at low doses and/or low dose rates.

METHODS AND MATERIALS

The Theory of Dual Radiation Action can be used to predict expected RBE values based on the spatial distribution of energy deposition at microscopic levels from these sources. Single-event lineal energy spectra for these isotopes have been obtained both experimentally and theoretically. A grid-defined wall-less proportional counter was used to measure the lineal energy distributions. Unlike conventional Rossi proportional counters, the counter used in these measurements has a conducting nylon fiber as the central collecting anode and has no metal parts. Thus, the Z-dependence of the photoelectric effect is eliminated as a source of measurement error. Single-event spectra for these brachytherapy sources have been also calculated by: (a) the Monte Carlo code MCNP to generate the electron slowing down spectrum, (b) transport of monoenergetic electron tracks, event by event, with our Monte Carlo code DELTA, (c) using the concept of associated volume to obtain the lineal energy distribution f(y) for each monoenergetic electron, and (d) obtaining the composite lineal energy spectrum for a given brachytherapy source based on the electron spectrum calculated at step (a).

RESULTS

Relative to 60Co, the RBE values obtained from this study are: 2.3 for 103Pd, 2.1 for 125I, 2.1 for 241Am, and 1.3 for 192Ir.

CONCLUSIONS

These values are consistent with available data from in vitro cell survival experiments. We suggest that, at least for these brachytherapy sources, microdosimetry may be used as a credible alternative to time-consuming (and often uncertain) radiobiological experiments to obtain information on radiation quality and make reliable predictions of RBE in low dose rate brachytherapy.

Dosimetry of a radioactive coronary balloon dilatation catheter for treatment of neointimal hyperplasia

Recent reports suggest that intraluminal irradiation of coronary arteries in conjunction with balloon angioplasty reduces proliferation of smooth muscle cells and neointima formation, thereby inhibiting restenosis. One possible irradiation technique is to inflate the balloon dilitation catheter with a radioactive solution. This has advantages over other proposed irradiation procedures, in that accurate source positioning and uniform dose to the vessel wall are assured. Several high-energy beta-minus emitters may be suitable for this application. We present experimental measurements and analytical calculations of the dose distribution around a 3-mm-diam by 20-mm-long balloon filled with 90Y-chloride solution. The dose rate at the surface of the balloon is approximately 0.14 cGy/s per mCi/ml (3.78 x 10(-11) Gy/s per Bq/ml), with the dose decreasing to 53% at 0.5 mm, and < 5% at 3.5-mm radial distance. 90Y and other possible isotopes are currently available at specific concentrations > or = 50 mCi/ml (1.85 x 10(9) Bq/ml), which enables the delivery of 20 Gy in less than 5 min. The dosimetric and radiation safety advantages of this system warrant further feasibility studies. Issues of concern include incorporating the beta-emitter into a suitable chemical form, and assessing organ and whole body doses in the (< 1 in 10(3)) event of balloon failure.

Derivations of relative biological effectiveness for the high-let radiations produced during boron neutron capture irradiations of the 9L rat gliosarcoma in vitro and in vivo

PURPOSE

Relative biological effectiveness (RBE) values for the high linear-energy-transfer particles produced during boron neutron capture therapy have generally been based on theoretical considerations or in vitro experiments. The purpose of this study was to independently determine RBE values for all of the boron neutron capture therapy dose components.

METHODS AND MATERIALS

Clonogenic cell survival data were obtained for 9L rat gliosarcoma cells irradiated in the Brookhaven Medical Research Reactor thermal neutron beam both in vitro and as an intracerebral tumor. These data were analyzed using the linear quadratic model for cell survival to derive measured RBE values for all beam components and for a number of different boron compounds.

RESULTS

In the absence of boron, the combined effects of the protons from the nitrogen capture, 14N(n,p)14C, and the fast neutron scatter, 1H(n,n')p, reactions generated RBEs of 3.7 in vitro and 3.2 in an in vivo/in vitro excision assay, compared to 250 kVp X rays using an end point of 1% cell survival. Apparent RBEs for the 10B(n,alpha)7Li reaction products were calculated from cell survival data following reactor irradiations in the presence of the amino acid p-boronophenylalanine, the sulfhydryl dodecaborate monomer or dimer, or boric acid. Apparent RBEs for the 10B(n,alpha)7Li reaction ranged from 1.2 to 9.8 depending on which boron compound was used. RBEs from the in vitro studies were consistently higher than from the in vivo/in vitro studies. Under any conditions, the apparent RBE for the 10B(n,alpha)7Li reaction with p-boronophenylalanine was higher than that with any other boron compound tested.

CONCLUSIONS

Generally accepted RBE values for the fast neutron and 14N(n,p)14C reaction components of the total dose are too low. The apparent RBEs calculated for the 10B(n,alpha)7Li reaction were compound-dependent and consistent with differences in the distribution of 10B relative to glioma cell nuclei.

Rapid dose calculations for stereotactic radiosurgery

A three-dimensional dose calculation algorithm is described for stereotactic radiosurgery using multiple noncoplanar beam arcs. Precalculated dose libraries of 20-deg arc segments, or mini arcs, are stored in computer memory which permits rapid calculation of complete, high resolution, three-dimensional isodose distributions and dose volume histograms. Three-dimensional patient contours and target volumes are obtained from CT scans and angiographic x rays. Rapid dose calculations are made possible by the use of arc libraries and an improved algorithm for mapping beam doses to the dose calculation grid. This permits more flexibility in designing optimum treatment plans, as five-six complete plans can be generated in less than 1 h. Thus many possible treatment options can be tested in the 3-4-h time period typically available in stereotactic procedures between CT scanning and treatment.

Microdosimetry for boron neutron capture therapy

Preclinical studies for boron neutron capture therapy (BNCT) using epithermal neutrons are ongoing at several laboratories. The absorbed dose in tumor cells is a function of the thermal neutron flux at depth, the microscopic boron concentration, and the size of the cell. Dosimetry is therefore complicated by the admixture of thermal, epithermal, and fast neutrons, plus gamma rays, and the array of secondary high-linear-energy-transfer particles produced within the patient from neutron interactions. Microdosimetry can be a viable technique for determining absorbed dose and radiation quality. A 2.5-cm-diameter tissue-equivalent gas proportional counter has been built with 50 parts per million (ppm) 10B incorporated into the walls and counting gas to simulate the boron uptake anticipated in tumors. Measurements of lineal energy (y) spectra for BNCT in simulated volumes of 1-10 microns diameter show a dose enhancement factor of 4.3 for 30 ppm boron, and a "y" of 250 keV/microns for the boron capture process. Chamber design plus details of experimental and calculated linear energy spectra will be presented.

Microdosimetric single-event spectra for megavoltage electrons

Experimental techniques have been developed for obtaining single-event microdosimetric spectra from hospital based linear accelerators. Therapeutic electrons of 12, 15, 18, and 20 MeV from Clinic 18 and 20 accelerators have been produced at ultralow dose rates. Details of the experimental methods have been described previously by the authors. Single-event lineal energy spectra for these beams, as measured by a Rossi chamber differ significantly from cobalt-60 both in shape and dose average lineal energy (yd) which, depending on electron energy, can be 20-40% lower. The spectral peaks for electrons are greatly enhanced compared to cobalt. The yd for electrons also differs from previously reported values for 10-15 MV photon beams. These differences are less pronounced than for differences with cobalt. Spectral peaks and shapes correlate well with the actual electron stopping powers in tissue. The theory of dual radiation action predicts changes in relative biological effectiveness at low doses for megavoltage photon and electrons. Although at clinical doses the predicted differences are not statistically significant.

Enhanced risk from low-energy screen--film mammography X rays

Modern screen-film mammography with molybdenum-anode X rays results in tissue doses being delivered primarily by photons with an energy of less than 20 keV. Such photons interact with tissue predominantly through the photoelectric effect, producing low-energy electrons that have different patterns of energy deposition at the cellular level compared with those from higher-energy X rays. These differences result in low doses of typical molybdenum-based mammography X rays having an estimated radiobiological effectiveness of approximately 1.3 compared with 80 kVp or 250 kVp X rays, and approximately 2 compared with higher-energy gamma rays. Thus the risk from mammography could be higher, by such factors, than previously estimated. This would result in the optimal age for beginning mammographic screening, derived from risk-benefit ratios, being increased by at least 1-2 years and possibly by as many as 10 years.

Input of treatment planning data via a video frame grabber

Video frame grabbers are powerful devices which perform rapid conversion of video images into digital format for subsequent computer processing. Recently, the cost of these devices has become competitive with sonic and magnetic digitizing tablets commonly used in radiation therapy treatment planning. Frame grabbers, plus all associated hardware/software can be interfaced to a variety of personal or microcomputers, and allow input of irregular treatment field and patient contour data into standard treatment planning systems. Such a system is described, which offers advantages of speed, accuracy, and resolution as compared to more conventional digitizing systems.

Acute radiation effects on cutaneous microvasculature: evaluation with a laser Doppler perfusion monitor

Laser Doppler perfusion monitoring is a noninvasive technique for measuring blood flow in epidermal microvasculature that makes use of the frequency shift of light reflected from red blood cells. Measurements in patients undergoing radiation therapy show increases in blood flow of ten to 25 times baseline at doses above 50 Gy, and increases are observed with doses as low as 2 Gy. Follow-up measurements show rapid decreases in flow levels after completion of therapy, but levels remain elevated even at 1 year.

Determination of the relative angle between orthogonal films for radioactive implants

Dose calculations for radioactive seed implants require knowledge of the three-dimensional source coordinates. These are usually determined from a pair of orthogonal radiographs. However, occasionally localization films are taken with portable, and/or nonisocentric x-ray units, and no assurances can be made as to whether or not the films were truly exposed in an orthogonal geometry. A three-dimensional magnification ring is described (consisting of three mutually orthogonal rings) which permits the treatment planner to accurately establish the exact source rotation angle used when exposing the films. This angle can in turn be used when reconstructing source coordinates, to insure accuracy in treatment planning.

Quantitative evaluation of a portal film contrast enhancement technique

A study was conducted to evaluate the subjective improvement in portal film image quality resulting from the contact copy contrast enhancement technique which was introduced six years ago. Five observers were asked to identify and orient polyvinyl chloride cylinder images on both original and contrast-enhanced portal films taken with a 10-MeV linear accelerator. Fixed reviewing periods (T) were alloted of 20, 40, and 60 s as well as unlimited viewing time in order to increase the clinical relevance of this comparison. A scoring system and a probability representation were used to compare the original and enhanced films as a function of T. The results show a substantial increase in object detectability for the enhanced films at the short viewing times (T = 20, 40, and 60 s). For longer times (T greater than or equal to 80 s) the object detectability for enhanced and original films is not statistically different.

The variability of clinical thermoluminescent dosimetry systems: a multi-institutional study

Thirty-two radiotherapy centers in the USA and Canada cooperated in a study of the variability of clinical thermoluminescent dosimetry (TLD) systems. The primary purpose of the survey was to ascertain the accuracy of TLD for the determination of in vivo dose measurements. Each participating institution provided two TLD packets for irradiation on a Clinac 4, at a prearranged time. Two batch irradiations were made. Thirty-two TLD packets, one from each institution, were uniformly irradiated to a dose of 22.35 cGy (known by us, but not by the participants). A second group of 32 packets were likewise irradiated to a dose of 179.0 cGy. Participants were told only that their TLD's would be irradiated to doses between 10 and 50 cGy, and 100 to 200 cGy. TLD's were then returned to the institutions of origin for readout, and the doses reported to us for analysis. Calibration factors, readout and annealing procedures, etc., were all established independently by each participant. Although these procedures varied widely between institutions, the mean values of the reported doses were within 5% and 3% of the expected values for the low and high doses, respectively. Standard deviations in the reported doses were 10% and 5%. Also of interest, however, is the finding that 22% (i.e., 14 out of 64) of the dose reportings were in error by more than 10%. The implications of these findings vis à vis radiotherapy are discussed.

A quantitative assessment of portal film contrast as a function of beam energy

Portal film contrast on a specially designed test phantom has been studied as a function of photon beam energy and object-to-film distance. The results provide important insights into the physical processes responsible for image contrast. In particular, theoretical calculations of Compton scatter reactions in the phantom can be used to predict visual film contrast. Good agreement between theory and experiment can be achieved by evaluating the double differential Compton cross sections [d sigma (E,theta)/dE d theta] in the test object without resorting to variable parameters or artificial normalization. These calculations demonstrate the importance of low-energy photons, object-to-film distance, and object size on portal film contrast.

Errors in wedged field isodose distributions

A combination of wedged field and beam rotation can result in treatment planning errors. A simple formula is given which describes this effect.

Radiobiological effectiveness (RBE) of megavoltage X-ray and electron beams in radiotherapy

Recent experiments indicate that significant differences exist in the microdosimetric properties (i.e., lineal energy distributions) of megavoltage X-ray and electron beams used in radiation therapy. In particular, dose averaged values of lineal energy for 18 MeV electrons are 10-30% lower than for 10 MeV bremsstrahlung X rays, which in turn are 30-60% lower than for 250 kVp X rays. Differences of this magnitude may manifest themselves in observable radiobiological effectiveness (RBE) differences between these radiations. Cell survival data have been obtained for line DLD-1 human tumor cells on all three of the above radiation sources. Results clearly demonstrate an RBE difference between orthovoltage and megavoltage radiation (P = 0.001). A small difference is also measured in RBE between megavoltage photons and megavoltage electrons, but the difference is not statistically significant (P = 0.25). All biological, dosimetric, and microdosimetric data were obtained under nearly identical geometric conditions. These data raise interesting questions vis à vis the applicability of microdosimetric theories in the interpretation of biological effects.

Collimator-produced energy degradation of megavoltage x rays

Monte Carlo computer calculations have been made to assess the effects of small collimators (i.e., output field sizes less than or equal to 1 cm) on megavoltage x-ray beams. Such collimators are often used in experiments to measure beam energy, half value layers, and other beam parameters. Our calculations demonstrate, however, that such procedures can introduce significant changes in these same parameters. Under certain conditions, more than 40% of the transmitted photons exiting the collimator do so with degraded energy. Optimum collimator design, however, can greatly reduce these effects. In particular, it is shown that the thickness of material required to effect a good collimator is not only dependent upon the photon energy, but also upon the size of the collimator hole.

Microdosimetry of 10-15 MeV bremsstrahlung x rays

Experimental techniques have been developed for obtaining microdosimetric spectra on a hospital-based linear accelerator. Teletherapy beams of 10 and 15 MeV bremsstrahlung x rays from a Varian Clinac-18 and Clinac-20, respectively, have been produced at ultralow dose rates (50-200 microGy/h) which enables direct measurements of lineal energy distributions with a conventional Rossi-type gas proportional counter. Extensive measurements have been made to insure that the dosimetric properties of these low dose rate beams are nearly identical to those produced under high dose rate clinical conditions. Analytical procedures have been developed to correct measured lineal energy spectra for pileup caused by the low duty factor of the linear accelerator. The lineal energy spectra of these megavoltage beams differ significantly from Co-60, with dose averaged lineal energies (yD) being 20%-30% lower than for Co-60. Although such differences may not be important at clinical doses, the theory of dual radiation action does predict a lower biological effectiveness for these beams at very low dose levels.

Induction of spinal cord paralysis by negative pi-mesons

As part of our investigations on late non-neoplastic injury induced by negative pi-mesons (pions), a series of studies have been performed using pion beams for the induction of spinal cord paralysis in the Fisher 344 rat. Groups of rats were exposed to 1, 5 or 15 daily doses of peak pions or X rays. Paralysis appeared earlier after treatment with pions than after X rays even in a comparison of groups with similar final incidences. A single dose RBE for spinal cord paralysis of 1.3 was found. The RBE rises to a value of 3.2 if the total dose is given as a series of 15 daily exposures. These RBEs are far larger than we have observed using other late injury end-points, such as tubular degeneration in the kidney or fibrosis and sclerosis in the support structures of the colon for which the single dose RBE is less than 1.2. The biological and/or physical basis for the high sensitivity of the spinal cord to peak pions has not yet been resolved, but these data have suggested caution in exposing the spinal cord to peak pions in our clinical trials.

A three-film technique for reconstruction of radioactive seed implants

Computer dose calculations for interstitial implants of radioactive seeds require a knowledge of the spatial coordinates of each seed in the implant. These coordinates are usually constructed from the seed images on either an orthogonal or a stereo pair of radiographs. Such a procedure, however, requires that each seed can be identified on each film. We have developed a computer algorithm which computes the locations of the seeds from a set of two stereo and one AP radiographs, even when seed identities on all films are indeterminable. This is done by means of a ray tracing technique. Under ideal conditions the projections from the three films uniquely define all seed locations. Under clinical conditions, however, where digitizing uncertainties and patient movement are inevitable, the algorithm must determine the most likely set of seed locations from the larger set of all possible seed locations. The criteria for making this selection, as well as clinical examples, are presented.

Multiple scattering distributions for therapeutic pion beams

Accurate treatment planning for therapeutic beams of negative pions requires knowledge of the multiple scattering of pions in biologically relevant materials. Complete spatial and angular scattering distributions have been measured for pions in scatterers of carbon, water and calcium. Measurements were made for targets varying in thickness from 0.5 to 21 g cm-2 and for pions with ranges of approximately 12 and 20 g cm-2. An array of scintillators, multiwire drift and multiwire proportional chambers was used to record the scattering of individual particles. These data are compared with the results of Molière scattering theory. The implications for pion treatment planning are discussed.

Preclinical studies of dynamic treatment modes in pion therapy

Preliminary results on a system for delivering dynamic pion radiotherapy treatments are reported. The desired treatment volume is scanned across a small, focused pion beam using a computer-controlled treatment couch. A computer-controlled rangeshifting device modulates the stopping pion depth distribution in coordination with the couch motion to conform the dose to the shape of the treatment volume. For certain shaped tumors, the system can result in substantial normal tissue dose sparing and better field flatness as compared to irradiation by static treatments with broad terms. The characteristics of this new system, plus preliminary results for typical dose distributions as measured with thermoluminescent dosimeters (TLDs), are presented.

Pion in vivo dosimetry using aluminum activation

The method of aluminum activation to 24Na has been shown feasible as a high-LET, in vivo dosimeter for clinical pion beams at the Clinton P. Anderson Meson Physics Facility in Los Alamos. A 3 X 3 in. phi NaI (Tl) well detector measures the 24Na activity following exposure by windowing the 2.75 MeV photopeak. Calculations of the 24Na activity agree well with experiment if one assumes a production ratio of 0.075 24Na/stopped pi- in aluminum, and an in-flight cross section of 26 mb. The activity is produced primarily by stopping pions although 15-25% of the activity is the result of neutrons. Thus, the induced activation is a good measure of high-LET dose. By comparison with high-LET dose measured by a 7.6 mu silicon detector and a Rossi chamber, the amount of high-LET dose per activation is found to be 1.35 X 10(-6) rad/(24Na/gm Al). A clinical setup has been installed and a sample patient measurement is compared with high-LET dose calculated by treatment planning programs.

OER and RBE for negative pion beams of different peak widths

Experimental data on survival curves for pion because of different peak widths under aerobic and hypoxic conditions are reported. Metabolic depletion of oxygen by the Chinese hamster cells (line V79) was used to obtain hypoxia. The results indicate that the RBE at the beam entrance (plateau) is approximately 1.0. When the Bragg peaks were broadened to widths of 1.3, 7.8, and 10.5 cm (at the 80% dose level), the RBE (50% cell survival) at the peak centres was 1.7, 1.6, and 1.2, respectively. The OER at the entrance was 2.4 compared with about 2.9 for X rays. The OER was independent of the survival level at which it was measured. The OER at the peak centres at widths of 1.3, 7.8, and 10.5 cm was 2.1, 2.4, and 2.2, respectively. These results indicate that, although the RBE at the centre of the 10.5 cm wide peak was significantly lower than at the centres of the 1.3 and 7.8 cm peaks, the OER values are similar for all peak widths used in this study.

A heavy particle comparative study. Part I: depth-dose distributions

The results of a comparative study of heavy particles of interest in radiotherapy, with peaks spread over a depth of 10 cm, are reported in four parts. The introduction to this study and the depth-dose distributions of the particles, (n, pi-, p, He, C, Ne, and Ar ions) are reported herein. The results indicate that protons give the best localization of dose. The degree of localization of dose with heavy ions is reduced with increasing charge on the ion. For ranges less than 15cm, heavier ions such as neon and argon still have favourable dose localization; however, for ranges in excess of 15 cm, heavy ions such as argon are unfavourable but superior to fast neutrons because penetration can be controlled by modulation of energy or range.