Comparison of radiosurgery treatment modalities based on physical dose distributions.

PURPOSE: As a means of selecting the optimal stereotactic radiosurgery (SRS) treatment modality, a comparison of physical dose distributions to defined targets and nontarget brain tissue has been made for a group of test cases selected to represent a range of treatment-planning situations from small, nearly spherical volumes to large irregular volumes. METHODS AND MATERIALS: Plans were developed for each case using photon beams from the Leksell Gamma Unit (LGU), multiarc bremsstrahlung photon beams from a linear accelerator (linac) and proton beams, with the objective of encompassing the target as closely as possible with the prescription isodose line, and minimizing dosage to normal tissue within the bounds of standard clinical practice. Dose-volume histograms (DVHs) were calculated for target and for nontarget brain tissue and compared for the various modalities. RESULTS: In general, protons delivered less dosage to normal brain than other modalities for large and peripheral lesions and LGU plans were more successful at conforming to highly irregular shapes than conventional linac plans. CONCLUSIONS: Differences were observed to depend on treatment modality, target characteristics (shape, size and location), and the amount of effort expended on treatment planning and the time allotted for treatment implementation.

Comparison of radiosurgery treatment modalities based on complication and control probabilities.

PURPOSE: The relative efficacy of Gamma Knife, Linac, and Proton treatment modalities for stereotactic radiosurgery (SRS) was investigated on the basis of normal tissue complication probability (NTCP) and tumor control probability (TCP), calculated for representative test cases. METHODS AND MATERIALS: Five radiosurgery patient cases were selected to cover a range of treatment-planning situations from small spherical volumes to large irregular volumes. A target volume consisting of contours drawn on CT transverse slices was prepared for each case. Plans were developed using the three treatment modalities for each case, with the objective of encompassing the target as closely as possible with a prescription isodose line and minimizing dose to normal tissue, within the constraints of current clinical practice. Dose-volume histograms (DVH) were calculated for the target and for normal tissue, and these histograms were used to calculate NTCP and TCP values for each plan. RESULTS AND CONCLUSIONS: Differences in NTCP and TCP values were found to depend on treatment modality, size, shape, and location of the target, the amount of effort devoted to treatment planning, and the complexity of the plan.

Precision and accuracy of stereotactic convergent beam irradiations from a linear accelerator.

PURPOSE: The accuracy and the precision for radiosurgery procedures at linear accelerator facilities were investigated. METHODS AND MATERIALS: The technique of convergent beam irradiation, that is a series of successive isocentric arc irradiations, is specifically considered in this paper. Accuracy and precision depend on a sequence of methods and equipment among which localization of the target, patient alignment, and the dose delivery are the most critical steps. The purpose of the investigation was to quantitatively assess their contribution to the overall accuracy. The definitions and methods used to quantify and control accuracy are described. Measurements were carried out at a phantom to analyze the localization and positioning errors. Errors which may occur with the dose delivery technique were studied by a computer simulation. RESULTS: The calculations showed that these errors are not the main contributors to the overall accuracy as long as the linac inaccuracies are in the order or less than 1 mm. The accuracy found in the localization and positioning methods was less than 1 mm. CONCLUSION: It was concluded that an overall accuracy in the order of 1 mm can be obtained also under routine conditions. The great importance of adequate quality control is emphasized.

Radiation induced failures of complementary metal oxide semiconductor containing pacemakers: a potentially lethal complication.

New multi-programmable pacemakers frequently employ complementary metal oxide semiconductors (CMOS). This circuitry appears more sensitive to the effects of ionizing radiation when compared to the semiconductor circuits used in older pacemakers. A case of radiation induced runaway pacemaker in a CMOS device is described. Because of this and other recent reports of radiation therapy-induced CMOS type pacemaker failure, these pacemakers should not be irradiated. If necessary, the pacemaker can be shielded or moved to a site which can be shielded before institution of radiation therapy. This is done to prevent damage to the CMOS circuit and the life threatening arrythmias which may result from such damage.

Improved linac dose distributions for radiosurgery with elliptically shaped fields.

Stereotactic radiosurgery techniques for a linear accelerator typically use circular radiation fields to produce an essentially spherical radiation distribution with a steep dose gradient. Target volumes are frequently irregular in shape, and circular distributions may irradiate normal tissues to high dose as well as the target volume. Improvements to the dose distribution have been made using multiple target points and optimizing the dose per arc to the target. A retrospective review of 20 radiosurgery patients has suggested that the use of elliptically shaped fields may further improve the match of the radiation distribution to the intended target volume. This hypothesis has been verified with film measurements of the radiation distribution obtained using elliptical radiation beam in a head phantom. Reductions of 40% of the high dose volume have been obtained with elliptical fields compared to circular fields without compromising the dose to the target volume.

Stereotactic target point verification of an X ray and CT localizer.

Stereotactic radiosurgery with a linear accelerator requires the accurate determination of a target volume and an accurate match of the therapeutic radiation dose distribution to the target volume. X ray and CT localizers have been described that are used to define the target volume or target point from angiographic or CT data. To verify the accuracy of these localizers, measurements were made with a target point simulator and an anthropomorphic head phantom. The accuracy of determining a known, high contrast, target point with these localizers was found to be a maximum of +/- 0.5 mm and +/- 1.0 mm for the X ray and CT localizer, respectively. A technique using portal X rays taken with a linear accelerator to verify the target point is also described.

Radiosurgery target point alignment errors detected with portal film verification.

Stereotactic radiosurgery with a linear accelerator requires an accurate match of the therapeutic radiation distribution to the localized target volume. Techniques for localization of the target volume using CT scans and/or angiograms have been described. Alignment of the therapeutic radiation distribution to the intended point in stereotactic space is usually accomplished using precision mechanical scales which attach to the head ring. The present work describes a technique used to verify that the stereotactic coordinates of the center of the intended radiation distribution are in agreement with the localized target point coordinates. This technique uses anterior/posterior and lateral accelerator portal verification films to localize the stereotactic coordinates of the center of the radiation distribution with the patient in the treatment position. The results of 26 cases have been analyzed. Alignment errors of the therapeutic radiation distribution in excess of 1 mm have been found using the portal film verification procedure.

Optimization of high dose-rate cervix brachytherapy; Part I: Dose distribution.

Computer controlled high dose-rate (HDR) brachytherapy afterloading machines are equipped with a single, miniaturized, high activity Ir-192 source that can be rapidly moved in fine increments among several channels. Consequently, by appropriate programming of source dwell positions and times, the dose distribution can be optimized as desired. We have explored the optimization potential of this new technology for two applications: (a) cervix brachytherapy, and (b) transvaginal irradiation. Cervix brachytherapy with a gynecologic ring applicator was simulated by 48 sources of relative activities ranging from 0.17 to 1.00 that were equally distributed between the tandem and the ring. The results confirmed that the optimized distribution of physical doses are superior to those achievable with standard brachytherapy sources and applicators. For example, with five-point optimization, the relative dose-rate in the rectum was only 47% of that in point A; for standard application the dose rate was 47% higher. For transvaginal application 27 sources of relative activities between 0.07-0.79 were placed in the ring and a single source of unit strength in the tandem. Using dose distribution homogeneity as an optimization criterion, the results (+/- 2.5%) were again superior to those obtained for commonly used double ovoid (+/- 15%), linear cylinder (+/- 27%), or a "T" source (31%).

Dose determination in high dose-rate brachytherapy.

Although high dose-rate brachytherapy with a single, rapidly moving radiation source is becoming a common treatment modality, a suitable formalism for determination of the dose delivered by a moving radiation source has not yet been developed. At present, brachytherapy software simulates high dose-rate treatments using only a series of stationary sources, and consequently fails to account for the dose component delivered while the source is in motion. We now describe a practical model for determination of the true, total dose administered. The algorithm calculates both the dose delivered while the source is in motion within and outside of the implanted volume (dynamic component), and the dose delivered while the source is stationary at a series of fixed dwell points. It is shown that the dynamic dose element cannot be ignored because it always increases the dose at the prescription points and, in addition, distorts the dose distribution within and outside of the irradiated volume. Failure to account for the dynamic dose component results in dosimetric errors that range from significant (> 10%) to negligible (< 1%), depending on the prescribed dose, source activity, and source speed as defined by the implant geometry.

Computer controlled stereotaxic radiotherapy system.

A computer-controlled stereotaxic radiotherapy system based on a low-frequency magnetic field technology integrated with a single fixation point stereotaxic guide has been designed and instituted. The magnetic field, generated in space by a special field source located in the accelerator gantry, is digitized in real time by a field sensor that is six degree-of-freedom measurement device. As this sensor is an integral part of the patient stereotaxic halo, the patient position (x, y, z) and orientation (azimuth, elevation, roll) within the accelerator frame of reference are always known. Six parameters--three coordinates and three Euler space angles--are continuously transmitted to a computer where they are analyzed and compared with the stereotaxic parameters of the target point. Hence, the system facilitates rapid and accurate patient set-up for stereotaxic treatment as well as monitoring of patient during the subsequent irradiation session. The stereotaxic system has been developed to promote the integration of diagnostic and therapeutic procedures, with the specific aim of integrating CT and/or MR aided tumor localization and long term (4- to 7-week) fractionated radiotherapy of small intracranial and ocular lesions.

Photon contamination in 8-20-MeV electron beams from a linear accelerator.

The amount of x-ray contamination near the surface of a phantom irradiated with electron beams was measured directly. Measurements were done to ascertain if photon contamination in the beam contributes a higher dose to the more superficial layers of an irradiated medium than indicated by conventional methods. A 1.4-kG magnetic field was used to deflect the electron beams generated by a Philips SL/75-20 linear accelerator. The electron energies studied were 8, 10, 12, 14, 17, and 20 Mev. After sweeping the electron beam, a significant amount of photon contamination was measured in all cases. The characteristic qualities of the photon contamination were measured directly in a water tank. They were found to agree with those of bremsstrahlung spectra generated in a thin target with a virtual source at the location of the scattering foil.

A rapid method for electron beam energy check.

Assessment of electron beam energy and its long term stability is part of standard quality assurance practice in radiation oncology. Conventional depth-ionization or depth-film density measurements are time consuming both in terms of data acquisition and analysis. A procedure is described utilizing ionization measurements at two energy specific depths. It is based on a linear relationship between electron beam energy and its practical range. Energy shifts within the range covered by the two measurement depths are easily resolved. Within a range of +/- 0.50 MeV (+/- 1.30 MeV) around the established mean incident energy of 5.48 MeV (20.39 MeV), the method accuracy is better than 0.10 MeV.

Stereotactic radiosurgery: dose-volume analysis of linear accelerator techniques.

Stereotactic radiosurgery of the brain may be accomplished with a linear accelerator by performing several noncoplanar arcs of a highly collimated beam focused at a point. The shape of the radiation distribution produced by this technique is affected by the beam energy, field size, and the number and size of the arcs. The influence of these parameters on the resulting radiation distributions was analyzed by computing dose volume histograms for a typical brain. Dose volume functions were computed for: (a) the energy range of 4-24 MV x rays; (b) target sizes of 1-4 cm; and (c) 1-11 arcs and dynamic rotation. The dose volume histograms were found to be dependent on the number of arcs for target sizes of 1-4 cm. However, these differences were minimal for techniques with 4 arcs or more. The influence of beam energy on the dose volume histogram was also found to be minimal.

Comparison of proton and x-ray conformal dose distributions for radiosurgery applications.

Highly focused dose distributions for radiosurgery applications are successfully achieved using either multiple static high-energy particle beams or multiple-arc circular x-ray beams from a linac. It has been suggested that conformal x-ray techniques using dynamically shaped beams with a moving radiation source would offer advantages compared to the use of only circular beams. It is also thought that, generally, charged particle beams such as protons offer dose deposition advantages compared to x-ray beams. A comparison of dose distributions was made between a small number of discrete proton beams, multiple-arc circular x-ray beams, and conformal x-ray techniques. Treatment planning of a selection of radiosurgery cases was done for these three techniques. Target volumes ranged from 1.0-25.0 cm3. Dose distributions and dose volume histograms of the target and surrounding normal brain were calculated. The advantages and limitations of each technique were primarily dependent upon the shape and size of the target volume. In general, proton dose distributions were superior to x-ray distributions; both shaped proton and shaped x-ray beams delivered dose distributions which were more conformal than x-ray techniques using circular beams; and the differences between all proton and x-ray distributions were negligible for the smallest target volumes, and greatest for the larger target volumes.

Scattering effects on the dosimetry of iridium-192.

Dosimetry calculations for iridium-192 sources generally assume that a sufficient medium surrounds both the iridium source(s) and the point of calculation so that full scattering conditions exist. In several clinical applications the iridium sources may be anatomically located so that the full scattering requirement is not satisfied. To assess the magnitude of this problem, relative measurements were made with a small ionization chamber in phantoms near air and lung-equivalent interfaces. Dose reduction caused by decreasing the volume of scattering material near these interfaces was then evaluated for a few clinical applications. The results show that reductions on the order of 8% may be expected at the interface with minimal dose reduction within the volume of the implant itself. In addition, the results indicate the verification of source strength of iridium sources in phantom require phantom dimensions determined by the source-chamber separation distance.

A pencil beam algorithm for proton dose calculations.

The sharp lateral penumbra and the rapid fall-off of dose at the end of range of a proton beam are among the major advantages of proton radiation therapy. These beam characteristics depend on the position and characteristics of upstream beam-modifying devices such as apertures and compensating boluses. The extent of separation, if any, between these beam-modifying devices and the patient is particularly critical in this respect. We have developed a pencil beam algorithm for proton dose calculations which takes accurate account of the effects of materials upstream of the patient and of the air gap between them and the patient. The model includes a new approach to picking the locations of the pencil beams so as to more accurately model the penumbra and to more effectively account for the multiple-scattering effects of the media around the point of interest. We also present a faster broad-beam version of the algorithm which gives a reasonably accurate penumbra. Predictions of the algorithm and results from experiments performed in a large-field proton beam are presented. In general the algorithm agrees well with the measurements.

Tissue maximum ratios (and other parameters) of small circular 4, 6, 10, 15 and 24 MV x-ray beams for radiosurgery.

Small, circular, x-ray beams are commonly used for radiosurgery applications. Dosimetric characteristics of 4, 6, 10, 15 and 24 MV circular x-ray beams ranging in size from 10 to 40 mm are reported. These characteristics include the measurement of TMR, beam profiles and relative output factors. Measurements of these parameters were performed in a solid water phantom using film, a small diode, small parallel-plate and cylindrical ionization chambers and TLD. Comparison of relative dose measurements of small, circular beams performed using these detectors showed that the small diode, film and TLD results consistently agreed for circular beams as small as 10 mm diameter. Beam profiles were measured using film dosimetry. Comparison of TMR values of a 10 mm diameter beam measured using film and a small parallel-plate ionization chamber showed no significant differences. Tertiary collimators designed with tapered, divergence-matching holes, and straight-drilled holes have been used for radiosurgery applications. Measurement of beam penumbra produced with either of these types of tertiary collimators showed minimal differences between them.

Hyperfractionated radiation therapy and concurrent 5-fluorouracil, cisplatin and mitomycin-C in head and neck carcinoma. A pilot study.

Seventeen patients were entered into a Phase I/II trial of concurrent hyperfractionated radiation therapy (7,440 cGy total dose; 120 cGy b.i.d.) combined with constant infusion of 5-fluorouracil (5-FU) (1,000 mg/m2/24 hours for 72 hours) and cisplatin (DDP) (50 mg/m2) for a total of three cycles. Thirteen patients had Stage IV disease; three, Stage III disease; and one, Stage II hypopharyngeal disease. Thirteen of 17 patients had positive cervical lymph nodes, and the mean size of the largest lymph node was 5.5 x 5.1 cm. The patients were not treated with planned adjunctive surgery except for one patient who had a radical neck dissection for massive, rapidly growing cervical adenopathy, which recurred promptly within 1 month before the initiation of protocol therapy. After the initial six patients were entered, mitomycin-C (Mito 8 mg/m2) was added during the second cycle. All the patients completed the planned course of radiotherapy with a median dose of 7,440 cGy and a mean dose of 7,248 cGy except for two patients who died--one from toxicity and the other, suicide. The predominant toxicity was mucositis, which was grade 3/4 in 11 of 15 patients, resulting in an average interruption of radiation therapy of 12 days. Weight loss was significant and was on the average 12% of baseline weight. Hematological toxicity was mild in the 5-FU/DDP group (only one grade 3 toxicity of six) and severe in the 5-FU/DDP/Mito-treated patients (five of eight patients having grade 3/4 toxicity including one leukopenic pneumonitis death). Additional toxicity included one parapharyngeal cellulitis, which responded to antibiotics. Noncompliance with the complex regimen was only seen in three patients. One patient refused b.i.d. radiation therapy, and one patient refused further chemotherapy after the first cycle. Additionally, one patient who had a severe ethanol withdrawal reaction during the first cycle of 5-FU/DDP did not receive further chemotherapy. The complete response rate of both primary site and neck by the protocol regimen alone was 71%. However, two patients, one from each group, did undergo salvage neck dissection, and the locoregional control is currently 73%, with a mean follow-up time of 18.4 months. The feasibility of combining hyperfractionated radiation therapy with aggressive concurrent chemotherapy was demonstrated. The response and local control rate justifies the added toxicity of concurrent chemotherapy and radiation therapy.

Alanine EPR dosimeter response in proton therapy beams.

We report a series of measurements directed to assess the suitability of alanine as a mailable dosimeter for dosimetry quality assurance of proton radiation therapy beams. These measurements include dose-response of alanine at 140 MeV, and comparison of response vs energy with a parallel plate ionization chamber. All irradiations were made at the Harvard Cyclotron Laboratory, and the dosimeters were read at NIST. The results encourage us that alanine could be expected to serve as a mailable dosimeter with systematic error due to differential energy response no greater than 3% when doses of 25 Gy are used.

SU-D-211-05: A Systematic Analysis of Penumbra Characteristics in Conformai Arc Delivery for SBRT and Its Utilization for Negative Margin Technique (NMT), a Novel Planning Strategy for Improving Dose Conformation.

PURPOSE: To systematically analyze penumbra characteristics in co-planar arc-beams and demonstrate how optimal aperture margin (OAM) can be systematically estimated. METHODS: Full arc-beam delivery was simulated with a Pinnacle RTP for a total of 32 cases. Considered factors were energy (6X, 6F, 10X, and 10F, where 'F' indicates flattening-filter-free), field-size (2×2, 4×4, 6×6, and 8×8), and two materials (50×50×50 cm×cm×cm water and 50×50×50 cmxcmxcm water with a 20 cm diameter sphere of 0.25 g/cc lung density at the center). The highest-dose-gradient (HDG) and its range in both radial-dose-distribution (RDD) and longitudinal-dose-distribution (LDD) were evaluated. For OAM estimation, start with simulated dose distributions in interest. Measure coverage size (CS) at the chosen PIL in the RDD. The difference between the target size (TS) and the CS is the margin needed. In radial-direction, choose a PIL where the CS is larger than the TS [resulting in 'negative margin (NM)'] to enhance conformation capability.Next, measure the CS at the chosen PIL in the LDD. Longitudinally, positive margin is expected in general. This process was applied to 6X and 10F cases in lung to illustrate its applicability. RESULTS: Overall, larger HDG was observed in 6 MV (than 10 MV), smaller fields, LDD (than RDD), and in water (than lung). 6F showed slightly larger HDG than 6X while 10X did than 10F. Dose fall-off started significantly farther from the CAX in RDD than LDD, supporting the NM concept. With the systematic margin estimation applied, expected PILs matched closely with isodose points having tight target coverage, indicating the applicability of the method. CONCLUSIONS: Among tested, the 6F beam provides the most optimal dose distribution for dose conformation. The range of optimal PIL decreases as field-size increases. It has been demonstrated optimal AMs can be systematically estimated as proposed in this study.