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PubMed2006

Light field and crosshair quality assurance test using a simple lens system.

We describe here a simple lens system to test the positioning of the field light source and mylar crosshair for radiation therapy linear accelerators. Ideally the light source for the field light and the crosshair should be centered on the axis of rotation of the collimator. The traditional method for testing this coincidence uses the shadow of the crosshair caused by the field light source. The shadow of the crosshair is dependent on the position of both the field light source and mylar crosshair. Geometrically it is possible for the field light source and the mylar crosshair to be off the axis of rotation of the collimator and still cause the shadow of the crosshair to be on the axis of rotation at some distance. Using a lens system the motion of the field light source and crosshair can be observed in sharp focus independently of one another as the collimator is rotated.

Equipment Design↗

PubMed1989

[Stereotactic single-dose irradiation of cerebral angioma].

Angiomas are vascular malformations conceived as congenital anomalies. Surgical excision is the treatment of choice; an alternative is embolization. Radiation therapy is indicated: (1) for angiomas in vital or sensitive regions of the brain where they cannot be excised without the risk of a disabling neurological deficit and (2) for residual angiomas after partial embolization or incomplete surgical removal. The most effective radiotherapy for angiomas is radiosurgery. "Radiosurgery" is a term used for a special concept of radiotherapy. It describes stereotactically guided single high-dose irradiation. Therefore, a linear accelerator was modified at the German Cancer Research Center in Heidelberg. The precise localization of the target point and a steep dose gradient outside the target volume allow application of high doses to the tumor without damage to the surrounding normal brain tissue. Since 1984, in Heidelberg 41 patients with cerebral angiomas have been treated by stereotactic single high-dose irradiation (doses: 10-28 Gy). Eighteen patients were angiographically examined with a follow-up period longer than 18 months. In 7 patients, the pathological vessels was completely obliterated and in 6 partially. There was no change in 6 patients - all of whom received target doses of below 12 Gy.

Adolescent↗

PubMed1978

[Linear accelerators in oncoradiology. Comparison of the effciency of four clinically utilized electron linear accelerators with energies up to 20 MeV (author's transl)].

Further development of linear accelerators for medical purposes results in leaving off the principle of circular acceleration. The present investigation, comparing four clinically utilized electron linear accelerators with different constructional systems, reveals a large scale of clinical applicability, advantages and disadvantages partly compensating one another. None of the four machines examined offers a total of decisive advantages as compared to the other ones. Some essential data from the side of radiation exit are tested: depth dose data and photon contamination during operation with electrons, field sizes and homogeneity, penumbral width and penetration power for electron and X radiations. The appreciation of electron therapy by the radiologist may be a factor as well the handling of the machine, but in many cases the space available in an existing locality or the confidence in the ready service of a contracting firm, considerations concerning failure periods, estimate of repair and subsequent costs are influencing the decision for a certain type of equipment.

Electrons↗

PubMed1999

[A multileaf collimator: additional quality assurance in a GE linear accelerator].

BACKGROUND: In comparison to a conventional collimator, a multileaf collimator demands a great deal of quality assurance procedures due to its large number of leaves. A concept for daily quality assurance is presented, mainly concerning the positioning accuracy of the leaves. MATERIAL AND METHODS: Two leaf configurations including maximal opening as well as overtravel of single leaves, at a maximal opening of the jaws, are transmitted online in daily exchange from our record- and verify system to the linac. Aiming at a special test phantom a visual control of the positioning accuracy is performed. The leaf positioning is documented by an electronic portal imaging system and is compared with a reference shot by superposition of a grid. RESULTS: This method of quality assurance offers a fast and effective possibility to guarantee the proper function of the whole system by simulating the routine treatment situation. CONCLUSIONS: Compared to a conventional collimator only a slightly greater workload is needed for quality assurance of a multileaf collimator.

Equipment Design↗

PubMed2004

Feasibility of synchronization of real-time tumor-tracking radiotherapy and intensity-modulated radiotherapy from viewpoint of excessive dose from fluoroscopy.

PURPOSE: Synchronization of the techniques in real-time tumor-tracking radiotherapy (RTRT) and intensity-modulated RT (IMRT) is expected to be useful for the treatment of tumors in motion. Our goal was to estimate the feasibility of the synchronization from the viewpoint of excessive dose resulting from the use of fluoroscopy. METHODS AND MATERIALS: Using an ionization chamber for diagnostic X-rays, we measured the air kerma rate, surface dose with backscatter, and dose distribution in depth in a solid phantom from a fluoroscopic RTRT system. A nominal 50-120 kilovoltage peak (kVp) of X-ray energy and a nominal 1-4 ms of pulse width were used in the measurements. RESULTS: The mean +/- SD air kerma rate from one fluoroscope was 238.8 +/- 0.54 mGy/h for a nominal pulse width of 2.0 ms and nominal 100 kVp of X-ray energy at the isocenter of the linear accelerator. The air kerma rate increased steeply with the increase in the X-ray beam energy. The surface dose was 28-980 mGy/h. The absorbed dose at a 5.0-cm depth in the phantom was 37-58% of the peak dose. The estimated skin surface dose from one fluoroscope in RTRT was 29-1182 mGy/h and was strongly dependent on the kilovoltage peak and pulse width of the fluoroscope and slightly dependent on the distance between the skin and isocenter. CONCLUSION: The skin surface dose and absorbed depth dose resulting from fluoroscopy during RTRT can be significant if RTRT is synchronized with IMRT using a multileaf collimator. Precise estimation of the absorbed dose from fluoroscopy during RT and approaches to reduce the amount of exposure are mandatory.

Calibration↗

PubMed2001

Comparative study between IMRT with NOMOS BEAK and linac-based radiosurgery in the treatment of intracranial lesions.

A comparative study was undertaken to examine intracranial irradiation using intensity-modulation radiation therapy (IMRT) and linear accelerator-based radiosurgery. The IMRT was examined using the Peacock system with a BEAK attachment. A clinical case involving a metastatic brain lesion, treated with 3 radiosurgery isocenters, was planned for IMRT. The radiosurgery was planned using the Leibinger planning system. The IMRT was planned using the CORVUS planning system. The CORVUS planning system uses an inverse planning algorithm, a recent development in radiotherapy. Isodose distributions and dose volume histograms were generated and compared. Analysis of the dosimetry shows that the dose conformity and homogeneity within the target using the RTOG guidelines are superior for IMRT. The advantages of IMRT using inverse planning system include the ease of planning and execution of treatment, especially for cases that involve concave targets that require multiple isocenters using radiosurgery.

Algorithms↗

PubMed2000

Blood irradiation with accelerator produced electron beams.

Blood and blood products are irradiated with gamma rays to reduce the risk of graft versus host disease (GVHD). A simple technique using electron beams produced by a medical linear accelerator has been studied to evaluate irradiation of blood and blood products. Variations in applied doses for a single field 20 MeV electron beam are measured in a phantom study. Doses have been verified with ionization chambers and commercial diode detectors. Results show that the blood product volume can be given a relatively homogeneous dose to within 6% using 20 MeV electrons without the need to rotate the blood bags or the beam entry point. The irradiation process takes approximately 6.5 minutes for 30 Gy applied dose to complete as opposed to 12 minutes for a dual field x-ray field irradiation at our centre. Electron beams can be used to satisfactorily irradiate blood and blood products in a minimal amount of time.

Blood↗

PubMed2006

Monte Carlo simulation of the photon beam characteristics from medical linear accelerators.

The MCNPX code has been employed on a personal computer to calculate the dosimetric characteristics of the photon beams from the 6 MV Siemens MX2 and the 10 MV Varian Clinac 2100C linear accelerators. A model of the treatment head includes the major geometric structure within the beam path. The model was used to calculate the energy spectra of the photon beam, percentage depth dose and the dose profiles. The accuracy of the calculated results is examined by comparing them with the measured dose distributions for the two machines. The computed and measured depth dose curves agree to within 2% for all the depths beyond the build-up region for both treatment machines. The calculations agree to within 2% of the measured profiles within the 100-50% dose level. It has been found that the MCNPX code is an effective tool for simulating the clinical photon beam.

Body Burden↗

PubMed1988

A system for stereotactic radiosurgery with a linear accelerator.

A small field irradiation technique to deliver high doses of single fraction photon radiation to small, precisely located volumes (0.5 to 8 cm3) within the brain has been developed. Our method uses a modified Brown-Roberts-Wells (BRS), CT-guided, stereotactic system and a 6 MV linear accelerator equipped with a special collimator (diameters of 12.5 mm to 30.0 mm projected to isocenter) located 23 cm from isocenter. Target localization via planar angiography has been added. Treatment consists of a series of arcing beams using both gantry and couch rotations. During treatment, the patient's head is immobilized independently of the radiotherapy couch and is precisely positioned without reference to room lasers or light field. A precise verification of alignment precedes each treatment. Extensive performance tests have shown that a target, localized by CT, can be irradiated with a positional accuracy of 2.4 mm in any direction with 95% confidence. If angiography is used for localization, the results are better. The dose 1.0 cm outside the target volume is less than 20% of the prescribed dose for a medium sized collimator.

Brain Neoplasms↗

PubMed2005

In vivo dosimetry with MOSFETs: dosimetric characterization and first clinical results in intraoperative radiotherapy.

PURPOSE: To investigate the use of metal oxide silicon field effect transistors (MOSFETs) as in vivo dosimetry detectors during electron beams at high dose-per-pulse intraoperative radiotherapy. METHODS AND MATERIALS: The MOSFET system response in terms of reproducibility, energy, dose rate and temperature dependence, dose-linearity from 1 to 25 Gy, angular response, and dose perturbation was analyzed in the 6-9-MeV electron beam energy range produced by an intraoperative radiotherapy-dedicated mobile accelerator. We compared these with the 6- and 9-MeV electron beams produced by a conventional accelerator. MOSFETs were also used in clinical dosimetry. RESULTS: In experimental conditions, the overall uncertainty of the MOSFET response was within 3.5% (+/-SD). The investigated electron energies and the dose rate did not significantly influence the MOSFET calibration factors. The dose perturbation was negligible. In vivo dosimetry results were in accordance with the predicted values within +/-5%. A discordance occurred either for an incorrect position of the dosimeter on the patient or when a great difference existed between the clinical and calibration setup, particularly when performing exit dose measurements. CONCLUSION: Metal oxide silicon field effect transistors are suitable for in vivo dosimetry during intraoperative radiotherapy because their overall uncertainty is comparable to the accuracy required in target dose delivery.

Breast Neoplasms↗

PubMed2005

A dose verification method using a monitor unit matrix for dynamic IMRT on Varian linear accelerators.

Dosimetry verification is an important step during intensity modulated radiotherapy treatment (IMRT). The verification is usually conducted with measurements and independent dose calculations. However, currently available independent dose calculation methods were developed for step-and-shoot beam delivery methods, and their uses for dynamic multi-leaf collimator (MLC) delivery methods are not efficient. In this study, a dose calculation method was developed to perform independent dose verifications for a dynamic MLC-based IMRT technique for Varian linear accelerators. This method extracts the machine delivery parameters from the dynamic MLC (DMLC) files generated by the IMRT treatment planning system. Based on the parameters a monitor unit (MU) matrix was separately calculated as two terms: direct exposure from the open MLC field and leakage contributions, where the leaf-end leakage contribution becomes more important in higher dose gradient regions. The MU matrix was used to compute the primary dose and the scattered dose with a modified Clarkson technique. The doses computed using the method were compared with both measurement and treatment planning for 14 and 25 plans respectively. An average of less than 2% agreement was observed and the standard deviation was about 1.9%.

Algorithms↗

PubMedDate not supplied

Buildup/surface dose and exit dose measurements for a 6-MV linear accelerator.

The central axis dose distribution in the buildup region for the Varian Clinac 6/100 6-MV x-ray beam was measured in a polystyrene phantom using a fixed volume (0.5 cm3) parallel-plate ionization chamber (2.4-mm plate separation). Results for the surface dose measurements ranged from approximately 8% of the maximum dose for a 5 X 5 cm field, up to 36% for a 40 X 40 cm field, 100-cm source-skin distance. The effect of a 0.6-cm-thick polycarbonate blocking tray and metal filters on the surface and buildup dose is also reported. In addition, ionization measurements were made to document the dose perturbations caused by the absence of backscattering material at the exit surface of a polystyrene phantom. Exit dose measurements showed a 15% reduction in dose with essentially no scattering material beyond the measurement point. Near full scatter condition could be restored by placing 5-10 mm (depending on field size) of unit density material directly behind the ion chamber's distal surface.

Humans↗

PubMedDate not supplied

Levels of leakage radiation from electron collimators of a linear accelerator.

The leakage radiation from electron applicators used with our linear accelerator has been measured. For the applicators 6 X 6 to 25 X 25 cm size, the leakage was measured in the plane of the patient and on the sides of the applicators with the available electron energies of 6, 9, 12, 15 and 18 MeV. The levels were significant. The highest leakage on the side was for the combination of 6 X 6-cm applicator and 9-MeV electrons (32%) and in the plane of the patient for 25 X 25-cm applicator with 18 MeV (10%) relative to the peak dose. Adding lead 1-2 mm, at appropriate locations inside the applicators has reduced the leakages to acceptable levels without affecting the beam parameters.

Biophysical Phenomena↗

PubMed1982

An electron beam pseudoarc technique for irradiation of large areas of chest wall and other curved surfaces.

An electron beam technique has been developed for the treatment of large areas of the chest wall and other curved surfaces employing the stationary field electron beam mode ordinarily available on a linear accelerator. The technique simulates an arc through the use of multiple fixed fields. The electron collimator is not used. Field shaping is achieved through the combined effects produced by the photon jaw settings, the arc limits, and secondary lead shielding on the patient's skin. Thus, there is no limitation on the circumferential extent of the field size that can be used. Electron beam energies of 6 MeV, 9 MeV, 12 MeV, 15 MeV and 18 MeV have been studied. Our findings indicate that, for these energies, this technique produces isodose curves parallel to the cylindrical surface at all depths beyond maximum build-up. This paper discusses the physical characteristics of the single beams, the degree of dose homogeneity achieved with the multiple fields, and the dosimetry technique developed to implement the therapy.

Electrons↗

PubMed1988

Practice patterns of palliative care for the United States 1984-1985.

This study reports the national averages for patterns of palliative radiation therapy observed in the United States for patients treated in 1984-1985 and compares those patterns to the pertinent literature. The data were collected in 1984-1985 by the Patterns of Care Study Survey of Palliative Care conducted in 49 institutions selected to provide valid national averages for the practice patterns reported. Data were collected from 784 patient records selected from five "strata" of practice. Demographic data and process data were tabulated and national averages were calculated from the data. Four metastatic sites were selected, weight bearing bones (401), non-weight bearing bones (102), brain metastasis (224), and lung-mediastinum (57). The median patient age was 63 years, equally divided by sex. In 52% of patients this was the first metastasis. Common Karnofsky performance scores ranged from 40 to 80%. Lung, breast, and prostate were the most common primaries. Two-thirds of the patients were treated by linear accelerators, one-third by cobalt. The median number of fractions was 10, median dose 3000 cGy, median fraction size 300 cGy, and median treatment duration 15 days. The goal of treatment was relief of pain (98%) and return of function (30%) for weight bearing bones, for brain metastasis it was preservation of function (68%), pain relief (33%), and relief of compression (25%). All sites showed TDF values that ranged from 33-85, and a TDF of approximately 65 was most common for weight bearing bones and brain metastasis with no consistent pattern of TDF selection for the other sites. Compliance by strata of practice with work-up criteria was excellent with isolated poor compliance seen in several strata.

Age Factors↗

PubMedDate not supplied

Detection of DNA damages and repair in human culture cells with simulated space radiation.

DNA damages and its repair of cultured WI38 human fibroblast cells and T98G human glioblastoma cells were studied by exposing to carbon ion beams of HIMAC accelerator. The exposed cells were incubated at 37 degrees C for appropriate intervals and the damages were analyzed by alkaline comet assay and quantitative RT-PCR with p53 mRNA. Highly inhomogeneous DNA damages were observed among the electrophoretic cell images of the comet assay. The degree of the damages was analyzed semi-quantitatively by using the Comet Index. The damaged fraction of WI38 cells was 85% immediately after 4 Gy (100keV/micrometer) irradiation and decreased to 50% after 120 min. incubation indicating a repair of cell DNA. Time dependent p53 gene expression as also analyzed by the quantitative RT-PCR method.

Carbon↗

PubMed1998

Correlation between cell death and induction of non-rejoining PCC breaks by carbon-ion beams.

We have shown a correlation between cell death and induction of non-rejoining chromatin breaks in two normal human cells and three human tumor cell lines irradiated by carbon-ion beams and X rays. Non-rejoining chromatin breaks were measured by counting the number of remaining chromatin fragments detected by the premature chromosome condensation (PCC) technique. Carbon-ion beams were accelerated by the Heavy Ion Medical Accelerator in Chiba (HIMAC). The cells were irradiated by two different mono-LET beams (LET = 13 keV/micrometer and 77 keV/micrometer ) and 200 kV X rays. The RBE values of cell death for carbon-ion beams relative to X rays were 1.1 to 1.4 for 13 keV/micrometer beams and 2.5 to 2.9 for 77 keV/micrometer beams. The induction rate of non-rejoining PCC breaks per cell per Gy was found to be highest for the 77 keV/micrometer beams for all of the cell lines. The results found in this study show that there is a good correlation between cell death and induction of non-rejoining PCC breaks for these human cell lines.

Astrocytoma↗

PubMed2000

Cost-minimization analysis: radiation treatment with and without a multi-leaf collimator.

PURPOSE: To compare the costs of radiation treatment on a linear accelerator with a multileaf collimator (MLC) versus treatment on a linear accelerator without an MLC. The study was designed to determine whether the increased throughput of fields and decreased block cutting made the MLC cost effective from an institutional perspective. METHODS AND MATERIALS: The number of fields, basic treatment equivalent, equivalent simple treatment visits, and blocks were prospectively collected for the four linear accelerators. Building, equipment, staffing, and service costs were all obtained in 1999 Australian dollars from the manufacturers and hospital department heads. The Joint Radiation Oncology Centre at Westmead and Nepean Hospitals, which are Australian public hospitals, runs as one unit, with the same staff, and currently operates five linear accelerators. Currently, four of the linear accelerators are used for general radiotherapy, operating for exactly the same hours; the final machine operates more limited hours and is used for specialized radiotherapy techniques and emergency cases. RESULTS: The two machines with MLCs, on average, treated 5,169 fields each, while the two machines without MLCs treated 4,543 fields in a 3-month period, a 12% increase in throughput. The two non-MLC machines required 155 premounted trays (PMTs) in total, while the MLC machines required 17 PMTs. Linear accelerators with MLCs were demonstrably more efficient, and while their capital costs were higher, the reduction in labor costs associated with block cutting and, particularly the increased throughput, more than offset these initial costs. The total cost of a radiation field with an MLC was found to be $A101.69 compared to $A106.98 without an MLC. A multiway sensitivity analysis showed the results to be robust. The worst-case scenario was a departmental savings of $A168,000 per year; the best-case scenario was a savings of $A680,000 per year. CONCLUSION: Under the conditions pertaining to the radiation oncology department in this group of hospitals, and in similar departments, the use of an MLC can be justified.

Australia↗