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At least 1,117 records · Page 62Linked to original sources

PubMedDate not supplied

Unwanted radiation produced by leakage neutrons from medical electron accelerators.

Monte Carlo calculations have been made to determine the energy delivered to a phantom by neutrons escaping from the head of a Varian Clinac 35 medical accelerator at an SSD of 1 m. The energy was sorted into two regions: inside and outside a volume defined by a circular beam of area 100 and 600 cm2. For the two beam sizes, the energies outside the treatment volume were 12 and 8.7 g-rad (per photon rad) respectively. Room scattering increased these values by about 20%. These energies are smaller than those delivered by scattered photons by more than ten times.

Models, Structural↗

PubMedDate not supplied

Use of a clinical accelerator in spectroscopic studies of the decay of 197Ir.

The 40-MeV direct beam bremsstrahlung irradiation facility of the Sagittaire clinical electron accelerator was used to produce 197Ir via the reaction 198Pt(lambda,p)197Ir. Gamma rays emitted in the Ir decay were detected with an intrinsic germanium system and multichannel analyzer. 106 of the spectrum peaks could be analyzed for energy, relative intensity, and half-life. Half-life results indicate that 197Ir decays from two isomers, one with half-life 6.1 +/- 0.1 min and another with half-life 8.9 +/- 0.1 min.

Gamma Rays↗

PubMed1999

The comparison of isodose plans: radiotherapy techniques in the treatment of thyroid ophtalmopathy.

Thyroid ophthalmopathy is listed among benign disorders as the fourth indication for radiotherapy. Radiation reduces symptoms and signs of the disease in 46-93% of patients. Except for the lenses, the orbital content is resistant to the usual radiation dose of 20 Gy in 10 fractions. A number of radiotherapeutic techniques which avoid the lenses have been suggested. The testing of different techniques was performed on the same patient by using a computerized simulation of lateral alternating and crossed photon beams from a 6 MV and 18 MV linear accelerator or cobalt device. The radiation from a 6 MV linear accelerator using lateral beams angled at 5 degrees posteriori gave the best results. By this technique both lenses receive below 10% of the prescribed dose with a safety zone of 20% of the field size. Isodose distribution in clinical target volume is within the range of 10%. The described technique is recommended as it enables a successful avoidance of both lenses with a satisfactory homogeneity of dose distribution in the clinical target volume.

Adult↗

PubMed2001

Dose rate outside primary barriers.

The author has investigated the use of the inverse square function to predict the photon dose rate outside primary barriers for medical accelerators. Measurements of the dose rate as a function of distance outside the barriers were made at five medical accelerator facilities. An analysis of the data showed that the radiation field appears to diverge from a point between the x-ray target and primary barrier. This behavior of the x-ray beam was attributed to scattering of photons in the shield barrier. It was concluded that the dose outside the barrier will be overestimated if the distance from the x-ray target to the point of calculation is utilized to carry out an inverse square correction.

Health Physics↗

PubMed2001

[A safe site for transposition of ovaries in radical hysterectomy for cervical carcinoma].

OBJECTIVE: To know the influence of twilight by using the radiotherapeutical technique box on hormonal function of ovaries. The aim of transposition of ovaries by radical hysterectomy of the cervical cancer is to move the ovaries out of radiotherapeutical target volume, and thus to protect their hormonal function. SETTING: Institute for the Care of Mother and Child, Prague, Czech Republic. METHODS: Computer simulation of radiation isodoses of linear accelerator on the transferred pictures of CT and calculation on the total dose, and radiobiological equivalent outside the target volume of radiation regarding the total dose of 46 Gy. RESULTS: In the pelvis region the minimal dose succeeded in acting as a castrating dose. The safe area is 2.5 cm above the margin of the radiation beam. CONCLUSION: It can be recommended to tuck the ovaries 3.5 cm above the margin of the pelvis.

Female↗

PubMed2006

[Prostate localization systems for prostate radiotherapy].

The development of sophisticated conformal radiation therapy techniques for prostate cancer, such as intensity-modulated radiotherapy, implies precise and accurate targeting. Inter- and intrafraction prostate motion can be significant and should be characterized, unless the target volume may occasionally be missed. Indeed, bony landmark-based portal imaging does not provide the positional information for soft-tissue targets (prostate and seminal vesicles) or critical organs (rectum and bladder). In this article, we describe various prostate localization systems used before or during the fraction: rectal balloon, intraprostatic fiducials, ultrasound-based localization, integrated CT/linear accelerator system, megavoltage or kilovoltage cone-beam CT, Calypso 4D localization system tomotherapy, Cyberknife and Exactrac X-Ray 6D. The clinical benefit in using such prostate localization tools is not proven by randomized studies and the feasibility has just been established for some of these techniques. Nevertheless, these systems should improve local control by a more accurate delivery of an increased prescribed dose in a reduced planning target volume.

Humans↗

PubMed2001

Independent dose calculations for the PEACOCK System.

An independent dose calculation method has been developed to validate intensity-modulated radiation therapy (IMRT) plans from the NOMOS PEACOCK System. After the plan is generated on the CORVUS planning system, the beam parameters are imported into an independent workstation. The beam parameters consist of intensity maps at each gantry angle and each arc position. In addition, CT scans of the patient are imported into the independent workstation to obtain the external contour of the patient. The coordinate system is defined relative to the alignment point chosen in the CORVUS plan. The independent calculation uses the pencil beam data viz tissue maximum ratio (TMR) and beam profiles for a single 1 x 0.8-cm beamlet formed by the NOMOS multileaf intensity-modulating collimator (MIMiC) leaf. The pencil beam data were measured for the 6-MV photon beam from Siemens PRIMUS linear accelerator using film dosimetry. The dose at a point is calculated using the depth and off-axis distance from a given pencil beam, corrected for its beam intensity. Isodose distributions are generated using the independent dose calculations and compared to the CORVUS plans. Isodose distributions show good agreement with the CORVUS plans for a number of clinical cases. The independent dose calculation algorithm is described in this paper.

Algorithms↗

PubMed1995

An investigation into the comparison between different dosimetric methods of measuring profiles and depth doses for dynamic wedges on a Varian 600C linear accelerator.

Three dosimetric methods for commissioning the dynamic wedges on a Varian 600C linear accelerator operating at 4.7 MV were compared. The techniques involved the use of ionization chambers, x-ray verification film and solid state detectors. Ionization chambers gave the most accurate results, as expected, but data acquisition was very time consuming. Film dosimetry was found to have good spatial resolution for beam profiles, but problems were encountered with the acquisition of depth doses. Solid state detectors generally showed reasonable agreement with the ionization chambers.

Film Dosimetry↗

PubMedDate not supplied

Neutron energies in medical electron accelerator rooms.

Two experimental methods are described for obtaining average energies of neutron spectra produced by an electron accelerator in a medical treatment room. In the first method, the dose equivalent is measured by means of indium activation foils in an Andersson-Braun moderator. Fluences are determined by the usual activation foil techniques. These data lead to the conversion factor, neutrons/(cm2s) per mrem/h, which uniquely defines the effective energy of the unknown spectrum. In the second method, the fast neutron broad-beam attenuation in polyethylene is obtained from measurement of the thermalized fluence versus depth curve, whose slope in the exponential region parallels that of the fast neutron spectrum. It was found for a 24-MV linear accelerator that the effective energy varied from 0.28 MeV at the isocenter under closed collimators to 0.016-0.024 MeV in the entrance maze. These results compare favorably with other measurements and calculations. It is concluded that the dose equivalent can be measured to within +/- 50% with an activation foil in an Andersson-Braun moderator and that the characteristic energy obtained by attenuation at the isocenter closely approximates the average energy of the neutron spectrum escaping the treatment head.

Electrons↗

PubMed1999

A model simulator for radiotherapy treatment planning and checking.

For a linear accelerator the direction and orientation of a beam is specified uniquely by a set of gantry, turntable and collimator angles. Sometimes, particularly for 3D planning, it is very difficult for the planner or plan checker to visualize the set-up of the treatment machine from the angles specified for each beam. This note describes a simple line-of-sight model that can be used for planning and checking the radiotherapy geometry prior to simulator verification.

Humans↗

PubMed1986

Microdosimetry of pulsed radiation fields employing the variance method.

The relationship between dose mean lineal energy and relative variance has been exploited previously to derive yD from the calculated variance in current measurements in steady and uniform radiation fields. Recently Kellerer and Rossi made the observation that utilization of two detectors can make the variance technique practicable in time-varying fields. We report here the first measurements of yD for 10 MeV X rays and 9 and 18 MeV electrons from a pulsed linear accelerator using the variance method. Two independent analog-to-digital converters were used to obtain data from two spherical proportional counters in synchrony with the beam pulse. The method is described in detail and results are reported for site diameters of 1/2, 1, and 2 microns. Data for an accurate determination of yD can be obtained with this technique in less than 1 min, making possible an essentially "on line" determination of yD or zD in a clinical situation.

Electrons↗

PubMed2002

[Properties of a commercial Hi-pSi detector for dosimetry of stereotactic collimators with very small diameters].

BACKGROUND: Conformal stereotactic radiosurgery and radiotherapy with linear accelerators and hole collimators yield a dose concentration in the target volume by rotation of the gantry. For small target volumes collimators with isocentre diameters of 4-45 mm are used. In this paper dosimetric measurements with a commercial high doped p-type silicon detector are demonstrated and compared to measurements with diamond detector and ionisation chamber. MATERIAL AND METHODS: The properties of the silicon detector SFD from Scanditronix were investigated with the radiation of a Gammatron S and a Varian 2100 CD at 6 MV. The results were compared with those of a calibrated ionisation chamber (0.3 cm3) and a diamond detector. MEASUREMENTS AND RESULTS: At the beginning the reproductibility of the registered dose and dose rate and the temperature dependence of the Si-detector were investigated at the Gammatron S. For the comparison the absorbed dose was measured with the ionisation chamber in air. The sensitivity decreases slightly with dose and dose rate. After a period of several days without radiation again higher doses were registered. The temperature dependence causes deviations of 0.25%/K. The signal-to-noise ratio and the spatial resolution were investigated with the linear accelerator. The signal-to-noise ratio is clearly lower compared with that of the diamond detector, whereas the resolution is nearly the same. CONCLUSIONS: The Si-detector is qualified for dosimetry of very small fields because of the insignificant dose and dose rate dependence and in spite of some disadvantages regarding dosimetric properties compared with the diamond detector. The advantage is the availability and the cost. Measurement with ionisation chambers are not useful for collimator diameters below 20 mm.

Humans↗

PubMed2004

A shielding design for an accelerator-based neutron source for boron neutron capture therapy.

Research in boron neutron capture therapy (BNCT) at The Ohio State University Nuclear Engineering Department has been primarily focused on delivering a high quality neutron field for use in BNCT using an accelerator-based neutron source (ABNS). An ABNS for BNCT is composed of a proton accelerator, a high-energy beam transport system, a (7)Li target, a target heat removal system (HRS), a moderator assembly, and a treatment room. The intent of this paper is to demonstrate the advantages of a shielded moderator assembly design, in terms of material requirements necessary to adequately protect radiation personnel located outside a treatment room for BNCT, over an unshielded moderator assembly design.

Boron Neutron Capture Therapy↗

PubMed2003

Using intraoperative radiation therapy--a case study.

The introduction of a mobile linear accelerator in the OR has made intraoperative radiation therapy (IORT) more plausible. An IORT treatment can deliver a single high dose of radiation to a tumor or tumor bed after surgical resection or surgical exposure of high risk areas. This article details a case study in which IORT was used on a patient with sigmoid carcinoma and the procedure outcomes.

Adenosarcoma↗

PubMed2003

Experimental and computational determination of neutron dose equivalent around radiotherapy accelerators.

An experimental and computational investigation of the neutron dose equivalent in the treatment room and maze of various radiotherapy accelerators in Canada was completed. A bubble detector was selected for the bulk of the measurements due to its relative insensitivity to gamma radiation. The rooms and accelerator heads were modelled using the MCNP4B Monte Carlo radiation transport code, modified with a photoneutron patch. The results of the investigation showed generally good agreement between the experiments. Monte Carlo and analytical approximations to the neutron dose equivalent, and suggest that if the analytical approximations are used carefully they may be substituted for more costly experimental or Monte Carlo determinations.

Body Burden↗

PubMedDate not supplied

Definition of source for head shielding requirements in linear accelerators may affect room shielding design.

Siemens Corporation defines the source of the Mevatron 20 linear accelerator as a line source approximately 2 m in length. Calculations for secondary barriers for leakage radiation are generally based on 0.1% of the useful beam 1 m from the target. The difference in source terms can lead to calculating thinner barriers than are required. The placement of a 30 X 10 X 10-cm block of lead behind the head was required to achieve acceptable radiation levels.

California↗

PubMedDate not supplied

A linear accelerator monitor unit totalizer.

An independent primary dose counter or totalizer has been developed for our linear accelerator. This counter is reset independently of the control console and thus will totalize the number of monitor units (MUs) delivered to the patient. This is especially useful for patients being treated at extended distances when it is necessary to reset the control console several times. The independent totalizer does not load down the control console dosimetry circuit and can be adapted to the digital logic used. The details of the system will be presented.

Humans↗

PubMedDate not supplied

Measurement of the restricted linear energy transfer of stray radiation close to the treatment volume of 12 and 18 MeV clinical photon beams.

The restricted dose mean linear energy transfer (LET) (L500,D) of the stray radiation field a few centimeters outside the treatment volume has been measured for 12 and 18 MV photons produced by a clinical Therac-20 (AECL) accelerator. The measurements were performed as a function of field size and distance from the edge of the treatment volume, using the method of the high-pressure ionization chamber. Contrary to what was found in a previous investigation for a clinical Co-60 unit and despite the presence of photoneutrons (in the case of 18 MV photons), the L500,D outside the beam does not increase significantly relative to the L500,D of the primary beam.

Energy Transfer↗