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At least 469 records · Page 26Linked to original sources
PubMedDate not supplied

[Progress in dosage optimization for stereotactic radiosurgery].

Stereotactic radiosurgery is a technique for treatment of intracranial lesions requiring high precision in all steps--from image acquisition to final irradiation. One of most difficult steps is the treatment planning phase, consisting of determination of irradiation parameters sufficient to cover the target volume by avoiding sensitive volumes. A manual and empirical definition can be very long and difficult, especially in the case of complex target volumes situated in sensitive zones. As in conventional radiotherapy, stereotactic radiosurgery has taken advantages from dosimetric optimization. The question is: "What is the configuration of irradiation parameters used in order to obtain the treatment plan by satisfying defined constraints?". The purpose of this article is to summarize optimization methods used in radiosurgery and to describe the technical alternatives proposed for this treatment as well as the possibilities of plan evaluation between different techniques. This purpose will be illustrated by the optimization methodology used in the Center Oscar Lambret of Lille, France for the radiosurgical treatment with linear accelerator.

Algorithms↗
PubMed1999

Monte Carlo modelling of electron beams from medical accelerators.

Monte Carlo simulation of radiation transport is considered to be one of the most accurate methods of radiation therapy dose calculation. With the rapid development of computer technology, Monte Carlo based treatment planning for radiation therapy is becoming practical. A basic requirement for Monte Carlo treatment planning is a detailed knowledge of the radiation beams from medical accelerators. A practical approach to obtain the above is to perform Monte Carlo simulation of radiation transport in the medical accelerator. Additionally, Monte Carlo modelling of the treatment machine head can also improve our understanding of clinical beam characteristics, help accelerator design and improve the accuracy of clinical dosimetry by providing more realistic beam data. This paper summarizes work over the past two decades on Monte Carlo simulation of clinical electron beams from medical accelerators.

Electrons↗
PubMed2002

Dose to radiation therapists from activation at high-energy accelerators used for conventional and intensity-modulated radiation therapy.

The increased beam-on times which characterize intensity-modulated radiation therapy (IMRT) could lead to an increase in the dose received by radiation therapists due to induced activity. To examine this, gamma ray spectrometry was used to identify the major isotopes responsible for activation at a representative location in the treatment room of an 18 MV accelerator (Varian Clinac 21EX). These were found to be 28Al, 56Mn, and 24Na. The decay of the dose rate measured at this location following irradiation was analyzed in terms of the known half-lives to yield saturation dose rates of 9.6, 12.4, and 6.2 microSv/h, respectively. A formalism was developed to estimate activation dose (microSv/week) due to successive patient irradiation cycles, characterized by the number of 18 MV fractions per week, F, the number of MU per fraction, M, the in-room time between fractions, td (min), and the treatment delivery time t'r (min). The results are represented by the sum of two formulas, one for the dose from 28Al 1.8 x 10(-3) F M (1-e(-03t'(r))/t'r and one for the dose from the other isotopes approximately 1.1 x 10(-6) F(1.7) Mt(d). For conventional therapy doses are about 60 microSv/week for an 18 MV workload of 60,000 MU/week. Irradiation for QA purposes can significantly increase the dose. For IMRT as currently practiced, lengthy treatment delivery times limit the number of fractions that can be delivered per week and hence limit the dose to values similar to those in conventional therapy. However for an IMRT regime designed to maximize patient throughput, doses up to 330 microSv/week could be expected. To reduce dose it is recommended that IMRT treatments should be delivered at energies lower than 18 MV, that in multienergy IMRT, high-energy treatments should be scheduled in the latter part of the day, and that equipment manufacturers should strive to minimize activation in the design of high-energy accelerators.

Algorithms↗
PubMedDate not supplied

Improvement of linear accelerator depth-dose curves.

A semiempirical analytic description of the accelerator depth-dose curve is described along with its physical explanation. The results of Monte Carlo calculations are presented and compared with experimental data to test this model. Calculations were made for different atomic number (Z) materials used as x-ray targets and flatteners, with the results showing that medium-Z materials are the logical choice. It is demonstrated empirically that Dmax is a simple function of the average energy (E) of the x-ray spectrum. The variation of E with Z of the target and flattener is demonstrated. As a practical example, Monte Carlo calculations and experimental data for old and new Clinac 35 accelerators are presented.

Electrons↗
PubMedDate not supplied

Evaluation of neutron dose equivalent levels at the maze entrance of medical accelerator treatment rooms.

This paper evaluates the accuracy of Kersey's method of calculation of the dose level due to photoneutrons at the maze entrance of medical accelerators. In this study measurements and calculations of the neutron dose equivalent were made for 13 medical accelerator facilities. The group of accelerators was composed of four different models from two manufacturers. The maze length for the treatment rooms varied from 3.0 to 8.54 m and 10 of the mazes had a single 90-deg turn with the remaining having two 90-deg turns. Moderated activation detectors and a portable neutron remmeter were used for measurements in the treatment room and maze, respectively. It was found that the maximum disagreement between the measured and calculated values was a factor of 2.3 with the calculated value exceeding the measured value. The majority of the measured values were within 25% of the calculated levels. It was concluded that Kersey's method is suitable for use in designing medical accelerator mazes.

Hospital Units↗
PubMedDate not supplied

Beam characteristics of a new model of 6-MV linear accelerator.

This paper describes the beam characteristics and dosimetry measurements performed on the 6-MV photon beam of a new model of linear accelerator, three of which were recently introduced and installed in our institution. Percent depth dose and tissue maximum ratio tables for a variety of field sizes and depths, as well as other parameters used for treatment planning are presented. These accelerators are the first of their kind using both hardware and software tools to control interlocks. Checking procedures for these interlocks are available from the authors upon request. Comparison of characteristic parameters between these three new 6-MV linear accelerators and with the 6-MV beams of two other accelerators is also made.

Calibration↗
PubMed1994

Wedge factors for rectangular fields.

The variation of wedge factor with field size was measured for a range of square and rectangular fields for 45 degrees and 60 degrees wedges. Measurements were performed on accelerators with both externally mounted wedges, the Varian 600C of nominal energy 6MV and internally mounted wedges, the Philips SL75/5 of nominal energy 6MV and the Philips SL25 of nominal energy 25MV. Analysis of the results confirm previous investigations reported in the literature of the variation of wedge factor with field size and in particular the significantly greater variation for internally mounted wedges. A wedge factor for a rectangular field based on the wedge factor of the open field equivalent square gives a maximum error of 1.5%. A wedge factor for a rectangular field based on the wedge factor of the square field of equal area reduces the maximum error to 0.5% for all three accelerators. Analysis of results reported in the literature show a similar reduction.

Particle Accelerators↗
PubMed1997

Off-axis output factors for 6MV and 18MV photons.

A series of in-air measurements showed that collimator scatter (Sc) did not change significantly for 6MV photons when the centre of the field was moved away from the central axis. This result enabled a model to be developed for the off-axis Effective Output Factor (EOF) which was then verified for 6MV and 18MV photons on a Varian 2100c accelerator and for 6MV photons on a Varian 600c accelerator. Thus off-axis output factors may be predicted, for a range of rectangular asymmetric fields, using only the Primary Off-Centre Ratio (POCR) in air and the on-axis output factor. Depth doses were also investigated off-axis and found to have no clinically significant differences compared with on-axis depth doses, for depths less than 7.5 cm for 6MV and 12.5 cm for 18MV photons. The model is simple to implement and avoids the need for a measurement for each patient, thus saving accelerator time.

Biophysical Phenomena↗