PubMed Health⌕ Search

Biomedical subjects

R Arráns

Publications and source records attributed to R Arráns.

10 recordsLinked to original sources
PubMed2003

Ionization chamber dosimetry of small photon fields: a Monte Carlo study on stopping-power ratios for radiosurgery and IMRT beams.

Absolute dosimetry with ionization chambers of the narrow photon fields used in stereotactic techniques and IMRT beamlets is constrained by lack of electron equilibrium in the radiation field. It is questionable that stopping-power ratio in dosimetry protocols, obtained for broad photon beams and quasi-electron equilibrium conditions, can be used in the dosimetry of narrow fields while keeping the uncertainty at the same level as for the broad beams used in accelerator calibrations. Monte Carlo simulations have been performed for two 6 MV clinical accelerators (Elekta SL-18 and Siemens Mevatron Primus), equipped with radiosurgery applicators and MLC. Narrow circular and Z-shaped on-axis and off-axis fields, as well as broad IMRT configured beams, have been simulated together with reference 10 x 10 cm2 beams. Phase-space data have been used to generate 3D dose distributions which have been compared satisfactorily with experimental profiles (ion chamber, diodes and film). Photon and electron spectra at various depths in water have been calculated, followed by Spencer-Attix (delta = 10 keV) stopping-power ratio calculations which have been compared to those used in the IAEA TRS-398 code of practice. For water/air and PMMA/air stopping-power ratios, agreements within 0.1% have been obtained for the 10 x 10 cm2 fields. For radiosurgery applicators and narrow MLC beams, the calculated s(w,air) values agree with the reference within +/-0.3%, well within the estimated standard uncertainty of the reference stopping-power ratios (0.5%). Ionization chamber dosimetry of narrow beams at the photon qualities used in this work (6 MV) can therefore be based on stopping-power ratios data in dosimetry protocols. For a modulated 6 MV broad beam used in clinical IMRT, s(w,air) agrees within 0.1% with the value for 10 x 10 cm2, confirming that at low energies IMRT absolute dosimetry can also be based on data for open reference fields. At higher energies (24 MV) the difference in s(w,air) was up to 1.1%, indicating that the use of protocol data for narrow beams in such cases is less accurate than at low energies, and detailed calculations of the dosimetry parameters involved should be performed if similar accuracy to that of 6 MV is sought.

Algorithms↗
PubMed2001

A Monte Carlo approach for small electron beam dosimetry.

BACKGROUND AND PURPOSE: In treatments where it is necessary to conform the field shape yielding a very small effective beam area, dosimetry and conventional treatment planning may be inaccurate. The Monte Carlo (MC) method can be an alternative to verify dose calculations. A conjunctival mucosa-associated lymphoid tissues lymphoma is presented, to show the importance of an independent assessment in critical situations. MATERIALS AND METHODS: In this work, the MC technique has been employed using the program BEAM (based on EGS4 code). Electron beam simulation has been performed and the results have been compared with those obtained with films. The patient dose distribution has been obtained by two methods: the full Monte Carlo (FMC) simulation and a conventional planning system (PLATO). RESULTS: Concerning dosimetry, some differences have been observed in the comparison of profiles obtained with film and those obtained with the MC method. Moreover, significant differences were found in the patient isodose distribution between both calculation methods. CONCLUSIONS: The results highlight that, in treatments where small beams are needed, conventional dosimetry and planning systems have some limitations. Therefore, an independent and more accurate assessment, such as MC, would be desirable.

Computer Simulation↗
PubMed1999

A conformal technique for a ring shaped conjunctive lymphoma treatment.

Radiotherapy is commonly utilised as standard treatment in the so called mucosa-associated lymphoid tissues (MALT), due to the low probability of distant relapse. The particularities of the lesion, make necessary both energy degradation and beam conformation. To keep homogeneity within acceptable limits, a lengthener attached to the electron applicator has been devised to closely fit the anatomy of the patient. Considering the small area of the outcoming field, film dosimetry is preferred, since the dimensions of an ionisation chamber and even of a semiconductor probe might be comparable to the field size.

Adult↗
PubMed1997

Computer-based anthropometrical system for total body irradiation.

For total body irradiation (TBI) dose calculation requirements, anatomical information about the whole body is needed. Despite the fact that video image grabbing techniques are used by some treatment planning systems for standard radiotherapy, there are no such systems designed to generate anatomical parameters for TBI planning. The paper describes an anthropometrical computerised system based on video image grabbing which was purpose-built to provide anatomical data for a PC-based TBI planning system. Using software, the system controls the acquisition and digitalisation of the images (external images of the patient in treatment position) and the measurement procedure itself (on the external images or the digital CT information). An ASCII file, readable by the TBI planning system, is generated to store the required parameters of the dose calculation points, i.e. depth, backscatter tissue thickness, thickness of inhomogeneity, off-axis distance (OAD) and source to skin distance (SSD).

Anthropometry↗
PubMed1997

Lateral scatter correction algorithm for percentage depth dose in a large-field photon beam.

Differences between the scatter conditions of dosimetry and treatment situation are more important in the case of large-field photon beams than in standard ones. In the former, the scattering volume is defined by the phantom cross section; in the latter, the radiation field size. Two factors should be considered: the thickness and the cross section of the phantom. Both of them have an effect on the Percentage Depth Dose (PDD) distribution. In a previous study we addressed the influence of backscatter thickness on dose delivered. The aim of this work is to measure the effect of cross section phantom on the PDD curves under our TBI treatment conditions. Results showed a strong dependence of the PDDs on this parameter. A semi-empirical expression has also been derived to calculate (within 0.5% uncertainty) the Lateral scatter Correction Factor (LCF). The model of LCF states a linear dependence on depth whilst slope of these curves depends exponentially on distance to the lateral surface. The algorithm is being applied to our practical Total Body Irradiation (TBI) procedure.

Algorithms↗
PubMed1995

Constancy of wedge factors in a Siemens Mevatron 74 linear accelerator.

The variation of the wedge factor (WF) with field size is an important piece of data which determines the radiation output in treatments using wedge filters. WF is closely related with the accelerator head layout, and the choice of the wedge tray mounted above or below the jaws plays a predominant role. In this work we have studied the WF variations in our linac and found that, in apparent contradiction with the literature, the WF remains constant with field size. Nevertheless, these results cannot be used directly in other linacs, and individual dosimetry must be carried out.

Particle Accelerators↗
PubMed1994

Midline dose algorithm for in vivo dosimetry.

The high level of accuracy required in radiotherapy treatment dosimetry makes necessary good treatment quality control. The common way is the use of in vivo dosimetry equipment that allows the direct measurement of dose delivered to the patient. Control of homogeneity and constancy of the incident beam on the patient can be achieved directly by means of entrance dose measurement; however, control of dose delivered to tumours and internal organs is difficult because of the impossibility of a direct measurement. In this case calculations are made using external measurements (entrance and exit sides of the patient) to obtain the dose delivered. In this work, an algorithm that allows the real-time knowledge of midline dose as a function of thickness and entrance and exit doses coming from semiconductor detectors is presented. By having the electrometer connected to the computer, these three values (entrance, midline, and exit dose) are displayed instantaneously when the algorithm is included in the acquisition program. The model has been developed both for standard (source to surface distance = 100 cm) and special treatment techniques such as total body irradiation (SSD = 314 cm). There is a good agreement of experimental and calculated values with differences below 0.04%.

Algorithms↗
PubMed1989

Neutron measurements around an 18 MV linac.

An estimate of the neutron production of medical electron accelerators is of interest in order to quantify the radiological risk for the staff operating such machines. First, we used a theoretical procedure, based on the Montecarlo method, in order to get some information about the neutron spectrum. Second, by using the neutron activation of indium foils, we have empirically obtained the neutron fluence at different locations in the accelerator room. Finally, some post-irradiation environmental levels of radiation are given.

Monte Carlo Method↗