1,721,145 research outputs found
Simulazioni con MCNP5 di distribuzioni di dose assorbita in fantocci TE esposti a fasci per terapia neutronica
No full textLe distribuzioni spaziali dei vari contributi di dose generati dai neutroni termici/epitermici/veloci di fasci per trattamenti di BNCT (Boron Neutron Capture Therapy) e di Fast Neutron Therapy in materiali tessuto-equivalenti sono determinate sperimentalmente con diverse tecniche dosimetriche (camere a ionizzazione, dosimetri a termoluminescenza, dosimetri a gel) e tramite misure di flusso (foglietti di attivazione).
Attualmente non esistono protocolli standard che definiscano le procedure per la misurazione della dose in tali campi di radiazione, e i metodi di misura sopra citati presentano incertezze sperimentali non trascurabili. In particolare, le difficoltà provengono dalla presenza, in materiale tessuto equivalente esposto a fasci di neutroni, di radiazioni secondarie di diversa qualità e LET, nonché dall’alta intensità dei flussi neutronici impiegati.
Risulta pertanto necessario affiancare ai metodi di misura sperimentale anche accurate simulazioni Monte Carlo relative alla distribuzione delle varie componenti di dose assorbita in materiale tessuto-equivalente esposto a fasci di neutroni. I risultati dei calcoli Monte Carlo forniscono uno strumento insostituibile per la caratterizzazione della sorgente di radiazione impiegata per la terapia e per la convalida dei piani di trattamento. In generale le simulazioni MC sono utili per la verifica della consistenza delle misure sperimentali e dei calcoli. Inoltre in alcune situazioni sperimentali (ad esempio per la determinazione della dose dovuta allo scattering) sono richiesti calcoli MC per completare le procedure necessarie per ottenere i valori di dose assorbita.
In questo lavoro sono presentati i risultati di calcoli Monte Carlo riguardanti distribuzioni di dose in fantocci di materiale tessuto equivalente esposti a fasci di neutroni epitermici. In particolare sono state studiate separatamente le distribuzioni di dose depositata da fotoni ed in seguito a scattering di neutroni epitermici e veloci. Si è inoltre calcolata la distribuzione della dose da boro, considerando nel fantoccio simulato la presenza di regioni con una concentrazione di boro paragonabile a quella presente nei tessuti tumorali di pazienti sottoposti a trattamenti BNCT. I calcoli sono stati effettuati utilizzando il codice MCNP5.
Le distribuzioni spaziali di dose ottenute sono consistenti con i risultati di misure puntuali effettuate con dosimetri a termoluminescenza e camere a ionizzazione accoppiate e con immagini di dose ottenute con dosimetri a gel
POSSIBILITA’ DI UTILIZZARE LA CONDENSAZIONE LATERALE DELLA GUTTAPERCA IN CANALI POCO ALLARGATI
No full text
An optimized Monte Carlo (PENELOPE) code for the characterization of gel-layer detectors in radiotherapy
No full textMonte Carlo (MC) simulation of radiation transport is considered to be one of the most accurate methods of radiation therapy dose calculation. A basic requirement for MC treatment planning is a detailed knowledge of the characteristics of radiation beam generated from medical linear accelerators (LINACs). One of the most important input parameters is the photon fluence of the beam, usually not determinable experimentally. Thus, an MC simulation code based on the PENELOPE package was developed in order to survey the influence of the incident spectrum on the in-phantom dose distributions. Different spectra for the incident photon fluence have been considered in order to establish the most adequate one. The resulting planned dose distributions have been compared with those determined experimentally with ionization chamber measurements and gel dosimeter layers analyzed with optical technique. The specific gel composition has been implemented in the MC simulation code. Comparisons between experimental measurements, approximated simulations (water) and specific simulations (gel composition) have been performed
Gel-layer dosimetry for dose verification in intensity-modulated radiation therapy
No full textIntensity Modulated Radiotherapy (IMRT) is a technique in which the radiation fluence within each of the treatment beams is not uniformly distributed. This allows the patient dose to follow the boundaries even of a target volume of complex shape, and, virtually, to spare critical healthy organs at risk. The agreement between planned and delivered IMRT dose is verified by means of standard dosimetric methods such as film dosimetry or semiconductors array dosimetry. In this paper we compare the output of a commercial device using an array of diodes for IMRT absolute dose verification with the output of a gel dosimeter, composed by a 10x8 cm2 rectangular layer of a tissue-equivalent gel matrix in which a proper chemical dosimeter has been incorporated. The dose distribution is derived from the images of visible light transmittance, detected with a CCD camera before and after the gel exposure. The analysis was carried out on a single IMRT field chosen among those archived at the Istituto Nazionale Tumori of Milan. The radiation field was examined in an area common to both dosimeters. The agreement between the two detectors was good, as shown by analysis of dose profiles, especially for doses above 15-20 cGy. Gel dosimeter was in good agreement with the planned dose too, with a percentage of dosimeter points passing a dose to agreement test ranging between 90 to 93%. Although preliminary, our data suggest that gel dosimetry is a reliable method for IMRT dose verification. Due to the good spatial resolution and to the tissue equivalent properties of its composition, it would be suitable also for 3D IMRT dose reconstruction and verification in the form of multiple piled-up gel layers
Study of the dosimetric robustness of PVA-GTA based Fricke gels against manufacture parameters
No full textPurpose. Fricke gel dosimeters (FGDs) based on poly-vinyl alcohol
(PVA) as gelling agent and glutaraldehyde (GTA) as cross-linker,
were developed with the aim to overcome the drawbacks affecting
the FGDs prepared with traditional natural gel matrices like gelatin
or agarose [1]. Here, we investigate the dosimetric robustness of
PVA-GTA FGDs against parameters influencing their preparation, like
pH and temperature of gelling.
Materials and Methods. PVA-GTA matrices undergo gelling by
heating instead of cooling. Gelling temperatures from 6 C to 40
C were considered. FGDs with PVA and GTA concentrations of
10% w/w and 1% w/w respectively, were arranged preparing Fricke
solutions with different amounts of sulfuric acid in the interval
18–100 mM. FGDs in spectrophotometry cuvettes (10 mm optical
path) were uniformly irradiated with a 137Cs source. The optical
absorbance (OA) properties and dose-response in the interval
0–35 Gy were investigated. Furthermore, FGDs in form of thin
layers (3 mm optical path, area 10 5 cm2
) were irradiated with
80 kV X-rays producing a steep dose gradient. Light transmittance
images were acquired before irradiation and at consecutive
times up to 6 h post-irradiation to derive the Fe3+ diffusion
coefficient.
Results. OA spectra and dose-response curves of FGDs were independent of the gelling temperature. A sensitivity of approximately
0.073 Gy-1 was obtained. By contrast, as in traditional FGDs, sulfuric
acid concentration significantly affected the OA spectra as well as
the sensitivity and linearity of the PVA-GTA FGDs. Both gelling temperature gradient and pH did not influence the Fe3+ diffusion coefficient, assessed equal to approximately 0.22 mm2
/h, i.e. more than
two times lower than in traditional FGDs.
Conclusions. The robustness of PVA-GTA-FGDs against preparation
parameters represent a significant advantage over traditional FGDs.
In particular, the response independence of the gelling temperature
may enable the manufacture of large FGD phantoms without any
gradient in sensitivit
Fricke-gel dosimetry in Boron Neutron Capture Therapy
No full textGel dosimetry allows three-dimensional (3D) measurement of absorbed dose in tissue-equivalent dosemeter phantoms. Gel phantoms are imaged using optical techniques. In neutron capture therapy (NCT), properly designed gel dosemeters can give 3D dose distributions, due to the various components of the secondary radiation, in phantoms exposed in the thermal or epithermal column of a nuclear reactor. In addition to the therapeutic dose arising from the reaction 10B(n,alpha)7Li, the other dose components are also obtainable, i.e. the gamma dose (due to reactor background and to the reaction 1H(n,gamma)2H of thermal neutrons with hydrogen, the dose due to protons emitted in the reaction 14N(n,p)14C of thermal neutrons with nitrogen and the dose due to recoil protons resulting from elastic scattering of epithermal neutrons
- …
