Biomedical subjects

P Francescon

Publications and source records attributed to P Francescon.

At least 19 recordsLinked to original sources

PubMed2006

Use of motion tracking in stereotactic body radiotherapy: Evaluation of uncertainty in off-target dose distribution and optimization strategies.

Spatial accuracy in extracranial radiosurgery is affected by organ motion. Motion tracking systems may be able to avoid PTV enlargement while preserving treatment times, however special attention is needed when fiducial markers are used to identify the target can move with respect to organs at risk (OARs). Ten patients treated by means of the Synchrony system were taken into account. Sparing of irradiated volume and of complication probability were estimated by calculating treatment plans with a motion tracking system (Cyberknife Synchrony, Sunnyvale, CA, USA) and a PTV-enlargement strategy for ten patients. Six patients were also evaluated for possible inaccuracy of estimation of dose to OARs due to relative movement between PTV and OAR during respiration. Dose volume histograms (DVH) and Equivalent Uniform Dose (EUD) were calculated for the organs at risk. In the cases for which the target moved closer to the OAR (three cases of six), a small but significant increase was detected in the DVH and EUD of the OAR. In three other cases no significant variation was detected. Mean reduction in PTV volume was 38% for liver cases, 44% for lung cases and 8.5% for pancreas cases. NTCP for liver reduced from 23.1 to 14.5% on average, for lung it reduced from 2.5 to 0.1% on average. Significant uncertainty may arise from the use of a motion-tracking device in determination of dose to organs at risk due to the relative motion between PTV and OAR. However, it is possible to limit this uncertainty. The breathing phase in which the OAR is closer to the PTV should be selected for planning. A full understanding of the dose distribution would only be possible by means of a complete 4D-CT representation.

Carcinoma↗

PubMed2005

CT-3D rotational angiography automatic registration: a sensitivity analysis.

Preprocessing, binning and dataset subsampling are investigated with regard to simultaneous maximisation of the speed, accuracy and robustness of CT-3D rotational angiography (3DRA) registration. Clinical diagnosis and treatment can both take advantage of this integration, because 3DRA allows the shape of vessel structures to be evaluated three-dimensionally with respect to standard 2D projective angiography. The method for optimising preprocessing, binning and subsampling consisted of independent variation of the corresponding parameters to maximise robustness and speed while maintaining subvoxel accuracy; the latter was computed as the sum of the mean squared errors initially present in the registrations with the errors relative to both binning and subsampling. The results suggest the choice of 256 bins, steps between 14 mm (coarse optimisation) and 2.5 mm (fine optimisation) and bone segmentation by threshold, for binning, subsampling and preprocessing, respectively. The application of this parameter set-up to 50 CT-3DRA registrations resulted in a saving, on average, of 40% of the time with respect to the method previously used, while registration error was maintained within 2 mm (1.97 mm, 90% confidence interval) and robustness was increased, so that no manual initial realignment was needed in 48 registrations. Validation by the registration of images acquired for a head phantom showed subvoxel residual errors. In conclusion, the proposed procedure can be considered a satisfactory strategy to optimise CT-3DRA registration.

Angiography↗

PubMed2005

Preliminary study on the use of nonrigid registration for thoraco-abdominal radiosurgery.

The inclusion of organ deformation and movement in radiosurgery treatment planning is of increasing importance as research and clinical applications begin to take into consideration the effects of physiological processes, like breathing, on the shape and position of lesions. In this scenario, the challenge is to localize the target in toto (not only by means of marker sampling) and to calculate the dose distribution as the sum of all the contributions from the positions assumed by the target during the respiratory cycle. The aim of this work is to investigate the use of nonrigid registration for target tracking and dynamic treatment planning, i.e., treatment planning based not on one single CT scan but on multiple CT scans representative of the respiration. Twenty patients were CT scanned at end-inhale and end-exhale. An expert radiation oncologist identified the PTV in both examinations. The two CT data sets per patient were nonrigidly registered using a free-form deformation algorithm based on B-splines. The optimized objective function consisted of a weighted sum of a similarity criterion (Mutual Information) and a regularization factor which constrains the transformation to be locally rigid. Once the transformation was obtained and the registration validated, its parameters were applied to the target only. Finally, the deformed target was compared to the PTV delineated by the radiation oncologist in the other study. The results of this procedure show an agreement between the center of mass as well as volume of the target identified automatically by deformable registration and manually by the radiation oncologist. Moreover, obtained displacements were in agreement with body structure constraints and considerations usually accepted in radiation therapy practice. No significant influence of initial target volume on displacements was found. In conclusion, the proposed method seems to offer the possibility of using nonrigid registrations in radiosurgery treatment planning, even if more cases need to be investigated in order to give a statistical consistency to parameter setup and proposed considerations.

Abdominal Neoplasms↗

PubMed2004

Interstitial radiosurgery with the photon radiosurgery system in the minimally-invasive treatment of selected deep-seated brain tumors.

The purpose of this study was to evaluate the results of interstitial radiosurgery (IR) with Photon Radiosurgery System (PRS) in 18 patients (P) with deep-seated brain primary or secondary tumors. Follow-up varied from 2 to 53 months (mean, 13.6 mo). Seven P with glioblastomas died due to tumor progression. Five P with metastases died for systemic disease while local control was achieved in all. Six P with low-grade astrocytomas were well and imaging showed tumor control. We conclude that PRS IR is effective in the treatment of metastases while it provides lower benefit in malignant gliomas. It could play a major role in low-grade astrocytomas.

Adolescent↗

PubMed2003

Dose verification of an IMRT treatment planning system with the BEAM EGS4-based Monte Carlo code.

Intensity modulated radiation therapy (IMRT) has been increasingly used in radiotherapy departments during the last several years. A major advantage of IMRT in comparison to traditional three-dimensional conformal radiotherapy is the higher capability in providing dose distributions that conform very tightly to the target even for very complex shapes such as, for instance, concave regions. This results in a significant sparing of adjacent normal tissues. Different types of algorithms are employed in the IMRT dose calculation, from the simple pencil beam method, such as the finite-size pencil beam algorithm, to the more sophisticated algorithms, such as the kernel-based convolution/superposition ones. With the latter ones, electronic disequilibrium and inhomogeneities are better dealt with in comparison to the correction-based models like pencil beam. Nevertheless, even these types of algorithms may have some approximations that can potentially affect the dose results, especially considering that in an IMRT plan small segments or beamlets may be present for which electronic disequilibrium and inhomogeneities effects are of paramount importance. The goal of this work was to determine the accuracy in monitor units (MU) and dose distribution calculation of the algorithm implemented in the commercial treatment planning system PINNACLE3 (P3), for two IMRT plans with 6 MV photon beams. This system is based on a convolution/superposition with the Collapsed Cone approximation algorithm. The "BEAM" Monte Carlo (MC) code was employed as a benchmark in comparing the MU calculation and the dose distribution of P3. The model used to calculate the MU, with the separation of collimator scatter from the phantom scatter, valid for broad beams, was verified for narrow and irregular segments. The attention was focused on the way P3 calculates output factors (OF). A difference of 8% compared to MC was found for a particularly narrow segment analyzed. A dependence of the results on field size was found. For the complete plan, the agreement of dose distribution and MU calculation with MC results (affected by a dose uncertainty less than 0.5%) is very good: the dose difference at isocenter is 2.1% (1 standard deviation) for a "Prostate" site and 2.9% (1 standard deviation) for the "Head and Neck" site.

Algorithms↗

PubMed2002

A simple method for bremsstrahlung spectra reconstruction from transmission measurements.

A new method for evaluation of bremsstrahlung spectra from transmission measurements has been developed. In this method some very well known facts relating to thick target bremsstrahlung spectra are a priori included in the calculation procedure. Some characteristics of the method are preliminarily illustrated on a 6 MV therapy linear accelerator.

Algorithms↗

PubMed2001

Employ of a new device for intra-operative radiotherapy of intracranial tumours.

BACKGROUND: This paper presents the operating experience acquired during 18 months of use of the Photon Radiosurgery System manufactured by Photo-electron Corp. as an intra-operative radiation therapy device. The device is based on a miniature x-ray source that delivers low energy x-rays from the tip of a 3.2 mm thick needle-like probe. The interstitial stereotactic employ of the source has already been reported, while there is no evidence of the open-field intra-operative use in the literature. METHOD: Open field intra-operative radiation therapy (IORT) is possible inserting the probe into spherical applicators with diameters ranging from 1.5 to 5 cm. The applicators are made to fit the surgical cavity in order to provide uniform distance between the x-ray source and the tumour bed. Delivery of the prescribed dose takes typically 10 to 45 minutes. Radiation characteristics of the source were measured by means of ionization chambers and radiochromic films positioned in a water phantom. Operating procedures aimed at quality assurance and radiation safety were developed. IORT was administered to 14 patients affected by malignant intracranial tumours. Doses from 10 to 15 Gy at 5 mm depth from the tumour bed were delivered after tumour removal. FINDINGS: This preliminary experience does not afford any clinical evidence of IORT efficacy for intracranial lesions: it permits one however to state the feasibility and safety of the procedure. INTERPRETATION: This system could favour a rapid and significant increase of the experience of intra-operative irradiation in the treatment of CNS tumours. A role in the treatment of extracranial neoplasms can be also foreseen but needs to be more extensively investigated.

Adult↗

PubMed2000

2D and 3D dose distribution determination in proton beam radiotherapy with GafChromic film detectors.

This paper presents the results obtained using radiochromic (MD-55 GafChromic) film for the 2D and 3D dosimetric reconstruction of the dose delivered by a proton beam under the real conditions of a programme of radiotherapy treatment for ocular tumours. Standard microdensitometric measurements were used to determine the variation in film optical density (O.D.) vs dose. Calibration curves were obtained by least-square fitting of the experimental OD values using a second order polynomial. This allows conversion of O.D. to dose. With this procedure it was possible to determine the distribution of the dose delivered by the proton beam in a phantom composed of layers of GafChromic film, with high surface spatial resolution and, through sections, the complete mapping of the dose delivered to a volume subjected to irradiation, as in a course of radiotherapy treatment.

Eye Neoplasms↗

PubMed2000

Photon dose calculation of a three-dimensional treatment planning system compared to the Monte Carlo code BEAM.

The purpose of this work is to compare the photon dose calculation of a commercially available three-dimensional (3D) treatment planning system based on the collapsed cone convolution technique against BEAM, a Monte Carlo code that allows detailed simulation of a radiotherapy accelerator. The first part of the work is devoted to the commissioning of BEAM for a 6 MV photon beam and to the optimization of the linac description to fit the experimental data. This step also involves a comparison with radiochromic film data on an inhomogeneous phantom built to simulate electronic nonequilibrium conditions. Commissioning the selected photon beams required a careful description of the treatment head and the fine tuning of physical parameters such as electron beam energy and radius. The second part shows the dose comparison for real patient's CT data sets: A mediastinal treatment and a breast treatment were simulated. Doses in terms of absolute values per monitor unit were calculated based on the BEAM simulation of the CT data sets. For comparisons of real-patient cases, differences between the treatment planning system and BEAM ranged from 0 to 2.6% and were within +/-2 standard deviations for the dose calculated at the prescription point. Dose-volume histogram analysis indicated that there is no consistent difference between the Monte Carlo and the convolution calculations. On the basis of the results presented in this study, we can conclude that the CCC algorithm is capable of giving results absolutely comparable to those of a Monte Carlo calculation, as far as common 3D radiotherapy planning is concerned.

Algorithms↗

PubMed1998

A simple method to verify in vivo the accuracy of target coordinates in linear accelerator radiosurgery.

PURPOSE: A simple method that verifies the coincidence of the isocenter with the center of the target volume in radiosurgery treatment conditions is described. The accuracy is compared to that of accepted computerized procedures employing fiducial markers. METHODS AND MATERIALS: The center of the beam is identified by a cylindrical localizer, fixed to the plate of the supplemental collimator, with a 2 x 50 mm tungsten rod coincident with the beam axis and is projected onto the x-ray portal verification films. Prior to irradiation, the coordinates of the intersection of the beams axes, which is in a known spatial relationship with the isocenter, are read directly on portal x-ray films and their coincidence with the coordinates set during patient positioning, is checked. RESULTS: The mean displacement in AP, Lat, and Vert coordinates respectively, over 84 patients, between the coordinates calculated by the computerized procedure employing fiducial markers and the coordinates calculated by using the rulers was 0.3 +/- 0.4 mm. CONCLUSIONS: From the results obtained with the two methods we can conclude that rulers method can be used as a fast indirect control of the position of the radiation isocenter. Moreover, the dimensions of the radiation field and the correct alignment of the tertiary circular collimator can be also documented.

Mathematics↗

PubMed1998

Calibration of a mosfet detection system for 6-MV in vivo dosimetry.

PURPOSE: Metal oxide semiconductor field-effect transistor (MOSFET) detectors were calibrated to perform in vivo dosimetry during 6-MV treatments, both in normal setup and total body irradiation (TBI) conditions. METHODS AND MATERIALS: MOSFET water-equivalent depth, dependence of the calibration factors (CFs) on the field sizes, MOSFET orientation, bias supply, accumulated dose, incidence angle, temperature, and spoiler-skin distance in TBI setup were investigated. MOSFET reproducibility was verified. The correlation between the water-equivalent midplane depth and the ratio of the exit MOSFET readout divided by the entrance MOSFET readout was studied. MOSFET midplane dosimetry in TBI setup was compared with thermoluminescent dosimetry in an anthropomorphic phantom. By using ionization chamber measurements, the TBI midplane dosimetry was also verified in the presence of cork as a lung substitute. RESULTS: The water-equivalent depth of the MOSFET is about 0.8 mm or 1.8 mm, depending on which sensor side faces the beam. The field size also affects this quantity; Monte Carlo simulations allow driving this behavior by changes in the contaminating electron mean energy. The CFs vary linearly as a function of the square field side, for fields ranging from 5 x 5 to 30 x 30 cm2. In TBI setup, varying the spoiler-skin distance between 5 mm and 10 cm affects the CFs within 5%. The MOSFET reproducibility is about 3% (2 SD) for the doses normally delivered to the patients. The effect of the accumulated dose on the sensor response is negligible. For beam incidence ranging from 0 degrees to 90 degrees, the MOSFET response varies within 7%. No monotonic correlation between the sensor response and the temperature is apparent. Good correlation between the water-equivalent midplane depth and the ratio of the exit MOSFET readout divided by the entrance MOSFET readout was found (the correlation coefficient is about 1). The MOSFET midplane dosimetry relevant to the anthropomorphic phantom irradiation is in agreement with TLD dosimetry within 5%. Ionization chamber and MOSFET midplane dosimetry in inhomogeneous phantoms are in agreement within 2%. CONCLUSION: MOSFET characteristics are suitable for the in vivo dosimetry relevant to 6-MV treatments, both in normal and TBI setup. The TBI midplane dosimetry using MOSFETs is valid also in the presence of the lung, which is the most critical organ, and allows verifying that calculation of the lung attenuator thicknesses based only on the density is not correct. Our MOSFET dosimetry system can be used also to determine the surface dose by using the water-equivalent depth and extrapolation methods. This procedure depends on the field size used.

Calibration↗

PubMed1998

Use of a new type of radiochromic film, a new parallel-plate micro-chamber, MOSFETs, and TLD 800 microcubes in the dosimetry of small beams.

The dosimetry of the fields usually employed in radiosurgery requires the use of small detectors to measure Total Scatter Factor (Sc,p), Tissue Maximum Ratio (TMR), Percentage Depth Dose (PDD), and Off Axis Ratio (OAR). In this paper new dosimeters are investigated: a new type of radiochromic film, a micro parallel-plate chamber (filled with both air and tetramethylsilane, TMS), MOSFETs, and TLD-800 microcubes. Their behavior has been compared with the response of radiographic film and with the values obtained with BEAM Monte Carlo simulation. The experimental data confirm that dosimetry with radiochromic films and TLDs gives consistent results for all beam diameters. The parallel-plate micro chamber underestimates the Sc,p for the smallest field diameters (4.4 mm and 6.7 mm); MOSFETs show an over-estimation for the Sc,p of the 4.4 mm, 6.7 mm, and 10.5 mm field diameters. BEAM Monte Carlo simulation employing a parallel beam and a standard 6 MV x-ray spectrum has been used to obtain a correction factor as a function of the field size for both the parallel-plate micro chamber and MOSFETs. High accuracy measurements of PDD and TMR have been made in a water phantom both with radiochromic film and with the micro parallel-plate chamber and have been compared with the data computed by BEAM Monte Carlo simulation. The latter dosimeter is preferred because of the quicker and simpler use and because it gives immediate readout. Measurements of OAR made with radiochromic films and with radiographic films give differences in the 80%-20% penumbra width within 0.6 mm for field diameters ranging from 4.4 mm to 19 mm.

Calibration↗

PubMed1995

Evaluation of linear accelerator radiosurgical techniques using biophysical parameters (NTCP and TCP).

PURPOSE: Several irradiation techniques are compared with regard to normal tissue complication probability and tumor control probability. METHODS AND MATERIALS: Normal tissue complication probability is calculated using a model based on the "critical element architecture." The probability of controlling an inhomogeneously irradiated tumor is calculated using a model that takes into account the heterogeneity of tumors (different radiosensitivity of clonogens within the tumor and the varying number of clonogens among patients with the same kind of tumor). The ratio of tumor control probability to normal tissue complication probability (therapeutic gain factor) at different levels of dose has been used as a score for the analysis of various irradiation techniques. RESULTS: The best irradiation techniques depends on many factors: irradiation genometry, irradiation field size, choice of the reference isodose, and it is dictated by the pathology of the lesions (noninfiltrating radioresistant tumors, infiltrating radioresistant tumors, noninfiltrating radiosensitive tumors, infiltrating radiosensitive tumors, arterovenous malformations). For the irradiation of the artero-venous malformations it is proposed to insert on the supplemental collimator a flattening filter to reduce the probability of inducing a poorly syncronized obliterative effect. CONCLUSION: We propose that for each kind of pathology to be treated radiosurgically, a proper irradiation strategy should be used.

Humans↗

PubMed1994

Linear accelerator radiosurgery of cerebral arteriovenous malformations: current status.

228 patients affected by cerebral arteriovenous malformations (AVMs) underwent linear accelerator radiosurgery. Follow-up ranges from 1 to 100 months (mean 42 months). Complete angiographic obliteration was achieved in 47% of treated patients at one year and 80% at 2 years. 17 haemorrhages were observed after treatment and 6 patients died from them. No bleeding took place after complete angiographic obliteration. 11 patients suffered for radionecrosis. In 6 patients complete recovery was obtained with corticoid medication. The aim of this study is to present our results and to evaluate the effect of irradiation on the risk of bleeding after radiosurgery. Patients were considered at risk in the time lapse after irradiation and before angiographic obliteration or other definitive treatment or death. Patients were followed from the date of radiosurgery and the number of haemorrhages were recorded every six months. In our series the bleeding risk in patients harbouring incompletely obliterated AVMs decreases from 8% in the first year after radiosurgery to 0% starting from the 24th month of the follow-up.

Adolescent↗

PubMed1994

Linear accelerator radiosurgery of cerebral arteriovenous malformations: an update.

One hundred eighty patients affected by cerebral arteriovenous malformations (AVMs) underwent radiosurgical treatment in our department. One hundred fifty-three patients have been treated with complete irradiation of the entire AVM nidus. In 27 patients (with large and/or three-dimensional irregular target volumes), only part of the nidus was covered with a dose adequate for obliteration. Follow-up ranged from 88 to 1 months (mean, 43.1 mo). Angiographic control was performed at 12, 24, and 36 months until complete obliteration was attained. The complete obliteration rate was 46% at 1 year and 80% at 2 years. We observed 15 hemorrhages after treatment, and five patients died from them. No bleeding took place after complete angiographic obliteration. The aim of this study is to evaluate the effect of irradiation on bleeding risk after radiosurgery and before complete obliteration. Inclusive parameters of patients considered at risk were as follows: 1) all patients in the time lapse between irradiation and demonstrated complete angiographic obliteration; 2) all patients in the time lapse between irradiation and definitive treatment either by surgery or embolization; and 3) all patients in the time lapse between irradiation and death. These groups include all irradiated patients who still had incompletely obliterated AVMs. They were stratified starting from 0 time (the date of radiosurgery), and the hemorrhages were evaluated every 6 months. In totally irradiated cases, the bleeding risk decreased from 4.8% in the first 6 months after radiosurgery to 0% starting from the 12th month of the follow-up.(ABSTRACT TRUNCATED AT 250 WORDS)

Adolescent↗

PubMed1993

Analysis of dosimetric measurements in linac radiosurgery calibration.

The aim of this paper is to analyse the dosimetric parameters of a linear accelerator used in radiosurgery treatments. The influence of these parameters on the resulting dose distribution are basic for delivering the predefined dose to the vascular or oncological target volume. Several dosimetric methods have been used to define the output factors for small fields. The thimble and the Markus chambers, TLD and film dosimetry are investigated; all these dosimetric systems give reliable and almost similar results if used in the correct way. In the determination of tissue maximum ratio (TMR) the response curves obtained by ionometric and film dosimetry were investigated. For TMR determination the use of the Markus chamber and the correction factors to be applied as a result of the small dimension of the field were also investigated.

Calibration↗

PubMed1993

From radiotherapy to stereotactic radiosurgery: physical and dosimetrical considerations.

The aim of this presentation is to analyse the mechanical and dosimetrical parameters of the linear accelerator used in stereotactic radiosurgery. The use of the thimble and Markus chambers, TL and film in small field dosimetry are investigated. To determine the optimal irradiation technique and dose distribution, the dose volume to healthy tissue is considered.

Humans↗

PubMed1990

Diagnostic features of body surface potential maps in patients with myocardial ischemia and normal resting 12-lead electrocardiograms.

Body surface maps recorded from 35 ischemic patients with normal resting 12-lead electrocardiograms were compared with those obtained from 36 age- and sex-matched normal subjects. From instantaneous maps of each subject 187 variables were derived relating to the configuration (80 variables) and magnitude (104 variables) of the potential distribution and duration of the electrocardiographic intervals (3 variables). By using stepwise discriminant analysis we selected 3 variables whose linear combination enabled us to correctly allocate 91% of the study population (jacknife procedure; specificity 92%, sensitivity 91%). To substantiate the validity of the results the discriminant function was tested on a new independent population consisting of 27 ischemic patients and 54 normal subjects from another laboratory. A proper allocation was obtained in 86% of the cases (specificity 87%, sensitivity 85%). The large number of correctly classified ischemic patients and the repeatability of the results indicate that the adopted criteria are good markers of ischemic heart disease.

Adult↗