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L Vlachos

Publications and source records attributed to L Vlachos.

At least 19 recordsLinked to original sources
PubMed2006

Dosimetric evaluation of a new collimator insert system for stereotactic radiotherapy.

The prototype of a stereotactic collimator set developed in our department is evaluated for clinical use. This set consists of three cylindrical blocks mounted on a tray which slides in the wedge insert of a Siemens Primus accelerator. Each block has a collimating hole along its long axis to produce radiation fields of circular cross-section at the isocentre plane with diameters of 15 mm, 20 mm and 25 mm. Different geometric and dosimetric quality assurance tests were performed and results are found within the limits set for stereotactic radiotherapy. Dosimetry results measured using Kodak EDR-2 radiographic film and a pinpoint ion chamber also show good agreement with corresponding results calculated by Monte Carlo simulation of the linear accelerator head and the collimators. Measured dosimetry data were used to adapt a conventional PLATO treatment planning system for stereotactic radiotherapy using the prototype collimator set. Treatment planning system calculations and film measurements for treatment of an intracranial lesion in an anthropomorphic head phantom using coplanar 180 degrees arcs are compared and found to agree within 2 mm. This supports the accuracy of dose delivery using the prototype stereotactic collimators. Despite their increased penumbra (2.5-3.5 mm relative to 2-2.5 mm for commercially available collimators) the ease of construction makes the proposed stereotactic collimators an interesting alternative for accomplishing cost effective stereotactic treatments.

Computer Simulation↗
PubMed2006

Radiobiological modelling of dose-gradient effects in low dose rate, high dose rate and pulsed brachytherapy.

This paper presents a generalization of a previously published methodology which quantified the radiobiological consequences of dose-gradient effects in brachytherapy applications. The methodology uses the linear-quadratic (LQ) formulation to identify an equivalent biologically effective dose (BED(eq)) which, if applied uniformly to a specified tissue volume, would produce the same net cell survival as that achieved by a given non-uniform brachytherapy application. Multiplying factors (MFs), which enable the equivalent BED for an enclosed volume to be estimated from the BED calculated at the dose reference surface, have been calculated and tabulated for both spherical and cylindrical geometries. The main types of brachytherapy (high dose rate (HDR), low dose rate (LDR) and pulsed (PB)) have been examined for a range of radiobiological parameters/dimensions. Equivalent BEDs are consistently higher than the BEDs calculated at the reference surface by an amount which depends on the treatment prescription (magnitude of the prescribed dose) at the reference point. MFs are closely related to the numerical BED values, irrespective of how the original BED was attained (e.g., via HDR, LDR or PB). Thus, an average MF can be used for a given prescribed BED as it will be largely independent of the assumed radiobiological parameters (radiosensitivity and alpha/beta) and standardized look-up tables may be applicable to all types of brachytherapy treatment. This analysis opens the way to more systematic approaches for correlating physical and biological effects in several types of brachytherapy and for the improved quantitative assessment and ranking of clinical treatments which involve a brachytherapy component.

Algorithms↗
PubMed2006

Technique charts for Kodak EC-L film screen system for portal localization in a 6MV X-ray beam.

Port films are used in radiotherapy for visual evaluation of the radiation fields and subsequent quantitative analysis. Common port films suffer from poor image quality compared to the simulator-diagnostic films and is desirable to determine the appropriate exposure required for the best image contrast. The aim of this work is to generate technique charts for the Kodak EC-L film screen system for use in a 6MV x-ray beam. Three homogeneous water phantoms were used to simulate head-neck, thorax and abdomen dimensions of adult human, correspondingly. The film screen system was calibrated in a 6MV x-ray beam and under various irradiation conditions. The film screen system behavior was studied as a function of phantom thickness, field size and air gap between the phantom and the film screen system. In each case the optimum film exposure which produces the maximum image contrast was determined. The generated technique charts for the EC-L film screen system and for a 6 MV x-ray beam are used in our radiotherapy department for daily quality assurance of the radiotherapy procedure.

Calibration↗
PubMed2005

Screening of patients after colectomy: virtual colonography.

BACKGROUND: Virtual colonography is a powerful new method of imaging the entire colon and is useful to assess polyps and diagnose colon cancer. We evaluated virtual colonography in the postoperative screening of patients who had colon cancer. METHODS: Fifty-three patients were examined with virtual colonography 12 to 48 months postoperatively. Forty-four patients had received segmental colectomy with restoration of the gastrointestinal tract, and nine patients underwent abdominoperineal resection and permanent colostomy. After proper cleaning of the colon and distention with air, spiral computed tomographic examination of the abdomen with a slice thickness of 5 mm (table speed [TS] 10 mm, reconstruction interval [RI] 2.5 mm) was performed in the supine and prone positions (including intravenous contrast medium infusion). Images were transferred to a separate workstation (Philips Easy Vision) for postprocessing, three-dimensional rendering, and endoluminal viewing. RESULTS: Eleven recurrences (16.41%) were identified in 10 patients by virtual colonography, but one recurrence was missed. Conventional colonoscopy was incomplete in six cases, and two patients with colostomy refused colonoscopy. In these eight cases (15%), virtual colonoscopy was completed without problems. A second tumor in one patient who had received abdominoperineal resection was demonstrated by virtual colonography, but conventional colonoscopy failed to demonstrate the lesion. Liver metastases were identified in only one patient. CONCLUSIONS: Virtual colonography seems to provide a good alternative in the follow-up of patients after colectomy. The technique is effective in the diagnosis of locoregional recurrences and distant metastases and is well accepted by patients, and results are equal to those of the conventional colonoscopy.

Adult↗
PubMed2005

Dose verification of single shot gamma knife applications using VIPAR polymer gel and MRI.

This work describes an experimental procedure with potential to assess the overall accuracy associated with gamma knife clinical applications, from patient imaging and dosimetry planning to patient positioning and dose delivery using the automated positioning system of a Leksell Gamma Knife model C. The VIPAR polymer gel-MRI dosimetry method is employed due to its inherent three-dimensional feature and linear dose response over the range of gamma knife applications. Different polymer gel vials were irradiated with single shot gamma knife treatment plans using each of the four available collimator helmets to deliver a maximum dose of 30 Gy. Percentage relative dose results are presented not only in the form of one-dimensional profiles but also planar isocontours and isosurfaces in three dimensions. Experimental results are compared with corresponding Gammaplan treatment planning system calculations as well as acceptance test radiochromic film measurements. A good agreement, within the experimental uncertainty, is observed between measured and expected dose distributions. This experimental uncertainty is of the order of one imaging pixel in the MRI gel readout session (<1 mm) and allows for the verification of single shot gamma knife applications in terms of acceptance specifications for precision in beam alignment and accuracy. Averaging net R(2) results in the dose plateau of the 4 mm and 18 mm collimator irradiated gel vials, which were MR scanned in the same session, provides a crude estimate of the 4 mm output factor which agrees within errors with the default value of 0.870.

Argon↗
PubMed2005

Increased renal glucose metabolism in Type 1 diabetes mellitus.

AIMS: In poorly controlled diabetes, increased renal glucose uptake has been implicated in the pathogenesis of diabetic nephropathy by promoting nonenzymatic glycosylation of proteins, activation of protein kinase C, and increased polyol pathway flux. However, whether glucose uptake by the diabetic kidney is actually increased, especially in patients with Type 1 diabetes, is unclear. METHODS: To examine this question, we used a combination of net balance and isotopic techniques to compare renal glucose uptake in 12 subjects with Type 1 diabetes before and after restoration of near normoglycaemia by infusion of insulin with that in 15 postabsorptive nondiabetic volunteers. RESULTS: Prior to insulin infusion, the diabetic subjects were markedly hyperglycaemic (arterial glucose 15.8 +/- 0.9 vs. 4.4 +/- 0.1 mm) and their renal tissue glucose uptake (i.e. total glucose disappearance across the kidney minus glycosuria) was increased more than 2 1/2-fold (388 +/- 43 vs. 148 +/- 12 micromol/min, P < 0.001). This was wholly explained by the mass action effects of hyperglycaemia since the diabetic subjects had normal renal blood flow (1575 +/- 82 vs. 1492 +/- 68 mL/min, P = 0.46) and reduced renal tissue glucose fractional extraction (1.7 +/- 0.2 vs. 2.3 +/- 0.1%, P = 0.027). Insulin infusion for three hours, which restored near normoglycaemia (arterial glucose 7.6 +/- 0.7 mm), reduced renal tissue glucose uptake toward normal (258 +/- 41 micromol/min, P = 0.006) without altering renal blood flow (1557 +/- 110, P = 0.63) or renal tissue glucose fractional extraction (2.1 +/- 0.3%, P = 0.35). Renal and hepatic glucose release, which had been increased (419 +/- 49 and 960 +/- 54 vs. 204 +/- 9 and 734 +/- 32 micromol/min, both P < 0.001), were suppressed by insulin to 138 +/- 22 and 520 +/- 53 micromol/min, respectively (both P < 0.001). CONCLUSIONS: In poorly controlled Type 1 diabetes, renal glucose uptake is markedly increased, which provides a link between hyperglycaemia and biochemical processes implicated in the pathogenesis of diabetic nephropathy. Its reversal by restoration of near normoglycaemia with insulin may explain the benefit of intensive insulin therapy in preventing diabetic nephropathy.

Adult↗
PubMed2004

Ovarian masses in young adolescents: imaging findings with surgical confirmation.

Functional cysts and benign neoplasms are the most common ovarian masses among young adolescents. Ovarian cancer on the other hand, although rare in this age group, is the most common genital tract malignancy. The purpose of this study was to define imaging characteristics of ovarian masses in adolescents between 12 and 21 years old and correlate imaging and surgical findings. Thirty-seven female adolescent patients aged between 12 and 21 years were operated on because of a diagnosed ovarian mass between 1997 and 2002. All patients underwent pelvic ultrasound, five had an abdominal CT scan, two had abdominal MRI, one abdominal X-ray and one intravenous pyelography. Ultrasound was used to define the size of the lesion and to characterize its gross morphologic condition as solid, simple cyst or complex cyst. The records were reviewed for age at presentation, presenting symptoms, diagnostic studies, surgical procedure and pathology findings, which were available for all patients. In our study 32 patients (86.5%) were symptomatic and five asymptomatic (13.5%). The most common presenting symptom was abdominal pain (59.5%). Thirty-four patients (91.1%) had benign lesions, two had malignant tumors (5.4%) and one patient had a borderline lesion (2.7%). The most common ovarian masses detected were germ cell tumors (27.5%) and functional cysts (25%). Twenty patients (54%) underwent operative laparoscopy and 17 patients (46%) exploratory laparotomy. Simple resection of the ovarian mass was achieved in the majority of cases (84%). Bilateral salpingo-oophorectomy was performed in only one case (2.7%).

Adolescent↗
PubMed2003

The effect of various regimens of hormone replacement therapy on mammographic breast density.

OBJECTIVES: To evaluate the effect of three distinct hormone replacement therapy regimens on mammography. METHODS: 121 postmenopausal women who had never received or were past users of hormone replacement therapy were studied prospectively. Women with an intact uterus were randomly allocated either to conjugated equine estrogens 0.625 mg plus medroxyprogesterone acetate 5 mg (CEE/MPA, n=34) or to 17beta-estradiol 2 mg plus norethisterone acetate 1 mg (E(2)/NETA, n=35). Hysterectomized women received CEE 0.625 mg (CEE, n=25). Women who either declined or did not qualify for treatment served as controls (n=27). Treatment was continuous and the study period lasted 12 months. Main outcome measures were the changes according to Wolfe classification between baseline and 12-month-mammograms. RESULTS: No increase in breast density was identified in any of the women in the control group. Two women (8%) in the CEE group showed an increase in breast density. Four women (11.8%) in the CEE/MPA and 11 women (31.4%) in the E(2)/NETA group revealed an increase in breast density. No woman in the therapy groups showed an involution of fibroglandular tissue while seven women (25.9%) in the control group exhibited involution of breast parenchyma. CONCLUSIONS: Our study suggests that hormone replacement therapy may suspend breast involution but does not increase breast density in the majority of patients. In the minority of patients who show a density increase, the magnitude of this increase varies according to the regimen employed.

Adult↗
PubMed2003

Evaluation of the performance of VIPAR polymer gels using a variety of x-ray and electron beams.

The aim of this investigation was the evaluation of the usefulness of N-vinyl pyrrolidone argon (VIPAR) polymer gel dosimetry for relative dose measurements using the majority of types and energies of radiation beams used in clinical practice. For this reason, VIPAR polymer gels were irradiated with the following beams: 6 and 23 MV photons (maximum dose: 15 Gy) and 6, 9, 12, 15, 18 and 21 MeV electrons (90% dose: 15 Gy). Using 6 MV x-rays, a linear gel dose response was verified for doses up to 20 Gy. Assuming linearity of response for the rest of the photon and electron beams used in this study, percentage depth dose measurements were derived. For all beams used and the range of relative doses studied, a satisfying agreement was observed between percentage depth dose measurements performed using the VIPAR gel-MRI method and an ion chamber, validating the assumption that a linear gel dose response holds for all photon and electron beams studied. VIPAR gels, therefore, can be used for relative dose distribution measurements using photons or electrons of any typical energy used in external radiotherapy applications. It is also demonstrated that two-dimensional dose distribution measurements through an irradiated (9 MeV electrons, 3 cm x 3 cm cone) VIPAR gel volume can be easily obtained.

Argon↗
PubMed2003

Stent therapy for malignant superior vena cava syndrome: should be first line therapy or simple adjunct to radiotherapy.

UNLABELLED: The goal of this paper is to present our experience with superior vena cava (SVC) stenting, as first line procedure for immediate relief, in patients with malignancy, and its potential influence in the subsequent radiotherapy (XRT). Over a 1-year period, 18 patients with SVC syndrome due to severe stenosis secondary to mediastinal malignancy were referred for stent insertion. A SVC score was used to measure treatment effectiveness. Stent insertion had been successful in 18/18 patients (technical success 100%). All patients experienced symptomatic relief within few hours of the procedure. There were no major complications. In all patients we were able to start radiotherapy (XRT) the next day, after stenting according to our new institutional protocol. All patients were able to comply with the XRT program, perfectly well. CONCLUSIONS: SVC stenting provides immediate significant relief of the very annoying SVC syndrome symptoms, thus facilitating excellent compliance of all the patients to the subsequently XRT protocols. We strongly recommend SVC stenting as first line procedure, in patients with SVC syndrome due to malignancy prior to radiotherapy.

Adult↗
PubMed2002

Abdominal aortic aneurysm and renal artery stenosis: renal function and blood pressure before and after endovascular treatment.

We describe three patients with abdominal aortic aneurysm (AAA) and renal artery stenosis (RAS). These patients were treated by placement of an aortic endograft and angioplasty or stenting of the renal artery. After the procedure renal function improved or remained stable in two patients and deteriorated slightly in one. Blood pressure was reduced in one hypertensive patient and remained normal in the other two normotensive patients. In conclusion, simultaneous treatment of AAA and RAS with aortic endograft placement and renal artery angioplasty with or without stent, is a safe and effective technique for selected high-risk patients.

Aged↗
PubMed2002

Renal artery stenting for renal insufficiency in solitary kidney in 26 patients.

OBJECTIVE: to present our experience with stent placement in renal arteries in solitary kidneys for treating renal insufficiency. DESIGN: retrospective analysis. MATERIALS: in 26 patients with solitary kidney (17 men, 9 women, mean age: 63 years), presented with renal insufficiency (se-creat >0.144 mmol/l), stent was placed in a stenosed renal artery. We analysed the clinical outcome, based on the level of creatinine at 3 months following the procedure. Clinical benefit was considered when there was a decrease compared to the baseline creatinine by >20% or a stabilisation of the creatinine value (+/-20% of the baseline). RESULTS: in 16 of the 26 patients (62%), clinical benefit was achieved. However, 38% of the study population, renal function continued to deteriorate. Baseline creatinine value was the single best predictor for clinical benefit achievement (odds ratio: 13; 95% confidence intervals: 1.6-107, p=0.01). CONCLUSION: renal stenting results in improvement or stabilisation of renal function in the majority of the patients with solitary kidneys and renal artery stenosis, presenting with renal insufficiency. Because best outcome was observed mainly in those patients with not progressed renal insufficiency, intervention should be focused on that group.

Creatinine↗
PubMed2002

Dosimetry comparison of 192Ir sources.

192Ir sources besides being widely utilized in the field of conventional brachytherapy also find use in contemporary peripheral and coronal intravascular applications. In this study, the same Monte Carlo simulation code and input data were used to investigate differences between the dose rate distributions of the most commonly used 192Ir sources in the cm and mm distance range. Findings are discussed in view of differences in source and encapsulation dimensions as well as structural details. Results are presented in the AAPM TG-43 formalism, as generalized by AAPM TG-60, for five 192Ir HDR source designs as well as an LDR seed and an LDR wire source. Dose rate constants of the sources at r0 = 1 cm and r0 = 2 mm were found proportional to the corresponding geometry factors along the transverse source bisectors and an equation of the form lambda r0(cGyh(-1) U(-1)) = 1.12 x G(r0,90 degrees) provides results within clinical accuracy (less than 2%) for any 192Ir source. Radial dose functions do not depend significantly on source and encapsulation geometry and agree within 2% with that of a point 192Ir source. Anisotropy is of importance for accurate dosimetry at the cm distance range but it does not affect dose rate in the mm distance range significantly. At such short radial distances the source geometry factor defines the shape of isodose lines. Dose uniformity at given distances from the sources is strongly dependent on source dimensions as indicated by dose rate profiles in polar and Cartesian coordinates.

Anisotropy↗
PubMed2002

Polymer gel dosimetry using a three-dimensional MRI acquisition technique.

In this work, three-dimensional (3-D) MRI techniques are employed in N-Vinylpyrrolidone-Argon-(VIPAR-) based polymer gel dosimetry. VIPAR gels were irradiated using a Nucletron microSelection 192Ir HDR brachytherapy remote afterloading system with single source dwell position and intravascular brachytherapy irradiation protocols. A single VIPAR gel and a single irradiation are adequate to obtain the full calibration curve needed. The 3-D dose distributions obtained with the 3-D MRI method were found to be in good agreement with the corresponding Monte Carlo calculations, for brachytherapy and intravascular irradiations. The method allows the reconstruction of isodose contours over any plane, with increased spatial resolution and accuracy following a single MR acquisition. VIPAR gel measurements using a 3-D MRI readout technique can be of particular use in the experimental dosimetry of brachytherapy sources, as well as for dose verification purposes when complex irradiation regimes and three-dimensional dose gradients are investigated.

Argon↗
PubMed2001

Transplant renal artery stenosis: hypertension and graft function before and after angioplasty.

We present a case, who after 15 years of renal transplantation developed severe deterioration of her hypertension without alteration in renal function. Colour Doppler sonography revealed a 90% stenosis near the anastomosis of the graft artery to iliac artery, which was successfully and uneventfully corrected by percutaneous balloon angioplasty. Following the procedure the blood pressure control dramatically improved and her antihypertensive regimen returned and remained at baseline for the subsequent year of observation. Renal function remained normal and stable before and after angioplasty.

Angiography, Digital Subtraction↗
PubMed2001

Narrow stereotactic beam profile measurements using N-vinylpyrrolidone based polymer gels and magnetic resonance imaging.

In this work, polymer gel-MRI dosimetry (using VIPAR gels), radiographic film and a PinPoint ion chamber were used for profile measurements of 6 MV x-ray stereotactic beams of 5 and 10 mm diameter. The VIPAR gel-MRI method exhibited a linear dose response up to 32 Gy. VIPAR gels were found to resolve the penumbra region quite accurately, provided that the in-plane image resolution of the related T2-map is adequate (< or = 0.53 mm). T2-map slice thickness had no significant effect on beam profile data. VIPAR measurements performed with a spatial resolution of 0.13 mm provided penumbra widths (80%-20% distance) of 1.34 and 1.70 mm for the 5 and 10 mm cones respectively. These widths were found to be significantly smaller than those obtained with the film (2.23 mm for the 5 mm cone, 2.45 mm for the 10 mm cone) and PinPoint (2.25 mm for the 5 mm cone, 2.52 mm for the 10 mm cone) methods. Regarding relative depth dose measurements, good correlation between VIPAR gel and PinPoint data was observed. In conclusion, polymer gel-MRI dosimetry can provide relatively accurate profile data for very small beams used in stereotactic radiosurgery since it can overcome, to some extent, the problems related to the finite size of conventional detectors.

Calibration↗
PubMed2001

Dosimetry of 192Ir wires for LDR interstitial brachytherapy following the AAPM TG-43 dosimetric formalism.

Implementation of the AAPM Task Group 43 dosimetric formalism for 192Ir wires used as interstitial sources in low dose-rate (LDR) brachytherapy applications is investigated. Geometry factors, dose-rate constant values, radial dose functions, and anisotropy functions, utilized in this formalism, were calculated for various lengths of all commercially available wire source designs by means of a well-established Monte Carlo simulation code and an improved modification of the Sievert integral method. Results are presented in the form of look up tables that allow interpolation for dose-rate calculations around all practically used wire lengths, with accuracy acceptable for clinical applications.

Anisotropy↗