In-vivo dosimetry for field sizes down to 6 × 6 mm2 in shaped beam radiosurgery with microMOSFET

The aim of this study is to evaluate microMOSFET as in-vivo dosimeter in 6 MV shaped-beam radiosurgery for field sizes down to 6 × 6 mm2. A homemade build-up cap was developed and its use with microMOSFET was evaluated down to 6 × 6 mm2. The study with the homemade build-up cap was performed considering its influence on field size over-cover occurring at surface, achievement of the overall process of electronic equilibrium, dose deposition along beam axis and dose attenuation. An optimized calibration method has been validated using MOSFET in shaped-beam radiosurgery for field sizes from 98 × 98 down to 18 × 18 mm2. The method was detailed in a previous study and validated in irregular field shapes series measurements performed on a head phantom. The optimized calibration method was applied to microMOSFET equipped with homemade build-up cap down to 6 × 6 mm2. Using the same irregular field shapes, dose measurements were performed on head phantom. MicroMOSFET results were compared to previous MOSFET ones. Additional irregular field shapes down to 8.8 × 8.8 mm2 were studied with microMOSFET. Isocenter dose attenuation due to the homemade build-up cap over the microMOSFET was near 2% irrespective of field size. Our results suggested that microMOSFET equipped with homemade build-up cap is suitable for in-vivo dosimetry in shaped-beam radiosurgery for field sizes down to 6 × 6 mm2 and therefore that the required build-up cap dimensions to perform entrance in-vivo dosimetry in small-fields have to ensure only partial charge particle equilibrium.

An optimized calibration method for surface measurements with MOSFETs in shaped-beam radiosurgery

Nowadays MOSFET dosimeters are widely used for dose verification in radiotherapy procedures. Although their sensitive area satisfies size requirements for small field dosimetry, their use in radiosurgery has rarely been reported. The aim of this study is to propose and optimize a calibration method to perform surface measurements in 6 MV shaped-beam radiosurgery for field sizes down to 18 × 18 mm(2). The effect of different parameters such as recovery time between 2 readings, batch uniformity and build-up cap attenuation was studied. Batch uniformity was found to be within 2% and isocenter dose attenuation due to the build-up cap over the MOSFET was near 2% irrespective of field size. Two sets of sensitivity coefficients (SC) were determined for TN-502RD MOSFET dosimeters using experimental and calculated calibration; the latter being developed using an inverse square law model. Validation measurements were performed on a realistic head phantom in irregular fields. MOSFET dose values obtained by applying either measured or calculated SC were compared. For calibration, optimal results were obtained for an inter-measurement time lapse of 5 min. We also found that fitting the SC values with the inverse square law reduced the number of measurements required for calibration. The study demonstrated that combining inverse square law and Sterling-Worthley formula resulted in an underestimation of up to 4% of the dose measured by MOSFETs for complex beam geometries. With the inverse square law, it is possible to reduce the number of measurements required for calibration for multiple field-SSD combinations. Our results suggested that MOSFETs are suitable sensors for dosimetry when used at the surface in shaped-beam radiosurgery down to 18 × 18 mm(2).

[Identification of viable myocardium in ischemic heart disease with severe left ventricular contractile dysfunction: comparison of myocardial scintigraphy with 99mTc-sestamabi and with 201-thallium]

The aim of this study was to compare quantitatively uptake of 99mTc-Sestamibi at rest and that of late redistribution of 201Tl in the same patients with severe ischaemic left ventricular dysfunction, and to correlate the uptake of the tracer to regional ventricular dysfunction studied by ECG grated 99Tc-Sestamini. A double isotope myocardial scintigraphy, 201 Thallium at rest/redistribution and 99Tc-Sestamibi at rest and on exercise, was performed in 28 patients with severe postinfarction ischaemic cardiomyopathy (EF= 29 +/- 4%). Quantitative analysis for each patient and each isotope were performed with respect to the number of hits expressed in percentage of the activity of a normal zone in 17 circumferential profiles distributed in 4 zones, that is to say in 476 segments. A score allowed counting of viable and non-viable segments and evaluation of contractile function of the 17 segments with respect to wall motion and systolic thickening. Total concordance of global uptake of the two isotopes was observed in 430 of the 476 segments (90.3%) (r= 0.814, p< 0.0001), but the 99mTc-Sestamibi uptake was less than 201 Th (71 +/- 23% vs 73 +/- 21%, p= 0.0001). With respect to left ventricular wall motion, uptake of 99mTc-Sestamibi was greater than that of 201Tl in normal or hypokinetic segments but less in akinetic and dyskinetic segments. The difference between the two isotopes was most marked in segments with very severe contractile dysfunction. The authors conclude that the uptake of 99mTc-Sestamibi is correlated with that of late distribution of 201Tl when left ventricular contraction is not too poor and should no longer be considered as only a marker of perfusion but can also be useful in the investigation of myocardial viability.

Chronic motor cortex stimulation for phantom limb pain: correlations between pain relief and functional imaging studies

Chronic motor cortex stimulation (CMCS) has provided satisfactory control of pain in patients with central or trigeminal neuropathic pain. We used this technique in 3 patients with intractable phantom limb pain after upper limb amputation. Functional magnetic resonance imaging (fMRI) correlated to anatomical MRI permitted frameless image guidance for electrode placement. Pain control was obtained for all the patients initially and the relief was stable in 2 of the 3 patients at 2 year follow-up. CMCS can be used to relieve phantom limb pain. fMRI data are useful in assisting the neurosurgeon in electrode placement for this indication.

Perimetric visual field and functional MRI correlation: implications for image-guided surgery in occipital brain tumours

OBJECTIVE

To compare the results of visual functional MRI with those of perimetric evaluation in patients with visual field defects and retrochiasmastic tumours and in normal subjects without visual field defect. The potential clinical usefulness of visual functional MRI data during resective surgery was evaluated in patients with occipital lobe tumours.

METHODS

Eleven patients with various tumours and visual field defects and 12 normal subjects were studied by fMRI using bimonocular or monocular repetitive photic stimulation (8 Hz). The data obtained were analyzed with the statistical parametric maps software (p<10(-8)) and were compared with the results of Goldmann visual field perimetric evaluation. In patients with occipital brain tumours undergoing surgery, the functional data were registered in a frameless stereotactic device and the images fused into anatomical three standard planes and three dimensional reconstructions of the brain surface.

RESULTS

Two studies of patients were discarded, one because of head motion and the other because of badly followed instructions. On the remaining patients the functional activations found in the visual cortex were consistent with the results of perimetric evaluation in all but one of the patients and all the normal subjects although the results of fMRI were highly dependent on the choices of the analysis thresholds. Visual functional MRI image guided data were used in five patients with occipital brain tumours. No added postoperative functional field defect was detected.

CONCLUSIONS

There was a good correspondence between fMRI data and the results of perimetric evaluation although dependent on the analysis thresholds. Visual fMRI data registered into a frameless stereotactic device may be useful in surgical planning and tumour removal.

[Therapeutic indications of symptomatology: fatigue, pain, abnormal movements and reeducation]

Multiple Sclerosis is a chronic progressive and handicapping pathology of the central nervous system. Despite the recent progress no curative molecule has fundamentally modified its prognosis. Within this context symptomatic treatments are inevitable. As part of this consensus conference we have made a systematic critical and objective review about the literature relating how to take care symptomatically of "fatigue", pain, movement disorders and rehabilitation.